Quantifying Greenhouse Gas Fluxes in Agriculture and...

Quantifying Greenhouse Gas Fluxes in Agriculture and Forestry: Methods for Entity-Scale Inventory

Office of the Chief Economist Climate Change Program Office Technical Bulletin 1939 July 2014

United States Department of Agriculture

Authors:StephenM.Ogle,ColoradoStateUniversity(LeadAuthor)PaulR.Adler,USDAAgriculturalResearchServiceJayBreidt,ColoradoStateUniversityStephenDelGrosso,USDAAgriculturalResearchServiceJustinDerner,USDAAgriculturalResearchServiceAlanFranzluebbers,USDAAgriculturalResearchServiceMarkLiebig,USDAAgriculturalResearchServiceBruceLinquist,UniversityofCalifornia,DavisPhilRobertson,MichiganStateUniversityMicheleSchoeneberger,USDAForestServiceJohanSix,UniversityofCalifornia,Davis;SwissFederalInstituteofTechnology,ETH‐ZurichChrisvanKessel,UniversityofCalifornia,DavisRodVenterea,USDAAgriculturalResearchServiceTristramWest,PacificNorthwestNationalLaboratory

Contents:

3 QuantifyingGreenhouseGasSourcesandSinksinCroplandandGrazingLandSystems..... .........................................................................................................................................................................3‐4

3.1 Overview...........................................................................................................................................................3‐53.1.1 OverviewofManagementPracticesandResultingGHGEmissions...........3‐63.1.2 SystemBoundariesandTemporalScale..............................................................3‐103.1.3 SummaryofSelectedMethods/ModelsSourcesofData...............................3‐103.1.4 OrganizationofChapter/Roadmap........................................................................3‐11

3.2 CroplandManagement..............................................................................................................................3‐123.2.1 ManagementInfluencingGHGEmissionsinUplandSystems.....................3‐123.2.2 ManagementInfluencingGHGEmissionsinFloodedCroppingSystems........

...............................................................................................................................................3‐253.2.3 Land‐UseChangetoCropland..................................................................................3‐28

3.3 GrazingLandManagement......................................................................................................................3‐293.3.1 ManagementActivityInfluencingGHGEmissions...........................................3‐303.3.2 Land‐UseChangetoGrazingLands........................................................................3‐36

3.4 Agroforestry..................................................................................................................................................3‐373.4.1 CarbonStocks..................................................................................................................3‐393.4.2 NitrousOxide...................................................................................................................3‐413.4.3 Methane.............................................................................................................................3‐413.4.4 ManagementInteractions...........................................................................................3‐42

Chapter 3

Quantifying Greenhouse Gas Sources and Sinks in Cropland and Grazing Land Systems

Chapter 3: Quantifying Greenhouse Gas Sources and Sinks in Cropland and Grazing Land Systems

3.5 EstimationMethods...................................................................................................................................3‐423.5.1 BiomassCarbonStockChanges...............................................................................3‐433.5.2 LitterCarbonStockChanges.....................................................................................3‐493.5.3 SoilCarbonStockChanges.........................................................................................3‐493.5.4 SoilNitrousOxide..........................................................................................................3‐583.5.5 MethaneUptakebySoils.............................................................................................3‐743.5.6 MethaneandNitrousOxidefromFloodedRiceCultivation.........................3‐773.5.7 CO2fromLiming.............................................................................................................3‐833.5.8 Non‐CO2EmissionsfromBiomassBurning........................................................3‐863.5.9 CO2fromUreaFertilizerApplications...................................................................3‐90

3.6 SummaryofResearchGapsforCropandGrazingLandManagement..................................3‐92Appendix3‐A:SoilN2OModelingFrameworkSpecifications...............................................................3‐97

3‐A.1DescriptionofProcess‐BasedModels.....................................................................3‐993‐A.2EmpiricalScalarsforBaseEmissionRates.........................................................3‐1063‐A.3Practice‐BasedScalingFactors................................................................................3‐108

Appendix3‐B:GuidanceforCropsNotIncludedintheDAYCENTModel....................................3‐113Chapter3References..........................................................................................................................................3‐116

Ogle,S.M.,P.R.Adler,F.J.Breidt,S.DelGrosso,J.Derner,A.Franzluebbers,M.Liebig,B.Linquist,G.P.Robertson,M.Schoeneberger,J.Six,C.vanKessel,R.Venterea,T.West,2014.Chapter3:QuantifyingGreenhouseGasSourcesandSinksinCroplandandGrazingLandSystems.InQuantifyingGreenhouseGasFluxesinAgricultureandForestry:MethodsforEntity‐ScaleInventory.TechnicalBulletinNumber1939.OfficeoftheChiefEconomist,U.S.DepartmentofAgriculture,Washington,DC.606pages.July2014.Eve,M.,D.Pape,M.Flugge,R.Steele,D.Man,M.Riley‐Gilbert,andS.Biggar,Eds.

USDAisanequalopportunityproviderandemployer.

Acronyms,ChemicalFormulae,andUnits

C CarbonCH4 MethaneCO2 CarbondioxideCO2‐eq CarbondioxideequivalentsCRP ConservationReserveProgramEPA U.S.EnvironmentalProtectionAgencyGHG GreenhousegasH2CO3 Carbonicacidha HectareIPCC IntergovernmentalPanelonClimateChangeK PotassiumLRR LandResourceRegionm MeterMg MegagramsN NitrogenN2 NitrogengasN2O NitrousOxideNH4+ AmmoniumNO NitricoxideNO3‐ NitrateNOx Mono‐nitrousoxidesNRCS NaturalResourcesConservationServiceNUE NitrogenuseefficiencyO2 OxygenPg PetagramPRISM Parameter‐ElevationRegressionsonIndependentSlopesModelSOC SoilorganiccarbonSOM SoilorganicmatterSSURGO SoilSurveyGeographicDatabaseTg TeragramsUSDA U.S.DepartmentofAgriculture

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3 QuantifyingGreenhouseGasSourcesandSinksinCroplandandGrazingLandSystems

Thischapterprovidesmethodologiesandguidanceforreportinggreenhousegas(GHG)emissionsandsinksattheentityscaleforcroplandandgrazinglandsystems.Morespecifically,itfocusesonmethodsforlandusedfortheproductionofcropsandlivestock(i.e.,grazinglands).Section3.1providesanoverviewofcroplandandgrazinglandsystemsmanagementpracticesandresultingGHGemissions,systemboundariesandtemporalscale,asummaryoftheselectedmethods/models,sourcesofdata,andaroadmapforthechapter.Section3.2presentsthevariousmanagementpracticesthatinfluenceGHGemissionsinuplandandwetlandcroppingsystemsandland‐usechangetocropland.Section3.3providesasimilardiscussionforgrazinglandsystemsandland‐usechangetograzingsystems.Section3.4discussesagroforestry,andSection3.5providestheestimationmethods.Finally,Section3.6includesasummaryofresearchgapswithadditionalinformationonthenitrousoxide(N2O)methodologyandsupplementalmethodologyguidanceintheAppendices.

3.1 Overview

Croplandandgrazinglandsystemsaremanagedinavarietyofways,whichresultsinvaryingdegreesofGHGemissionsorsinks.Table3‐1providesadescriptionofthesourcesofemissionsorsinksandthesectioninwhichmethodologiesareprovidedalongwiththecorrespondingGHGs.

Table3‐1:OverviewofCroplandandGrazingLandSystemsSourcesandAssociatedGreenhouseGases

SourceMethodforGHGEstimation Description

CO2 N2O CH4

Biomassandlittercarbonstockchanges

Estimating herbaceousbiomasscarbon stockduringchangesinlanduseisnecessarytoaccountfortheinfluenceofherbaceousplantsoncarbondioxide(CO2)uptakefromtheatmosphereandstorageintheterrestrialbiosphereforatleastaportionoftheyearrelativetothebiomasscarbonandassociatedCO2uptakeinthepreviouslandusesystem.Agroforestrysystemsalsohavealongertermgainorlossofcarbonbasedonthemanagementoftreesinthesesystems.

Soilorganiccarbonstocksformineralsoils

Soilorganiccarbon stocksareinfluencedbylanduseandmanagementincroplandandgrazinglandsystems,aswellasconversionfromotherlandusesintothesesystems(Aaldeetal.,2006).Soilorganiccarbonpoolscanbemodifiedduetochangesincarboninputsandoutputs(Paustianetal.,1997).

Soilorganiccarbonstocksfororganicsoils

Emissionsoccurinorganicsoilsfollowingdrainageduetotheconversionofananaerobicenvironmentwithahighwatertabletoaerobicconditions(ArmentanoandMenges,1986),resultinginasignificantlossofcarbontotheatmosphere(Ogleetal.,2003).

DirectandindirectN2Oemissionsfrommineralsoils

N2Oisemittedfromcroplandbothdirectlyandindirectly.Directemissionsarefluxesfromcroplandorgrazinglandswheretherearenitrogenadditionsornitrogenmineralizedfromsoilorganicmatter.Indirectemissionsoccurwhenreactivenitrogenisvolatilizedasammonia(NH3)ornitrogenoxide(NOx),ortransportedviasurfacerunofforleachinginsolubleformsfromcroplandorgrazinglands,leadingtoN2Oemissionsinanotherlocation.

SourceMethodforGHGEstimation Description

CO2 N2O CH4

DirectN2Oemissionsfromdrainageoforganicsoils

Organicsoils (i.e.,histosols) areaspecialcaseinwhichdrainageleadstohighratesofnitrogenmineralizationandincreasedN2Oemissions.Themethodassumesthatorganicsoilshaveasignificantorganichorizoninthesoil,andtherefore,themaininputsofnitrogenarefromoxidationoforganicmatter.

Methaneuptakebysoils

Agronomicactivityuniversallyreducesmethanotrophyinarablesoilsby70%ormore(Mosieretal.,1991;Robertsonetal.,2000;Smithetal.,2000).Recoveryofmethane(CH4)oxidationuponabandonmentfromagricultureisslow,taking50to100yearsforthedevelopmentofeven50%offormer(original)rates(Levineetal.,2011).

MethaneandN2Oemissionsfromricecultivation

ThereareanumberofmanagementpracticesthataffectCH4 andN2Oemissionsfromricesystems.Themethodaddresseskeypracticesincludingtheinfluenceofwatermanagement,residuemanagementandorganicamendmentsonCH4emissionsfromrice(Lascoetal.,2006;Yanetal.,2005)andassociatedimpactsonN2Oemissions.

CO2fromliming

AdditionoflimetosoilsistypicallythoughttogenerateCO2emissionstotheatmosphere(deKleinetal.,2006).However,prevailingconditionsinU.S.agriculturallandsleadtoCO2uptakebecausethemajorityoflimeisdissolvedinthepresenceofcarbonicacid(H2CO3).Therefore,theadditionoflimewillleadtoacarbonsinkinthemajorityofU.S.croplandandgrazinglandsystems.

Non‐CO2emissionsfrombiomassburning

BiomassburningleadstoemissionsofCO2aswellasotherGHGsorprecursorstoGHGsthatareformedlaterthroughadditionalchemicalreactions.Note:CO2emissionsarenotaddressedforcropresiduesorgrasslandburning,becausethecarbonisre‐absorbedfromtheatmosphereinnewgrowthofcropsorgrasseswithinanannualcycle.

CO2fromureafertilizerapplication

UreafertilizerapplicationtosoilscontributesCO2emissions totheatmosphere.TheCO2emittedisincorporatedintotheureaduringthemanufacturingprocess.IntheUnitedStates,thesourceoftheCO2isfossilfuelusedforNH3production.TheCO2capturedduringNH3productionisincludedinthemanufacturer’sreportingsoitsreleaseviaureafertilizationisanadditionalCO2emissiontotheatmosphereandisincludedinthefarm‐scaleentityreporting.

3.1.1 OverviewofManagementPracticesandResultingGHGEmissions

GuidanceisprovidedinthissectionforreportingofGHGemissionsassociatedwithentity‐levelfluxesfromfarmand/orlivestockoperations.TheguidancefocusesonmethodsforestimatingtheinfluenceoflanduseandmanagementpracticesonGHGemissions(andsinks)incropandgrazinglandsystems.Methodsaredescribedforestimatingbiomassandsoilcarbonstockchanges,soilN2Oemissions,CH4emissionsfromfloodedrice,CH4sinksfrommethanotrophicactivity,CO2emissionsorsinksfromliming,biomassburningnon‐CO2GHGemissions,andCO2emissionsfromureafertilizerapplication(seeTable3‐2).

Table3‐2:OverviewofCroplandandGrazingLandSystemsSources,MethodandSection

Section Source Method

3.5.1‐3.5.2

Biomasscarbonstockchanges

HerbaceousbiomassisestimatedwithanIPCCTier2methodusingentityspecificdataasinputintotheIPCCequationsdevelopedbyLascoetal.(2006)andVerchotetal.(2006).WoodyplantgrowthandlossesinagroforestryorperennialtreecropsareestimatedwithanIPCCTier3method,usingasimulationmodelapproachwithentityinput.

3.5.3Soilorganiccarbonstocksformineralsoils

AnIPCCTier3methodisusedtoestimatetheSOCatthebeginningandendoftheyearformineralsoilswiththeDAYCENTprocess‐basedmodel.ThestocksareenteredintotheIPCCequationsdevelopedbyLascoetal.(2006),Verchotetal.(2006)toestimatecarbonstockchanges.

3.5.3Soilorganiccarbonstocksfororganicsoils

CO2emissionsfromdrainageoforganicsoils(i.e.,Histosols)areestimatedwithanIPCCTier2methodusingtheIPCCequationdevelopedbyAaldeetal.(2006)andregionspecificemissionfactorsfromOgleetal.(2003).

DirectN2Oemissionsfrommineralsoils

ThedirectN2OmethodsareestimatedwithanIPCCTier3method.Formajorcommoditycrops,acombinationofexperimentaldataandprocess‐basedmodelingusingtheDAYCENT1modelandDNDC2(denitrification‐decomposition)areusedtoderiveexpectedbaseemissionratesfordifferentsoiltextureclassesineachU.S.DepartmentofAgricultureLandResourceRegion.Forminorcommoditycropsandincaseswherethereareinsufficientempiricaldatatoderiveabaseemissionrate,thebaseemissionrateisbasedontheIPCCdefaultfactormultipliedbythenitrogeninput(deKleinetal.,2006).Theseemissionratesarescaledwithpractice‐basedscalingfactorstoestimatetheinfluenceofmanagementchangessuchasapplicationofnitrificationinhibitorsorslow‐releasefertilizers.

DirectN2Oemissionsfromdrainageoforganicsoils

DirectN2Oemissionsfromdrainageoforganicsoils,i.e.,Histosols,areestimatedwiththeIPCCTier1method(deKleinetal.,2006).

IndirectN2Oemissions

IndirectsoilN2OemissionsareestimatedwiththeIPCCTier1method(deKleinetal.,2006).

3.5.5Methaneuptakebysoils

Methaneuptakebysoilisestimatedwithanequationthatusesaveragevaluesformethaneoxidationinnaturalvegetation—whethergrassland,coniferousforest,ordeciduousforest—attenuatedbycurrentlandusepractices.ThisapproachisanIPCCTier3method.

MethaneandN2Oemissionsfromfloodedricecultivation

IPCCTier1methodsareusedtoestimateCH4andN2Oemissionsfromfloodedriceproduction(deKleinetal.,2006;Lascoetal.,2006).

1TheversionofDAYCENTcodedandparameterizedforthemostrecentU.S.nationalGHGinventory(U.S.EPA,2013)wasusedtoderiveexpectedbaseemissionrates.2DNDC9.5compiledonFeb25,2013wasusedtoderiveexpectedbaseemissionrates.

Section Source Method

3.5.7 CO2fromlimingAnIPCCTier2methodisusedtoestimateCO2emissionsfromapplicationofcarbonatelimes(deKleinetal.,2006)withU.S.‐specificemissionsfactors(adaptedfromWestandMcBride,2005).

3.5.8Non‐CO2emissionsfrombiomassburning

Non‐CO2GHGemissionsfrombiomassburningofgrazinglandvegetationorcropresiduesareestimatedwiththeIPCCTier2method(Aaldeetal.,2006).

3.5.9CO2fromureafertilizerapplication

CO2emissionsfromapplicationofureaorurea‐basedfertilizerstosoilsareestimatedwiththeIPCCTier1method(deKleinetal.,2006).

3.1.1.1 DescriptionofSector

Croplandsincludeallsystemsusedtoproducefood,feed,andfibercommodities,inadditiontofeedstocksforbioenergyproduction.Croplandsareusedfortheproductionofadaptedcropsforharvestandincludebothcultivatedandnon‐cultivatedcrops(U.S.EPA,2013).Cultivatedcropsaretypicallycategorizedasroworclose‐growncrops,suchascorn,soybeans,andwheat.Non‐cultivatedcrops(orthoseoccasionallycultivatedtoreplenishthecrop)includehay,perennialcrops(e.g.,orchardsandvineyards),andhorticulturalcrops.ThemajorityofU.S.croplandisinuplandsystemsoutsideofwetlandsasdefinedinSection4.1.1,Wetlands,anduplandcroppingsystems(i.e.,dryland)mayormaynotbeirrigated.Ricecanbegrownonnaturalorconstructedwetlands,butwewillrefertothesesystemsasfloodedricetoavoidconfusionwithChapter4.Inaddition,wetlandscanalsobedrainedforcropproduction,whichagainisconsideredacroplandbecausetheprincipaluseiscropproduction.Somecroplandsaresetasideinreserve,suchaslandsenrolledintheConservationReserveProgram(CRP).Croplandsalsoincludeagroforestrysystemsthatareamixtureofcropsandtrees,suchasalleycropping,shelterbelts,andriparianbuffers.

Grazinglandsaresystemsthatareusedforlivestockproduction,andoccurprimarilyongrasslands.Grasslandsarecomposedprincipallyofgrasses,grass‐likeplants,forbs,orshrubssuitableforgrazingandbrowsing,andincludebothpasturesandnativerangelands(U.S.EPA,2013).Furthermore,savannas,somewetlandsanddeserts,andtundracanbeconsideredgrazinglandsintheUnitedStatesifusedforlivestockproduction.Grazinglandsystemsinclude:(1)managedpasturesthatmayrequireperiodicclearing,burning,chaining,and/orchemicalstomaintainthegrassvegetation;and(2)nativerangelandsthattypicallyrequirelimitedmanagementtomaintainbutmaybedegradedifoverstockedorotherwiseoverused.

CropandgrazinglandmanagementinfluencesGHGemissions(Smithetal.,2008b),whichcanbereducedbyadoptingconservationpractices(CAST,2004;2011).Operatorsofcroplandsystemsuseavarietyofpracticesthathaveimplicationsforemissions,suchasnutrientadditions,irrigation,limingapplications,tillagepractices,residuemanagement,fallowingfields,forageandcropselection,set‐asidesoflandsinreserveprograms,erosioncontrolpractices,watertablemanagementinwetlands,anddrainageofwetlands.OperatorsofgrazingsystemsalsohaveavarietyofmanagementoptionsthatinfluenceGHGemissions,suchasstockingrate,forageselection,useofprescribedfires,nutrientapplications,wetlanddrainage,irrigation,limingapplications,andsilvopastoralpractices.

3.1.1.2 ResultingGHGEmissions

CroplandandgrazinglandsaresourcesofN2OandCH4emissionsandhavealargepotentialtosequestercarbonwithchangesinmanagement(Smithetal.,2008b).Infact,N2Oemissionsfrom

managementofagriculturalsoilsareakeysourceofGHGemissionsintheUnitedStates(U.S.EPA,2013).N2Oemissionsresultfromtheprocessesofnitrificationanddenitrification,whichareinfluencedbylanduseandmanagementactivity.Landuseandmanagementcanalsoinfluencecarbonstocksinbiomass,deadbiomass,andsoilpools.Carbonstockscanbeenhancedorreduceddependingonlanduseandmanagementpractices(CAST,2004;IPCC,2000;Smithetal.,2008b).Consequently,cropandgrazinglandsystemscanbeeitherasourceorsinkforCO2,dependingonthenetchangesinbiomass,deadbiomass,andsoilcarbon.Burningbiomassisapracticethatcaninitiallyreducebiomasscarbonstockbutcanprovidesufficientstimulustoenhanceensuingecosystemcarbonstorage.Ingeneralthough,burningcausesadeclineinsoilorganiccarbonstocksduetolossofcarboninputfromplantlitterandroots.Burningwillalsoleadtonon‐CO2GHGemissions—CH4,N2O,andotheraerosolgases(CO,NOx)—thatcanbelaterconvertedtoGHGsintheatmosphereoroncedepositedontosoil.

SoilsincropandgrazinglandsystemscanalsobeasourceorsinkforCH4dependingontheconditionsandmanagementofsoil.CH4canberemovedfromtheatmospherethroughtheprocessofmethanotrophyinsoils.Methanotrophyoccursunderaerobicconditionsandiscommoninmostsoilsthatdonothavestandingwater.Incontrast,CH4isproducedinsoilsthroughtheprocessofmethanogenesis,whichoccursunderanaerobicconditions(e.g.,soilswithstandingwatersuchassoilsusedforfloodedriceproduction).Bothoftheseprocessesaredrivenbytheactivityofmicroorganismsinsoils,andtheirrateofactivityisinfluencedbylanduseandmanagement.

3.1.1.3 Managementinteractions

TheinfluenceofcropandgrazinglandmanagementonGHGemissionsisnottypicallythesimplesumofeachpractice’seffect.Theinfluenceofonepracticecandependonanotherpractice.Forexample,theinfluenceoftillageonsoilcarbonwilldependonresiduemanagement.Theinfluenceofnitrogenfertilizationratescandependontheapplicationofnitrificationinhibitors.AvarietyofexamplesisgiveninSection3.2andSection3.3.Becauseofthesesynergies,estimatingGHGemissionsfromcropandgrazinglandsystemswilldependonacompletedescriptionofthepracticesusedintheoperation,includingpastmanagementtocapturelegacyeffectsonGHGemissions,aswellasancillaryvariablessuchassoilcharacteristicsandweatherorclimateconditions.

3.1.1.4 RiskofReversals

AnytrendinGHGemissionsassociatedwithachangeincropandgrazinglandmanagementcanbereversediftheoperatorrevertstotheoriginalpractice.ReversalswillnotnegatetheGHGmitigationforCH4orN2Othatoccurredpriortothereversion.IfemissionsarereducedforCH4orN2O,theemissionreductionispermanentandcannotbechangedbysubsequentmanagementdecisions.

Reversalscanoccurwithcarbonsequestrationinbiomassandsoils.CO2canberemovedfromtheatmospherethroughcropandforageproductionandsequesteredinbiomassorsoilsfollowingtheadoptionofaconservationpractice,suchasno‐till(CAST,2004;USDA,2011).Ifcarbonisincreasinginthebiomassorsoils,thenthepracticeeffectivelyreducestheamountofCO2intheatmosphere.However,netCO2canbereturnedtotheatmosphereifthereisareversioninmanagementtothepreviouspracticethatcausesadeclineinthebiomassorsoilcarbonstocks.Forexample,enrollmentoflandintheCRPhasincreasedtheamountofcarboninsoils(i.e.,increaseinsoilcarbonstock),andthusmitigatesCO2emissionstotheatmosphereassociatedwithotheremissionssources,suchasfossilfuelcombustion(USDA,2011).However,tillingformerCRPlandswillleadtoadeclineinsoilcarbonstocks,therebyreversingthetrendforCO2uptakefromtheatmosphereandleadingtoCO2emissiontotheatmosphere.Ingeneral,GHGemissionsinvolving

carbonstocksinbiomass,deadbiomass,orsoilscanbeconsideredreversible,dependingonfuturemanagementdecisions.Consequently,reversalsinvolvingcarbonstocksnotonlyaffectfutureemissiontrends,butalsohaveconsequencesonpastmitigationeffortsbyreturningpreviouslysequesteredCO2totheatmosphere.

3.1.2 SystemBoundariesandTemporalScale

Systemboundariesaredefinedbythecoverage,extent,andresolutionoftheestimationmethods.ThecoverageofmethodsinthisguidancecanbeusedtoestimateGHGemissionsourcesthatoccuronfarmandranchoperations,includingemissionsassociatedwithbiomasscarbon,littercarbon,andsoilscarbonstockchanges;CH4andN2Ofluxesfromsoils;emissionsfromburningofbiomass;andCO2fluxesassociatedwithureafertilizationandadditionofcarbonatelimes.GHGemissionsalsooccurwithproductionofmanagementinputs,suchassyntheticfertilizersandpesticides,andtheprocessingoffood,feed,fiber,andbioenergyfeedstockproductsfollowingharvest;butmethodsarenotprovidedtoestimatetheseemissions.Moreover,emissionsfromenergyuse,includingthoseoccurringontheentity’soperation,arenotaddressedinthemethods.

Themethodsprovidedforcropandgrazinglandsystemshavearesolutionofanindividualparceloflandorfieldandincludethespatialextentofallfieldsintheentity’soperation.Fieldsareareasusedtoproduceasinglecroporrotationofcrops,ortoraiselivestock(i.e.,pasture,rangeland).Fieldsareoften,butnotalways,dividedbyfences.Emissionsareestimatedforeachindividualfieldthatisusedforcroplandandgrazinglandontheoperation,andthentheemissionsareaddedtogethertoestimatethetotalemissionsfromthecropandgrazinglandsystemsintheentity’soperation.Thetotalsarethencombinedwithemissionsfromforestandlivestocktodeterminetheoverallemissionsfromtheoperationbasedonthemethodsprovidedinthisguidance.EmissionsareestimatedonanannualbasisforasmanyyearsasneededforGHGemissionsreporting.

3.1.3 SummaryofSelectedMethods/ModelsSourcesofData

TheIntergovernmentalPanelonClimateChange(IPCC)(IPCC,2006)hasdevelopedasystemofmethodologicaltiersrelatedtothecomplexityofdifferentapproachesforestimatingGHGemissions.Tier1representsthesimplestmethods,usingdefaultequationsandemissionfactorsprovidedintheIPCCguidance.Tier2usesdefaultmethods,butemissionfactorsthatarespecifictodifferentregions.Tier3usescountry‐specificestimationmethods,suchasaprocess‐basedmodel.ThemethodsprovidedinthisreportrangefromthesimpleTier1approachestothemostcomplexTier3approaches.Higher‐tiermethodsareexpectedtoreduceuncertaintiesintheemissionestimates,ifsufficientactivitydataandtestingareavailable.

Tier1methodsareusedforestimatingCO2emissionsfromureafertilization,CH4emissionsfromfloodedrice,indirectsoilN2Oemissions,anddirectsoilN2Oemissionsfromdrainedorganicsoils.Thesemethodsarethemostgeneralizedglobally,andlackabilitytocapturespecificconditionsatlocalsites,andconsequentlyhavemoreuncertaintyforestimatingemissionsfromanentity’soperation.SoilN2Oemissions,CO2emissionsorsinksfromliming,biomasscarbonstockchanges,soilcarbonstockchangesfordrainedorganicsoils,andbiomassburningnon‐CO2GHGemissionsallhaveelementsofTier2methods,butmayrelypartlyonemissionfactorsprovidedbytheIPCC(2006).ThesemethodsincorporatesomeinformationaboutconditionsspecifictoU.S.agriculturalsystemsandtheinfluenceonemissionrates,butagainlackspecificityforlocalsiteconditionsinmanycases.SoilcarbonstockchangesformineralsoilsareestimatedusingaTier3methodwithaprocess‐basedsimulationmodel(i.e.,DAYCENT).CH4sinksfrommethanotrophicactivityarealsoestimatedwithaTier3method,duetotheabsenceofIPCCguidanceforestimatinglanduseandmanagementeffectsonCH4uptakeinsoils.TheTier3methodassociatedwithsoilcarbonstockchangesinmineralsoilshasthegreatestpotentialforestimatingtheinfluenceoflocalconditionson

GHGemissions.Theapplicationhasageneralsetofparametersthathavebeencalibratedacrossanationalsetofexperiments.However,themodeldoesincorporatedriversassociatedwithlocalconditions,includingspecificmanagementpractices,soilcharacteristics,andweatherpatterns,providingestimatesofGHGemissionsthataremorespecifictotheentity’soperation.FutureresearchandrefinementsofthecroplandandgrazinglandmethodswilllikelyincorporatemoreTier3methodsinthefuture,andthusprovideamoreaccurateestimationofGHGemissionsforentityreporting.

Allmethodsincludearangeofdatasourcesfromvaryinglevelsofspecificityonoperation‐specificdatatonationaldatasets.Operation‐specificdatawillneedtobecollectedbytheentity,andgenerallyareactivitydatarelatedtothefarmandlivestockmanagementpractices(e.g.,tillagepractices,grazingpractices,fertilizerusage).Nationaldatasetsarerecommendedforancillarydatarequirementsthatareusedinmethods,suchasclimatedataandsoilcharacteristics.However,theentitydoeshavetheoptiontouseoperation‐specificdataforclimate(i.e.,weatherdata)andsoils.

3.1.4 OrganizationofChapter/Roadmap

Thecroplands/grazinglandsportionofthisreportisorganizedintofourprimarysections.Sections3.2and3.3provideadescriptionofmanagementimpactsonGHGemissionsincropandgrazinglandsystems.Section3.2isfurthersubdividedintosectionsfocusedonuplandagriculture,floodedmanagementforcropproduction,andtheinfluenceofland‐usechange.Section3.3issubdividedintoageneraldescriptionofmanagementpracticesandtheinfluenceofland‐usechange.ThefirsttwosectionsprovidethescientificbasisforhowmanagementpracticesinfluenceGHGemissions.Thesetwosectionsalsodiscussmanagementoptionsthatrequirefurtherstudy.Section3.4providesanoverviewofagroforestrysystems.AgeneraldescriptionofthevariousGHGemissionsandsinksthatresultfrommanagementpracticesandpotentialmanagementinteractionsisprovidedinthissection.

Section3.5describesthemethods.Eachmethodincludesageneraldescription(includingequationsandfactorsifappropriate),activitydatarequirements,ancillarydatarequirements,limitationsofthemethod,anduncertaintiesassociatedwiththeestimation.AsinglemethodisprovidedforeachoftheGHGemissionsources(andsinks),basedonthebestavailablemethodforapplicationinanoperationalsystemforentity‐scalereporting.Asinglemethodwasselectedtoensureconsistencyinemissionestimationbyallreportingentities.Moreadvancedapproachesmaybeadoptedinthefutureasthemethodsmature.

Section3.6providesasummaryofresearchgaps.Thegapshighlightkeyresearchareasthatrequirefurtherstudyforoneoftworeasons.ThefirstreasonisthatapracticelackssufficientevidenceoraclearimpactonGHGemissionsbasedonexistingresearch.Thisgapismostoftenrelatedtoalackofmechanisticunderstandingoftheprocessesinfluencedbythepractice.Thesepracticesmaybeincludedinfuturerevisionstothemethodsiffurtherstudyleadstoaconsensusthatthepracticehasanimpactonemissions.Thesecondreasonforidentifyingtheneedforfurtherstudyisthatthepracticeisincludedinestimationmethods,butthereisneedforfurtherresearchtoreduceuncertainty.Thissecondgapmayinvolvefurthermechanisticstudy,butcouldalsorequirefurthermethodsofdevelopmentorrefinement.

Finally,Appendix3‐AprovidesamorecomprehensivedescriptionofthesoilN2Omodelingframeworkspecifications.Thisappendixincludesadiscussionoftheprocess‐basedmodelsusedinthemethodology;theempiricalscalarsforthebaseemissionrates;andthepractice‐basedscalingfactors.Appendix3‐BprovidesalternativemethodologiesincaseswhereanentityismanagingcropsnotincludedintheDAYCENTmodel.

3.2 CroplandManagement

HowcroplandismanagedcanhaveasignificanteffectonGHGemissionsandremovals.ThissectionprovidesasummaryofthecurrentstateofthescienceanddescribeshowmanagementpracticesdriveGHGemissionsorsinksinuplandcroplandsystems.

3.2.1 ManagementInfluencingGHGEmissionsinUplandSystems

ThecroplandmanagementpracticespresentedbelowfocusprimarilyonmitigationpotentialforsoilN2O,CH4emissions,andcarbonsequestration.EachsubsectiondescribesthepracticeandtheunderlyingGHGphenomenonthatinfluencemitigationpotential.

3.2.1.1 NutrientManagement(ManufacturedandOrganic)

Nutrientmanagementreferstotheadditionandmanagementofsyntheticandorganicfertilizerstocroplandsoils,primarilytoaugmentthesupplyofnutrientstothecrop.Nitrogenisgenerallythemostimportantnutrientfromanagronomicstandpoint,becauseitisusuallytheprimarynutrientlimitingcropyieldsandoftenmustbeaddedmorefrequentlyandingreateramountsthanothernutrientssuchasphosphorusandpotassium(ERS,2011;RobertsonandVitousek,2009).NitrogenisalsotheprimarynutrientofconcernwithregardtoGHGemissions,becauseoncefertilizernitrogenentersthesoilitcanbedirectlyconvertedtoN2Obysoilbiologicalprocessesand,insomecases,chemicalreactions(FirestoneandDavidson,1989;Kooletal.,2011;Venterea,2007).WhilerelativelylittleofthefertilizernitrogenappliedisconvertedtoN2O,theseemissionsaregenerallyalargecomponentofthetotalGHGbudgetofcroplands(e.g.,Mosieretal.,2005;Robertsonetal.,2000)becauseN2Ohas310timestheglobalwarmingpotentialofCO2(IPCC,2007).Otherformsofnitrogenoriginatingfromfertilizersmayalsobelosttotheenvironment,includingNH3,nitricoxide(NO),andnitrate(NO3‐).Oncetransportedtodownwindordownstreamecosystems,theseothernitrogenspeciescanbeconvertedtoN2O;suchemissionsarereferredtoas“indirect”N2Oemissions(Beaulieuetal.,2011;deKleinetal.,2006).

NutrientmanagementcanalsoaffectGHGemissionsotherthanN2O,mostnotablythesequestrationofcarbonuponmanureadditionandcropresidueretentionoraddition.Theadditionoforganiccarbonamendments,suchasmanureorresidues,canincreasesoilcarbonwithintheboundariesofthelandparcelreceivingtheamendment(Ogleetal.,2005).However,soilcarbonlossesmayoccurfromthesourcefield(Schlesinger,2000)dependingonthemanagement(Izaurraldeetal.,2001).Manufacturednitrogenadditionscanalsoleadtocarbonsequestration(Ladhaetal.,2011)whereadditionsleadtoincreasedresiduereturntosoil.

Fertilizerrate,timing,placement,andformulationstronglyaffectN2Ofluxes.Ingeneral,anypracticethatincreasescropnitrogenuseefficiency(NUE)wouldbeexpectedtoreduceN2Oemissions,becauseappliednitrogenthatistakenupbycropsorcovercropsisnotavailabletothesoilprocessesthatgenerateN2O,atleastintheshortterm;thisalsomaypreventnitrogenleaching.Thus,strategiestoreduceN2OemissionscanalsoreducethelossofNO3‐andotherformsofreactivenitrogenfromcroppingsystems.

However,practicesthatimproveNUEwillnotalwaysreduceN2Oemissions.Differentfertilizerformulations,forexample,canresultindifferentN2OemissionsirrespectiveofNUEeffects(e.g.,GagnonandZiadi,2010;Gagnonetal.,2011).Likewise,bandedfertilizerplacementcanincreaseNUE(e.g.,Yadvinder‐Singhetal.,1994)butalsocanincreaseratherthandecreaseN2Oemissions(e.g.,Engeletal.,2010),andtillagemanagementcanalsoincreaseNUEwithoutreducingN2Oemissions(Grandyetal.,2006).Thus,NUEisgenerallyimportantbutnotbyitselfsufficienttopredictormanageN2Oemissions.Fertilizerrate,timing,placement,andformulationcanaffectNUEandN2Oemissionsindependently.

FertilizerRate:Morethananyotherfactor,theamountofnitrogenfertilizerappliedtosoilaffectstheamountofN2Oemitted;inmanycasesothernitrogen‐usestrategies(timing,placement,andformulation)providetheirbenefitbyeffectivelyreducingfertilizernitrogenavailableinthesoil.Inthissense,fertilizerrateintegratestheeffectsofmultiplepracticesandisthebasisfortheIPCCTier1N2Oaccountingmethod(deKleinetal.,2006),wherebyN2Oemissionsareassumedtobeasimplefractionofnitrogeninputs.

Irrespectiveofotherpractices,however,fertilizerrateitselfcanberefinedtoreduceN2Oemissionssolongasratesarenotreducedtothepointthatyieldsdecline.Otherwisemarketleakage—theneedtomakeupyieldselsewherewithmoreintensivefertilizeruseandconcomitantN2Oloss—maylimitthebenefitofreducinglocalfertilizerrates.Thequestionthenbecomeswhethernitrogenfertilizerratescanbereducedwithoutreducingyieldsinaparticularfield.Atleastforcorn,recentchangesinrecommendedfertilizerratesformanyMidwestStatessuggestthatthereislatitudeforreducingfertilizernitrogenratesforsomefarmers.Sincethe1970s,mostfertilizernitrogenrecommendationshavebeenbasedonyieldgoals,whichuseexpectedmaximumyieldmultipliedbynitrogenyieldfactorstocalculatefertilizerrecommendations(Stanford,1973).Precedinglegumecrops,manureinputs,andsoilnitrogentestsarethenusedtofurtherrefineorreducerecommendednitrogenapplicationrates(AndraskiandBundy,2002).

Analternativetotheyield‐goalapproachistheMaximumReturntoNitrogenapproach(Sawyeretal.,2006),wherebytherateofnitrogenfertilizerappliedisbasedonthemaximumfertilizerratethatgeneratessufficientadditionalyieldtojustifythefertilizercost.Theratesaredeterminedfromcropnitrogenresponsecurves.Typically(butnotalways)thisrateissignificantlylessthanthatrecommendedbytheyieldgoalapproach.MaximumReturntoNitrogencalculatorsforcornhavebeenadoptedinatleastsevenStatesintheMidwest.Thiscalculatorandsimilardecisionsupporttoolshavethepotentialforreducingtheamountoffertilizernitrogenappliedtocropsandmorepreciselymatchcropnitrogenrequirements,withoutaffectingthenetreturns(Archeretal.,2008),andwithconcomitantdecreasesinN2Oemissions(Millaretal.,2010).

Hundredsoffertilizeradditionexperimentsworldwidehaveshownthattypically0.5to3percentofnitrogenaddedtosoilisemittedasN2O(Bouwmanetal.,2002;Linquistetal.,2011;StehfestandBouwman,2006).Site‐to‐sitevariationiswellrecognizedandistobeexpectedbasedonsoils,climate,andfertilizerpractices—includingrate.Recentevidencesuggeststhatemissionratesmaybeevenhigheratnitrogeninputlevelsthatexceedcropdemand(Hobenetal.,2011;Maetal.,2010;McSwineyandRobertson,2005;VanGroenigenetal.,2010).

FertilizerTiming:Amajorchallengeinmanagingnitrogenfertilizerforcropproductionissynchronizingnitrogenavailabilityinthesoilwiththecrop’sdemandfornitrogen.Ingeneral,cropdemandfornitrogenisminimalearlyinthegrowingseasonandincreasesseveralweeksafterplanting.

Inmanycases,itmaybemostconvenientand/orcost‐effectivefortheproducertoapplynitrogenfertilizerpriortoplantingorsoonafterplantemergence.InmanypartsoftheU.S.CornBelt,however,applicationofnitrogenfertilizercommonlyoccursinthefallpriortothegrowingseason(Biermanetal.,2011;Ribaudoetal.,2011).Intheabsenceofanactiveandwell‐developedrootsystemtoutilizethefertilizernitrogen,thesepracticesincreasethepotentialforsoilmicrobialandchemicalprocessestotransformtheappliednitrogenintoN2OandothermobileformssuchasNO3,whichcancontributetoindirectN2Oemissions.

Improvingthesynchronybetweensoilnitrogenavailabilityandcropnitrogendemandcanbeachievedbyswitchingfromfalltospringnitrogenapplication;applyingnitrogenseveralweeksafterplantingwith“sidedress”fertilizerapplicationsthataretimedtocoincidewithplantgrowthstages;andusingmultiple“split”applicationsdistributedintimeoverthegrowingseason.Eachof

thesestrategieshasthepotentialtoreduceN2Oemissions,butthisisnotalwaysthecase.Switchingfromfalltospringnitrogenfertilizer,forexample,hasbeenshowntoreduceN2Oemissionsinsomecases(Burtonetal.,2008a;Haoetal.,2001)butnotalways(Burtonetal.,2008a).Similarly,switchingfrompre‐planttopost‐plantapplicationshasbeenshowntoreduceN2Oemissionsinsomestudies(Matsonetal.,1998),butonlypartofthetimeornotatallinotherstudies(Burtonetal.,2008b;Phillipsetal.,2009;Zebarthetal.,2008b).Somestudieshavefoundreducednitrateleaching,whichimpliesreducedindirectN2Oemissions,withfertilizerapplicationlaterintheseason(e.g.,Errebhietal.,1998).

FertilizerPlacement:ThemannerinwhichnitrogenfertilizerisappliedtosoilcanaffectitsavailabilityforcropuptakeandthereforeitssusceptibilitytosoiltransformationandN2Oproduction.ThreeaspectsoffertilizerplacementaresignificanttoN2Oemissions:(1)broadcastapplicationversusbandingwithinthecroprow;(2)thesoildepthtowhichnitrogenisapplied;and(3)addingfertilizeruniformlyacrossafieldversusapplyingataspatiallyvariablerate.

ThereissomeevidencethatapplyingnitrogenfertilizerinnarrowbandscanimprovecropNUE(MalhiandNyborg,1985).However,bandingalsocreateszonesofhighlyconcentratedsoilnitrogen,whichcanincreaseN2Oproductioncomparedwithbroadcastapplications(Engeletal.,2010).OtherstudieshavefoundnodifferencesinN2Oemissionsinbroadcastversusbandedapplications(Burtonetal.,2008a;Sehyetal.,2003).DirectcomparisonsofapplicationdeptheffectsonN2Oemissionshavealsoshowninconsistentresults(e.g.,BreitenbeckandBremner,1986b;Druryetal.,2006;Fujinumaetal.,2011;Hosenetal.,2002;Liuetal.,2006).However,variablerateapplicationusesdifferentnitrogenratesfordifferentareasoffield,basedonexpectedvariationsincropnitrogendemand.Thisisanewtechniquethatappearspromisingbasedonitsabilitytosubstantiallyimprovefertilizeruseefficiencyatthefieldscale(Mamoetal.,2003;Scharfetal.,2005),andatleastoneearlystudyhasshownreducedN2Oemissionswhennitrogenratewasvariedtomatchcropyieldpotential(Sehyetal.,2003).

FertilizerFormulationandAdditives:ThemostcommonlyusedformsofsyntheticnitrogenfertilizerintheUnitedStatesincludeanhydrousammonia(35percentoftotaluse),urea(24percent),andliquidsolutions,includingureaammoniumnitrate(29percent)(ERS,2011).AvailableevidencesuggeststhatN2Oemissionsfollowingapplicationsofanhydrousammoniaaregreaterthanemissionsfollowingbroadcasturea,althoughinsomestudiesthismaybepartlyduetofertilizerplacement.Infivestudies,anhydrousammoniaresultedin40to200percentgreaterN2Oemissionscomparedwithbroadcasturea(BreitenbeckandBremner,1986a;Fujinumaetal.,2011;Thorntonetal.,1996;Ventereaetal.,2005).Onestudy(Burtonetal.,2008a)foundnodifferenceinN2Oemissionsbetweenanhydrousammoniaandbroadcastureawhenbothwereappliedatalowerrate(80kgNha‐1year‐1)comparedwiththeotherstudies(≥120kgNha‐1).Consequently,theremaybeathresholdintheapplicationratebeforethereisasignificanteffectonemissions.

Thechemicalformofnitrogenfertilizerinfluenceslossesofnitrogenfromthreemajorpathways:surfacevolatilization,soilmicrobialprocesses,andNO3‐leaching.Allfertilizersaresusceptibletodenitrificationoncenitrifiedto(orappliedas)NO3‐.Ammonium‐basedfertilizers,includinganhydrousammonia,urea,andorganicsourcessuchasmanure,arealsosusceptibletoN2Olossduringnitrification.Urea,anhydrousammonia,andmanureareadditionallysusceptibletosurfacevolatilizationasNH3undersomeconditions.VolatilizedNH3andleachedNO3‐contributetoindirectN2Oloss.

Chemicaladditiveshavebeendevelopedtoreleasefertilizernitrogenintothesoilmoregraduallyandtodelaythenitrificationofnitrogenfromammonium(NH4+)toNO3‐inordertoimprovethesynchronybetweencropnitrogendemandandsoilnitrogenavailability.Polymer‐coatedureaslowlyreleasesnitrogenwithincreasingsoiltemperatureandwater,andisintendedtomake

nitrogensupplymoresynchronouswithplantnitrogendemandandreducenitrogenlosses.EffectsonN2Oproduction,however,appearmixed,withsomestudiesshowingreducedN2Oforpolymer‐coatedurea(e.g.,Hyattetal.,2010)andothersshowingnoimpactorevenhigheremissions(Ventereaetal.,2011a).Arecentmeta‐analysisof13studiesofmostlyvolcanicandwetland‐derivedsoilsfoundthatpolymer‐coatedureareducedN2Oemissionsby35percentonaveragecomparedwithconventionalfertilizers,butresultsaredifficulttogeneralizebecausemostofthesoilsincludedintheanalysiswerenottypicalforU.S.croppingsystems(Akiyamaetal.,2010).

Fertilizersformulatedwithnitrificationinhibitorscanpotentiallyreduceemissionsfromnitrificationanddenitrification,aswellasNO3‐leaching.SomeU.S.fieldstudiesshowsubstantialreductionsinN2Oemissionswhenfertilizerswithnitrificationinhibitorsareaddedcomparedwithconventionalfertilizers(e.g.,Halvorsonetal.,2010a),whileothersshowlittleornoimpact(e.g.,ParkinandHatfield,2010a).Ameta‐analysisofsome28studiesworldwidereportedanaveragereductionof38percent(Akiyamaetal.,2010),butagainresultsaredifficulttogeneralizeduetothesmallsamplesizeandsoilsthatarenottypicalofU.S.croppingsystems.

Onereasontheimpactsoffertilizersdesignedtoreduceemissionsareinconsistentisthattheformofnitrogenappliedinteractswithotherfactorstocontrolnitrogenlosses.Amongthesefactorsisweather,whichdirectlyaffectstheprocessesthatleadtogaseousnitrogenlossesandNO3‐leaching,andindirectlyaffectstheseprocessesbycontrollingplantnitrogenuptake.Soilpropertiessuchastextureandhydraulicstatusarealsoimportant.Ingeneral,nitrificationisimportantinwell‐aeratedsoils,whiledenitrificationismoreimportantinpoorlydrainedsoils.Thenitrogensourcealsointeractswithothermanagementpractices.Forexample,polymer‐coatedureasubstantiallyreducedN2Oemissionsunderno‐tillbutnotfulltillcultivationforirrigatedcorninColorado(Halvorsonetal.,2010a).

OrganicFertilizerEffectsonN2OEmissions:LandapplicationofanimalmanurehasbeenrelatedtoN2Oemissions.Mosieretal.(1998)andPetersen(1999)measuredincreasesinN2Oemissionswithmanureapplication.KaiserandRuser(2000)measuredannualemissionsoftheaddednitrogeninslurryrangingfrom0.74to2.86percent,andDeKleinetal.(2001)foundthatannualN2O‐Nlossesrangedfromzerotofivepercentoftheorganicnitrogenappliedtosoils.Others(e.g.,BartonandSchipper,2001)foundN2OemissionsfollowingtheadditionofmanureslurriesexceededemissionsfromanequivalentamountofmanufacturedN,likelyduetotheslurry’screatingenhancedconditionsfordenitrification.However,GHGemissionsalsooccurifmanureismanagedinpits,lagoons,orsolidstorage.

Injectionofmanureisacommonpracticetoavoidsurfacerunoffandreduceobjectionableodorsfrommanureapplication.BothFlessaandBesse(2000)andWulfetal.(2002)suggestedthatinjectionofswinemanurewouldcreatemorefavorableconditionsforN2OandCH4formationbecauseofthereducedaerationwithinthesoil.However,Dendoovenetal.(1998)didnotfinddifferencesineitherN2OorCH4emissionsfrominjectedorsurface‐appliedswineslurryontoaloamysoil.Thesefindingssuggestthattherate,timing,placement,andformulationofmanureisimportanttoN2Oproduction,similartomanufacturednitrogenfertilizer,butthereisaneedforadditionalresearch.

CO2EmissionsGeneratedfromUreaFertilizerApplications:Unlikeothernitrogenfertilizers,urearesultsinthedirectproductionofCO2inadditiontowhateverN2Omightbesubsequentlyproducedbymicrobes(deKleinetal.,2006).Sinceureais20percentC,everymetrictonofureaappliedtosoilresultsinthedirectemissionof20kgCO2‐C;alternatively,everykilogramofnitrogenappliedasurearesultsinthedirectemissionsof0.43kgCO2‐C.UreaismanufacturedbyreactingNH3andCO2toformammoniumcarbamate,whichisthendehydratedtoformureaprills.IntheUnited

StatestheCO2inureaiscapturedfromthefossilfuelusedtomanufactureNH3,sothesoilCO2producedrepresentsafossilfuelemission.

ManagementSystemInteractions:NitrogenmanagementpracticescaninteractwithothercroplandmanagementcomponentsinregulatingGHGemissions.Asemphasizedabove,anyfactorthataffectscropNUEhasthepotentialtoaffectN2Oemissions.Therefore,optimizingotherpractices—includingtillageandthemanagementofsoilpH,pests,irrigation,drainage,andotherfactors—willtendtoincreasenitrogenfertilizeruptakebythecropandthereforereduceN2Oemissions.Forthisreason,nutrientmanagementeffectsonGHGemissionsshouldbeconsideredinthecontextoftheentiresetofcroplandmanagementpractices.Forexample,thereisevidencethatfertilizerplacementcaninteractwithtillagemanagementincontrollingN2Oemissions(Ventereaetal.,2005),andthatinadequatemanagementofothernutrients(e.g.,phosphorusandpotassium)canreduceNUE(Snyderetal.,2009).EffortstominimizeorremediatewaterqualityimpactsofnitratetransportfromfarmtoaquaticsystemsmayalsoreduceindirectN2Oemissions.Forexample,theuseofsubsurfacebioreactorstoremovenitratefromdrainagewaterhasbeneficialimpactsonindirectN2O.However,todatethesebioreactorshavenotbeenimplementedatlarge(field)scalesandtherearealsoquestionsaboutreleaseofN2OandCH4duringthetreatmentprocessthatneedtobeansweredbeforetheirneteffectonGHGscanbeassessed(Elgoodetal.,2010).Also,environmentalandclimatefactors,whicharegenerallynotundermanagementcontrol,mayaffectN2Oemissions;forexample,nitrogenfertilizerappliedjustbeforelargerainfalleventscanstimulateincreasedemissions(Lietal.,1992).

3.2.1.2 TillagePractices

Differenttillagepracticesaregenerallyclassifiedintooneofthreecategories:fulltillage,reducedtillage,ornotillage.Tillageintensityisbasedonimplements,numberofpasses,andthepercentageofsurfaceanddepthoftillagedisturbance.Toolsareavailabletodeterminetillageintensity(e.g.,theSTIRModel;seeUSDANRCS,2008).No‐tillagepracticesarecharacterizedbytheuseofseeddrillsandfertilizerorpesticideapplicatorswithnoadditionaltillageeventsorimplements.Surfaceresiduesarenotincorporatedintothesoilwhenfollowingno‐tillagepractices,andthereislimiteddisturbancetothesoilprofile;consequentlyno‐tillagemanagementincreasessoilcoverandimprovesaggregatestability(Sixetal.,2000).Incontrast,examplesoffulltillage(oftenreferredtoasconventionaltillage)includeoneormorepasseswiththefollowingtillageimplements:moldboardplow,diskplow,diskchisel,twistedpointchiselplow,heavydutyoffsetdisk,subsoilchiselplow,andbedderordiskripper.Systemsarealsoclassifiedasfulltillageiftherearetwoormorepasseswithoneofthefollowingimplements:chiselplow,singledisk,tandemdisk,offsetdisk‐lightduty,one‐waydisk,heavy‐dutycultivator,ridgetill,orrototiller.Systemswithothertillagepractices,suchasasinglepasswitharidgetillimplement,mulchtill,orchiselplow,leadtointermediatedisturbanceofthesoilandareclassifiedasreducedtillage.

Changesintillagepracticescaninfluenceverticaldistributionofcarboninthesoilprofileandtotalsoilcarbonstocks(Paustianetal.,1997).Historically,fulltillagehasresultedinthereductionofsoilcarbonstocks(Laletal.,2004).Asynthesisofpreviousanalysesestimatedthatlong‐termfulltillagecandecreasesoilcarbonstocksby30percent(Ogleetal.,2005;Westetal.,2004).ChangingfromfulltillagetonotillagecanreversehistoriclossesofsoilC.No‐tillagepracticescanleadtoaccumulationofsoilcarbonintheuppersoilprofile(0to30cm),withlittletonochangeinthelowersoilprofile(30to60cm)(Syswerdaetal.,2011).Theopposite,adecreaseintheuppersoilhorizonwithanincreaseinthelowersoilhorizon,cansometimesoccurwithachangefromnotillagetofulltillage(Bakeretal.,2007).However,changesinthelowersoilprofiletendtobemorevariable,therebyrequiringalargersamplesizetodetectsignificantdifferences(KravchenkoandRobertson,2011).Areductionincarboninputassociatedwiththeinfluenceofno‐tillmanagementoncropproductionmayalsoleadtolossesofsoilcarbon,particularlyincoolerandwetterclimates

(Ogleetal.,2012).However,whiledifferencesintheresponseofsoilcarbonstockstotillageoccuramongfieldexperiments,comprehensiveanalysesofavailablefielddataindicatethat,onaverage,soilcarbonstocksincreasewithachangefromfulltillagetoreducedtillageornotillage,especiallywithlong‐termadoptionofnotillage(Ogleetal.,2005;Sixetal.,2004;WestandMarland,2002).

Decreasedtillageintensityincreasessoilcarbonbecauseofreduceddisturbancetosoilaggregates,reducedexposureofsoilorganicmattertoweatheringprocesses,anddecreaseddecompositionrates(Paustianetal.,2000).Theextenttowhichsoilcarbonaccumulationoccursafterareductionintillageintensityisdeterminedbythehistoryoflandmanagement,soilattributes,regionalclimate,andcurrentcarbonstocks(WestandSix,2007).Ingeneral,greatersoilcarbonaccumulationwillbeobservedinC‐poorsoils(i.e.,duetolong‐termcultivation)withaclayeytextureunderhighbiomasscroppingsystemsintemperatehumidandwarmclimates(FranzluebbersandSteiner,2002;Planteetal.,2006;Sixetal.,2004).Insomecases,intermittenttillage,duringlong‐termreducedornotillage,isneededtoreducesoilcompaction,forweedcontrol,ortoreducepestsorpathogens.Whileintermittenttillagecancauseadecreaseinsoilstocks,upto80percentofsoilgainsfromno‐tillagepracticescanbemaintainedwhenimplementingnotillagewithintermittenttillage(Conantetal.,2007;Ventereaetal.,2006).

TheeffectoftillagemanagementchangesonsoilN2Oemissionsisvariableandnotfullyunderstood.Increases(Rochette,2008),decreases(Mosieretal.,2006),andnochanges(Grandyetal.,2006;Lemkeetal.,1998)insoilN2Oemissionshavebeenobserved.However,thosedifferencesarenottotallyrandomandpastmeta‐analyseshaveconcludedthatclimateregime,durationofpractice,andnitrogenfertilizerplacementhaveinfluencedtillageeffectsonN2Oemissions(Sixetal.,2004;vanKesseletal.,2012).Othervariablessuchassoiltexturemayalsobeimportant.

RegionalclimatehasalsobeenidentifiedasamajordriverforthechangeinN2Oemissionswithadoptionofno‐tillagepractices,withemissionsincreasinginhumidclimatesanddecreasingindryclimates(Sixetal.,2004).However,timesinceadoptionofnotillagemightalsoplayarolewithhigheremissionsinitiallyafteradoptionofnotillageinbothhumidanddryclimates,butovertimeemissionsfromno‐tillagesystemsmaydeclineinhumidclimatesrelativetopreviousemissionsfromfulltillagesystems.Nevertheless,variousfieldstudieshaveshownmixedresults,bothsupportingandcontradictingthefinding.StudiesindrierclimatesoftheGreatPlainshaveshownadecreaseinemissionsevenwhenno‐tillagepracticeshadbeenadoptedforlessthan10years(Kessavalouetal.,1998;Mosieretal.,2006).Long‐termnotillageinmoistclimatesofMinnesotaandCanadaledtobothhigherandloweremissionsofN2O(Druryetal.,2006;Ventereaetal.,2005).

AnotherimportantfactorinfluencingN2Oemissionsundernotillage,andonethatfarmerscanactivelymanage,isfertilizerplacement(vanKesseletal.,2012).Ventereaetal.(2005)foundthatwhennitrogenfertilizerwasplacedonthesurface,N2Oemissionsweregreaterundernotillagethanfulltillage,butthereversewasfoundwhennitrogenfertilizerwasplacedbelow10centimeters.FertilizerplacementingeneralhasbeenfoundtohavedifferingresultsonN2Oemissions,asdiscussedinSection3.2.1.1.However,thefindingsofVentereaetal.(2005)aswellasotherstudies(e.g.,Groffman,1985;VentereaandStanenas,2008)indicatethatdeepernitrogenplacementtendstodecreaseN2Oemissionswhenaccompanyingno‐tillorreduced‐tillagepractices,atleastrelativetofulltillagecroppingsystemsatthesamelocation.TheconflictingresultsassociatedwithN2Oemissionsfromfertilizerapplicationsmaybepartlyexplainedbythetillagepractice.

Inaddition,Lemkeetal.(1998)determinedthatsoilclaycontentexplained92percentofthevariationinN2OemissionsbetweenfulltillageandnotillageacrossmultiplesitesinAlberta.Similarly,Burfordetal.(1981)foundthatemissionsfromno‐tillagepracticesweregreaterthan

fromfulltillageonsoilswithhigherclaycontentsatastudysiteintheUnitedKingdom.ItisarguedthatsoilswithhigherclaycontentshavehighermoisturecontentandthereforehaveagreaterpotentialforincreasedN2Oemissionsundernotillage.Indeed,Rochette(2008)attributedhigherratesofN2Ofluxfromminimumversusstandardtillagetogreatersoilcompaction,poorsoildrainage,reducedgasdiffusivity,andair‐filledporosityinhighclaysoils.

3.2.1.3 CropRotations,CoverCrops,andCroppingIntensity

Croprotationreferstothesequenceofcropsplantedinafield,withinoracrossyears.Croprotationsvarybylocationandgrowingregion,andmaybepracticedforavarietyofreasonssuchasimprovedeconomicreturns,pestmanagement,diseasecontrol,nutrientmanagementandwateravailability.Asimplerotationmaybeasequenceofcornandsoybeansthatisrepeatedovertime,whilemorecomplexrotationsmightincludeperennialcropssuchasalfalfawithcornandsunflowerrotationoverfiveyears,withthreeyearsofalfalfaandoneyeareachofcornandsunflower.Theactualrotationscanalsovaryfromastrictordertothesequence,particularlyinresponsetomarketdemand,i.e.,opportunisticrotations.Rotationswithhighbiomass‐yieldingcropsorperennialhaycropsorgrasscovercanincreasesoilcarbonstocks(Ogleetal.,2005).

Croppingintensitycanvaryacrossyears,duetovariationsinfallowfrequencyanduseofmultiplegrowingseasonswithmorethanonecropplantedandharvestedinasingleyear.Forexample,insemi‐aridenvironments,croprotationsoftenincludeayear‐longfallowperiodinordertoincreasetheamountofwaterstoredinthesoilprofileforthesubsequentcrop.Thislimitstheamountoforganicmatterinputtothesoil,andwiththeseverewaterlimitation,thesecroppingsystemsproducesmallamountsofbiomass,leadingtoareductioninsoilcarbonstocks(Doranetal.,1998).Consequently,intensifyingcropproductionbyreducingfallowfrequency,whichwillgenerallyinvolveadoptionofno‐tillagepractices,willincreasecarboninputacrossthewholerotationandpossiblytheamountofsoilorganiccarbon(Sherrodetal.,2003;2005).

Wintercovercropscanalsobeusedtoprovideplantcoveroutsideofthenormalgrowingseason.Priortoplantingthefollowingsummercrop,thecovercropiseitherlefttodecomposeasagreencoverorharvestedforforage.Ingeneral,theinclusionofacovercropinacroprotationwillleadtoanincreaseinsoilcarbonduetotheincreasedcarboninputderivedfromthecovercrop(Kongetal.,2005),especiallycovercroproots(KongandSix,2010).Covercropscanalsobeusedeffectivelyfornitrogenmanagement.InthefallandspringtheycancapturesoilnitrogenthatwouldotherwisebetransformeddirectlytoN2ObysoilmicrobesorleachtogroundwaterandcontributetoindirectN2Oemissions(i.e.,offsiteemissionsduetonitrogenlossesfromthesite).Additionally,whenkilledpriortoplantingthemaincrop,theirdecompositioncanprovidenitrogenthatwilldisplacesomeportionofcropfertilizationrequirements(whethermanufacturedororganic).Therefore,covercropscanreduceindirectN2Oemissionsandpossiblyoffsetfertilizationrates.However,therearenostudiesdemonstratingthataddingnitrogentosoilsincovercropsratherthanthroughfertilizationwillreducedirectN2Oemissions.Inthefuture,covercropbiomassmayalsobeharvestedforcellulosicethanolfeedstock,leavingrootstoenhancesoilcarbonstockssimilartoperennialplantsgrowninrotation(Ogleetal.,2005).

Theeffectsofcroprotationandintensityonsoilorganiccarboncanalsointeractwithothermanagementpractices,suchasresiduemanagement,tillage,andirrigation(Eghballetal.,1994).Consequently,managementinteractionsamongpracticesincludingtillageandirrigationwillbeimportantindeterminingtheinfluenceofcroprotationsonGHGemissions.Additionally,cropselectionasacomponentofcroprotationcanhaveamajoreffectonN2Oemissions(CavigelliandParkin,2012)insofarascropscanvaryintheirnitrogenuseefficienciesandnitrogenfertilizerneeds.Thisisparticularlythecasewhenlong‐livedperennialcropsaresubstitutedforannualcropsinforageorcellulosicbiofuelcroppingsystems(Robertsonetal.,2011).

3.2.1.4 Irrigation

Typesofirrigationsystemsincludesurfaceorfloodirrigation,(micro‐)sprinklerirrigation,subsurfacedripirrigation,andsubirrigation.Ingeneral,irrigationincreasessoilwatercontent,evapotranspirationrates,andrelativehumidity;decreasessoilandairtemperatures;andcanleadtoincreasedregionalprecipitation(LobellandBonfils,2008;Pielkeetal.,2007).ThesechangesaffectimportantprocessessuchasplantgrowthratesandsoilmicrobialactivitythatcontrolnetGHGfluxes.

Assoilwatercontentapproachessaturation,oxygen(O2)diffusionisinhibited,resultinginanaerobicconditionsthatcanenhanceCH4emissions(ChanandParkin,2001;Delgadoetal.,1996),oratleastreducetheCH4sinkstrengthofotherwiseaerobicsoils(Livesleyetal.,2010).SaturatedconditionsalsoenhancedenitrificationratesandpotentiallyN2Oemissions(Delgadoetal.,1996;Jambertetal.,1997;Livesleyetal.,2010),butnotethatpeakN2OemissionsfromdenitrificationoftenoccuratwatercontentslowerthansaturationbecausewhenO2isextremelylimiting,N2OislikelytobefurtherreducedtoN2beforediffusingfromthesoilsurfacetotheatmosphere(Davidson,1991;Dunfieldetal.,1995).Furthermore,nitrificationratespeakatapproximately50percentofsaturation,andwatercontentsclosetofieldcapacity(60to70percentofsaturation)areexpectedtosupportmaximumtotalN2Oemissionrates(Davidson,1991).Inaddition,irrigationcanincreaseindirectN2OemissionsbyenhancingNO3‐leachingandrunoffifmorewaterisaddedthanisevaporated(Gehletal.,2005;Spaldingetal.,2001).

WettingofdrysoilstypicallyincreasesCO2emissions(FiererandSchimel,2002).However,irrigationalsoincreasesplantgrowthratesand,therefore,soilorganiccarbonlevelstypicallyincreaseafteruplandcroppingisconvertedtoirrigatedcropping,althoughlossofsoilcarbonfromerosioncanalsoincreaseunderirrigation(Follett,2001;Laletal.,1998).Furthermore,irrigationcanaffectinorganiccarbonlevels,butcurrentavailabledatashowcontrastingresults(Blanco‐Canquietal.,2010;Denefetal.,2008;Entryetal.,2004).

FloodandSurfaceIrrigation:Floodirrigationinvolvesfloodingtheentirefieldwithwater.Undercontinuouslyfloodedconditions,soilsarehighlyanoxic,thusfacilitatinghighmethanogenesisanddenitrificationrates(Mosieretal.,2004).However,highdenitrificationratesdonotnecessarilyimplyhighN2OemissionsbecausetheextremelyanoxicconditionsfacilitatefurtherreductionofN2OtoN2beforeitisemittedfromthesoil(Mahmoodetal.,2008).ThisissupportedbyobservationsshowinghigherN2Oemissionsfromintermittentcomparedtocontinuouslyfloodedricesystems(Katayanagietal.,2012;Xuetal.,2012),althoughitremainsdifficulttopredicttherelativeportionofdenitrifiednitrogenthatisemittedasN2OrelativetoN2.

Surfaceirrigationalsoinvolvessupplyinglargeamountsofwatertothesurfaceofsoils,butinthiscasethewaterisaddedthroughfurrowsadjacenttocropbeds.Thesesystemsareoftennotveryefficient,becausewaterlossesfromevaporationandseepagecanbelarge.TheimpactoffurrowirrigationonGHGemissionsdependsonhowoftenandtheextenttowhichfurrowsarefilledwithwater.WettinganddryingcyclesarelikelytoemitlargepulsesofNOandN2O(Davidson,1992),aswellasCO2(FiererandSchimel,2002).Spatialvariabilitycanalsobehigh,suchasthehigherN2Oemissionsfromfurrowscomparedwithbedsthathavebeenobservedforirrigatedcottoncropping(Graceetal.,2010).Inaddition,microtolandscapescaleheterogeneityinenvironmentalconditions,duetotopographyandotherfactors,contributetomultiscalevariabilityinN2Oemissions(Hénaultetal.,2012;Yatesetal.,2006).ThisspatialandtemporalheterogeneityinenvironmentalconditionsandfluxratesmakesitverydifficulttoquantifyGHGfluxesfromthesetypesofsystemswithhighlevelsofaccuracyandprecision.

SprinklerSystems:Sprinklersystemsdeliverwatertovegetationandthesoilfromabovethesurfaceusingoverheadsprinklersorguns.Thisisusuallymoreefficientthansurfaceirrigation,but

evaporativelossesfromwaterinterceptedbyvegetation,litter,andthesoilsurfacecanstillbesubstantial.Duringandshortlyafterirrigationevents,soilmaybecomesaturatedandemitpulsesofN2O,butbecausethesoilisnotcontinuouslysaturated,N2Oemissionsareexpectedtobelowercomparedwithsurfaceirrigation(NelsonandTerry,1996).BothN2Oemissionsandsoilcarbonlevelsareexpectedtoincreasewithsprinklerirrigationcomparedwithuplandcropping.

SurfaceandSubsurfaceDripIrrigation:Surfacedripirrigationsupplieswaterfromdriplinesplacedadjacenttocroprows.Evaporativelossesshouldbelesscomparedwithabove‐surfacesprinklersystems,becauselesswaterisinterceptedbygrowingvegetation.However,evaporativelossescanstilloccurtotheextentthatsurfacelitterandsoillayersabsorbwaterfromthedripsprinkler.TheimpactsofsurfacedripirrigationonGHGfluxesareexpectedtobesimilartothoseofsprinklersystems,althoughthereisearlyevidencethatbothsurfaceandsubsurfacedripirrigationleadstolessemissionsofCH4andN2O(Kallenbachetal.,2010;Kennedyetal.,2013).

Subsurfacedripirrigationtargetswaterdeliverytotherootzoneusingburiedpipesandtubing.Thesesystemscanbeveryefficient,becausewaterisconcentratedintherootzoneataslow,steadyrate,henceminimizingoreliminatingevaporationlossesandavoidingsaturationofthewholesoilprofile.Consequently,thesesystemsarenotexpectedtobelargeCH4sources(DelGrossoetal.,2000a).Soilwatercontenthaslesstemporalvariationwithsubsurfacedripirrigationcomparedwithsprinklerandsurfacesystems,sopulsesofN2OandCO2emissionsarealsoexpectedtobeofsmallermagnitude(Kallenbachetal.,2010).Similarly,subsurfacedripirrigation/fertigationofhighvaluescrops,suchastomatoes,hasbeenshowntoreduceN2Oemissionscomparedwithfurrowirrigation(Kennedyetal.,2013).

Subirrigation:Subirrigationisusedinareaswithrelativelyhighwatertablesandinvolvesartificiallyraisingthewatertabletoallowthesoiltobemoistenedfrombelowtherootzone.Becausewaterissuppliedtorootsfrombelow,evaporationlossesarenotenhancedastheywouldbewithsurfaceirrigationsystems.ThissystemcandecreaseNO3‐leaching(Elmietal.,2003)butmayincreaseN2Olossesfromdenitrification(Munozetal.,2005).

ManagementInteractions:Irrigationsystemsinteractwithothercropmanagementstrategiessuchaschangesincroprotation,croppingintensity,tillage,andfertilizeramounttocontrolnetGHGfluxes.IrrigationtendstoamplifytheeffectsofthesefactorsonN2OandCH4emissionsatthesametimeasthepracticesincreasecropyieldsandsoilcarbonstocks.However,theresponseofsoilcarbontoirrigationiscomplexanddrivenbyinteractingfactors.Whenwaterandnutrientstressarereducedthroughirrigationandfertilization,theportionoftotalplantproductionallocatedbelowgroundcandecrease,butabsolutebelowgroundproductionandsoilorganiccarboncanincrease(Bhatetal.,2007).Howevernotallexperimentsshowincreasedsoilcarbonwithirrigation(Denefetal.,2008).Consequently,theirrigationbenefitsofincreasedyieldsandpotentialcarbonstoragemaybecounter‐balancedwiththeincreasedN2OandCH4fluxes.

However,therearealsooptionsforlimitingemissions,particularlywithfertilization.Fertigationaddsnutrientstotheirrigationsystemtodeliverwateralongwithsolublenutrientstotherootzone.Thesesystemshavethepotentialtobeveryefficientfrombothnutrientandwateruseperspectives(Spaldingetal.,2001),becausetheslowandtimedsupplyofnutrientsandwaterismoresynchronouswithplantdemandandtheyareconcentratedintherootzone.Consequently,N2Oandothernitrogenlossesareminimizedwhileplantgrowth,carboninputs,andcarbonsequestrationcanbemaximized.Similarly,CH4emissionsareminimizedbecausesoilsaturationisavoided.

3.2.1.5 ErosionControl

Soilerosionprocessesincludesoildetachment,transport,anddeposition.Soilerosioncanpotentiallyreducesoilcarbonstocksandincreasenetcarbonfluxtotheatmospherethroughdecreasedplantproductivityandsubsequentdecreasedorganicmatterinputtosoilandincreaseddecompositionoftheerodedsoilfraction(Lal,2003).However,soilerosioncanalsopotentiallyincreasenetsoilcarbonstocksanddecreasenetcarbonfluxtotheatmospherethroughdynamicreplacementofsoilcarbononerodedlandscapesanddecreaseddecompositionratesinzonesofsoildeposition(Hardenetal.,1999;Stallard,1998).

Lal(2003)estimatedthat20percentofcarboninerodedsoilisemittedtotheatmosphere,duetooxidationofsoilorganiccarbonfollowingthedisruptionofsoilaggregatescausedbydetachmentandtransport.However,inananalysisof1,400soilprofiles,VanOostetal.(2007)foundnegligiblecarbonlossasadirectresultofsoildetachmentandtransport.Atsiteswherethetransportedsoilwasdeposited,therewasaslight(~onepercent)decreaseinsoilcarbondecompositionrates,resultinginslightlyhighersoilcarbonaccumulation.Moreimportantly,itwasfoundthatonaverage,25percentoferodedcarbonwasreplacedontheerodedsitesovera50‐yearperiod(Hardenetal.,2008).Thecombinationofthesefindingssupportsanapproximate26percentsinkcapacityoferodedsoil(VanOostetal.,2007).

Theaccumulationofsoilcarbononerodedlocationswithinlandscapesisreferredtoasdynamicreplacement(Hardenetal.,1999).Dynamicreplacementoccursasaresultofsoilcarbonbuildingtowardasteadystateofsoilcarboncontent,constrainedbysoiltypeandclimate(WestandSix,2007).Steadystateoccurswhensoilcarbonaccumulationequalssoilcarbonlosses.BothVanOostetal.(2007)andLalandPimentel(2008)notethatthedynamicreplacementratemaybelowinareaswithlowercroplandproductioninputs.Forexample,dynamicreplacementmaybelowincropsystemswithlowresidueproduction,suchascottonandtobaccointheUnitedStates,whichhavelowercarbonaccumulationratesthanhighresidueinputscrops(Ogleetal.,2005).

Notethatwhilewatererosioncangenerateasmallcarbonsink,thebenefitofacarbonsinkisoffsetbyothernegativeimpactsfromsoilerosion.Forexample,soilerosioncanresultinwaterpollutionduetosedimentloading,airpollutionfromairborneparticulatematter(PM10),anddecreasedsoilfertilityresultinginsubsequentyielddeclines.

3.2.1.6 ManagementofDrainedWetlands

Drainageofwetlandseffectivelycreatesanuplandcroppingsystembyloweringwatertableswithtilesorditchestoproduceannualcrops.Themostobviouseffectofwetlanddrainageisincreasedoxidationandtillageofsoils.Forexample,conversionofnativewetlandsandgrasslandsintocroplandhasbeenshowntodepletenativesoilcarbonstocksby20tomorethan50percent(BlankandFosberg,1989;Eulissetal.,2006;Mann,1986).Inturn,CO2emissionsincreasewithhigherdecompositionrates,particularlyinorganicsoils,i.e.,Histosols(Allen,2012;ArmentanoandMenges,1986).LossoftheorganiclayerhascausedtremendoussubsidenceinU.S.croplands(Stephensetal.,1984)suchastheFloridaEverglades(Shihetal.1998)andtheCaliforniaDeltaregion(Broadbent,1960;Weir,1950),whereratesvaryfrom0.46to2.3cmyear‐1(DeverelandRojstaczer,1996;Devereletal.,1998;RojstaczerandDeverel,1995).SimilarsubsidencerateshavealsooccurredinotherregionssuchastheFloridaEverglades.

Manipulationofwaterlevelscanhavemultipleeffectsonnutrientcyclinginwetlands.Drainagealsomayresultinmoreoptimalsoilmoistureconditions(e.g.,40to60%water‐filledporespace)thatenhanceformationofN2Oasabyproductofnitrificationanddenitrificationreactions(Davidsonetal.,2000).DrainageincreasesnitrogenmineralizationrateswithconversionfromanaerobictoaerobicconditionsandenhancesN2Oemissions(Duxburyetal.,1982;Kasimir‐

Klemedtssonetal.,1997).Incontrast,drainagedecreasesCH4emissionsbyreducingthefrequencyanddurationofsoilsaturationrequiredforCH4productionaswellasenhancingfrequencyofmethanotrophicactivity(Dorretal.,1993;Gleasonetal.,2009;PhillipsandBeeri,2008).However,insituationswherewetlandsareinacropproduction,butnotdirectlydrained,CH4productioncanactuallybeenhancedduetoincreasedrunofffromadjacentcroplandsorconsolidationdrainage,whichincreaseswaterdepthandhydroperiods(Gleasonetal.,2009).

ManagingthewatertablebyraisingthedepthofdrainagetotheextentpossiblehasbeenaneffectivemeasuretoreducelossofCO2andotherGHGsfromdrainedorganicsoils(Jongedyketal.,1950;Shihetal.,1998).RecentresearchsuggeststhatevenperiodicfloodingoforganicsoilsthataredrainedforcropproductionmaybeeffectiveinreducingCO2emissions(Morrisetal.,2004).Thereislimitedinformationontheeffectofdrainageinmineralsoilswithahighwatertable(i.e.,hydricsoils),buttheinfluenceonGHGemissionsislikelylesssignificantthanindrainedorganicsoils.Itisimportanttonotethatwetlandsareaffordedsomeprotectionbylaws(e.g.,CleanWaterAct)andconservationprogramsthatrecognizetheimportanceofwetlands,suchasforwildlifehabitat,andprovideagriculturalproducersincentivestoavoiddrainingwetlands(e.g.,the“Swampbuster”provisionoftheFoodSecurityAct).

3.2.1.7 LimeAmendments

Agriculturallimeconsistsprimarilyofcrushedlimestone(CaCO3)anddolomite(CaMg(CO3)2)invaryingproportions.Agriculturallime,hereinafterreferredtoaslime,isappliedtosoilstodecreasesoilacidity.Limeiscommonlyappliedtoagriculturallandswherenitrogenousfertilizersarecontinuouslyusedandwhereprecipitationexceedsevapotranspiration.

TheapplicationoflimetosoilscancreateasinkorsourceofCO2totheatmosphere(Hamiltonetal.,2007),dependingonthestrengthoftheweatheringagent.Weatheringoflimebycarbonicacid(H2CO3),formedwhenCO2isdissolvedinwater,resultsintheuptakeofonemoleofCO2foreverymoleoflime‐derivedcarbondissolved(Eq.1).Carbonicacidweatheringproducesbicarbonate(HCO3‐)thatcontributestoalkalinityingroundwater,streams,andrivers(OhandRaymond,2006;Raymondetal.,2008).Alternatively,whenlimereactswiththestrongernitricacid(HNO3),whichisproducedwhennitrifyingbacteriaconvertNH4+basedfertilizerandothersourcesofNH4+tonitrate(NO3‐),carboninlimeisdissolvedandreleaseddirectlytotheatmosphere(Eq.2).

CaCO3+H2O+CO2=Ca2++2HCO3‐ Eq.1

CaCO3+2HNO3=Ca2++2NO3‐+H2O+CO2 Eq.2

Fieldmeasurementsandmodelinganalysesindicatethatmorelimeisdissolvedbycarbonicacidthanbynitricacid.Forexample,WestandMcBride(2005)estimatedthat62percentoflimewasdissolvedbycarbonicacidweathering,Hamiltonetal.(2007)estimated75to88percent,andOhandRaymond(2006)estimated66percent.Biasietal.(2008)usedchamberfluxmeasurementstoestimate15percentlossoflime‐derivedcarbonbydissolutionwithstrongacidsandinferredthat85percentisdissolvedbycarbonicacid.

WestandMcBride(2005)alsoestimatedtheprecipitationofHCO3‐backtoCaCO3onceHCO3‐reachestheocean,therebyreleasingCO2totheatmosphere.However,thelongtimeperiod(manydecadestocenturies)overwhichprecipitationwouldoccurintheocean(Hamiltonetal.,2007)effectivelyresultsincarbonsequestrationforannualaccountingpurposes.

Currentconsensusofleacheddrainagesamples,streamgaugedata,andmassbalancemodelingindicatesthatabout66percentofcarboninappliedlimeisessentiallytransferredfromonelong‐livedpool(CaCO3ingeologicformations)toanother(HCO3‐inoceans),andisthereforenotcountedasnewsequestration.However,theatmosphericCO2newlycapturedbythisprocessdoes

representsequestrationwhencorrectedforthe33percentreleasedtotheatmosphereasCO2;thisresultsinanet33percentsinkstrengthpercarboninlime.ThisestimateissimilartothatofOhandRaymond(2006)andWestandMcBride(2005),andiswithintherangeofHamiltonetal.(2007).Whilelimecanincreasesoilcarbonviaeffectsonsoilmicrobialactivity(Fornaraetal.,2011),inmostsoilsliminghasnodirectcarboneffect(Pageetal.,2009).

3.2.1.8 ResidueManagement

Cropresiduesaretheresidualremainingafterharvestoftheeconomicpartofthecrop.Theamountofcropresiduevarieswiththecropandtheharvestoperationmethod.Forexample,cottonharvestcontributesverylittleabovegroundresiduetothesoilduetotheplant’slowleafareaindexandsmallamountofplantmaterialafterleafdrop.SoybeanandotherlegumecropsalsohavesmallamountsofabovegroundresiduethatrapidlydecomposebecauseoflowC:Nratios.Incontrast,cropslikecorncanleavesubstantialamountsofresidueonthesoilsurfaceunlessthewholeplantisharvestedforsilageortheresidueiscollectedforbeddingorotherpurposes.

Abovegroundresiduemanagemententailsfivepotentialstrategies:(1)leavetheresidueonthesoilsurfacetodecayandbeincorporatedintothesoil(requiresno‐tillmanagement);(2)incorporatetheresidueintothesoilviatillage;(3)removetheresiduethroughaharvestingoperation(i.e.,silageorcellulosicbiomassharvest);(4)allowlivestocktograzeontheresidue;or(5)burntheresidue.EachofthesemanagementpracticeshasthepotentialtoaffectGHGemissions.LeavingcropresidueonthesurfaceandincorporatingitintothesoilafterdecaybymicroorganismsaffectsCO2releasefromthesoilduetotheenhancedbiologicalactivity,andpotentiallyincreasesN2Oemissionsthroughanalterationofthenitrogenbalanceinthesoil.Asimilarprocessoccurswhenresidueisincorporatedintothesoilviatillage.Notethattillagealsocausesreductionsinsoilcarbonstocks,andadditionalCO2isreleasedthroughburningfueltoruntillageequipment.HarvestingtheresiduereleasesCO2fromburningfuelintheengineslinkedwiththeharvestingprocess,althoughresidueharvestedforbiofuelproductionmaycreatenetfossilfueloffsetcredits.BurningcropresiduesinthefieldreleasesCO2,CH4,andN2O(aswellasCOandNOx)emissionstotheatmosphere.Ingeneral,butnotalways,residueremovalreducessoilcarbonstocks(GreggandIzaurralde,2010;Wilhelmetal.,2007).

ManagementinteractionsarealsoimportantwhenconsideringtheinfluenceofresiduemanagementonGHGemissions.Forexample,theinfluenceofresiduemanagementonsoilorganiccarbonwillbeaffectedbythetillagepractices(Malhietal.,2006).

3.2.1.9 Set‐Aside/ReserveCropland

The1985FarmBillestablishedtheConservationReserveProgram(CRP)topayproducerstoconverthighlyerodiblecroplandorotherenvironmentallysensitiveagriculturalareasintovegetativecover.Theseareascouldbeconvertedintograssland,nativebunchgrasses,pollinatorhabitat,shelterbelts,filterorbufferstrips,orriparianbuffers.Areasareremovedfromproductionandseededwithannualandperennialspeciestoformacoverthatwouldbeundisturbedforaminimumof10years.Inreturn,producersorlandownersreceivedapaymentforenrollingtheselandareasintotheCRP.ThroughouttheagriculturalhistoryoftheUnitedStates,therehavebeentimesinwhichagriculturallandsweresetasidetoreduceagriculturalsurpluses;however,thetimeperiodofremovalwastypicallyshort‐term(onetotwoyears)andmaintainedinaweed‐freestate.

TheprimaryaimsofCRParetodecreaseerosion,restorewildlifehabitat,andsafeguardgroundandsurfacewaterquality.Animportantancillaryaimiscarboncapture:CRPlandssequestercarboninsoilandlong‐livedplants,andthusrepresentavaluablemitigationopportunity.Inameta‐analysisofpairedsoils,Ogleetal.(2005)foundthat20yearsofset‐asideresultedintemperateregionsoils’accumulating82to93percentofthecarbonlevelsunderoriginalnative

vegetation,onaverage.PostandKwon(2000)concludedfromaglobalmeta‐analysisthat,onaverage,soilcarbonsequestrationratesonlandconvertedfromagriculturalproductiontograsslandis33gCm‐2year‐1.At39pairedCRP‐cropsitesinWisconsin,Kucharik(2007)foundsequestrationratesof50gCm‐2year‐1onMollisolsand44gCm‐2y‐1onAlfisols.Follettetal.(2009)estimatethatCRPsoilssequester~50gCm‐2year‐1onaverage.TheCouncilforAgriculturalScienceandTechnology(2011)estimatesthatCRPlandsarecurrentlyresponsiblefor6.3Tgofsoilcarbonsequestrationperyear.Gebhartetal.(1994)reportedamean18.8percentincreaseonfiveCRPsitesduringasix‐yearperiod.However,therearestudiesshowinglittleornoincreaseinC,leadingtouncertaintyintheeffectofset‐asidelandinareserveprogram(JelinskiandKucharik,2009;Karlenetal.,1999;Reederetal.,1998).Forexample,Karlenetal.(1999)comparedCRPlandwithperennialgrassestocroplandacrossfiveStatesandfoundthatonlyonesiteofthefiveshowedasignificantdifferenceintotalorganiccarboncontentinthesoilafterbeinginCRP.

IncreasesinsoilcarbonresultingfromsettingasidecroplandinCRPcanbereversedbyconvertingtheselandsbackintoproduction.Gilleyetal.(1997)foundthatthepositivechangesinCRPlanddisappearedimmediatelywhenthesoilsweretilleduponconversionbackintocropproduction.However,manystudiesindicatethatiflandunderCRPisreturnedtocultivation,someorallofthesoilcarboncanpotentiallyberetainedifthelandiscultivatedwithno‐tillpractices(BowmanandAnderson,2002;Daoetal.,2002;Olsonetal.,2005).Inadditiontochangesinsoilcarbonstocks,changeswillalsooccurinN2Oemissionsdependingonthenutrientmanagementpractices.Gelfandetal.(2011)measuredanetcarboncostof10.6MgCO2‐eqha‐1(289gC‐eqm‐2)forthefirstyearofno‐tillsoybeansfollowing20yearsofCRPgrassland,andasignificantportionofthenetemissionwasduetoN2Oproducedintheconversionyear.

3.2.1.10 Biochar

Biocharisasoilamendmentthatispromotedforitsabilitytoimprovecropproductionandsequestercarboninsoils(Atkinsonetal.,2010;Lehmann,2007a;2007b).Biocharischarcoalproducedwhenwoodorotherplantbiomassisburnedunderlow‐oxygenconditions,knownaspyrolysis.Whenappliedtosoils,biocharcanpersistforlongperiodsoftime;itschemicalstructuremakesitresistanttomicrobialattackundermostsoilconditions.However,itspersistencecanvarygreatlyforreasonsnotyetcompletelyunderstood.BiocharisacommoncomponentofmostU.S.agriculturalsoils(Skjemstadetal.,2002),leftfromfiresthatoccurredpriortoconversionoftheoriginalforestorprairie.Addingbiochartosoilshasbeenproposedasawaytosequestercarbon(Lehmann,2007a)becauseofthispotentialtopersistforcenturies(KimetuandLehmann,2010;Nguyenetal.,2008).Butbiochar’slongevityinsoildependsonanumberoffactorsincludingpyrolysisconditions(e.g.,pyrolysistemperature)andthechemicalcompositionofthebiocharfeedstock(Spokas,2010).Climateandsoilfactorssuchasmineralogyandpre‐existingorganicmattercontentalsoaffectbiochar’spersistenceinsoil.

Anadditionalbenefitofbiocharisitspositiveeffectsonagriculturalsoilfertility(Atkinsonetal.,2010;Lairdetal.,2010),largelybyprovidingadvantagessimilartootherformsofsoilorganicmatter:improvedsoilstructure,waterholdingcapacity,andcation‐exchangecapacity.BiocharhasalsobeenshowntoreducesoilN2Oemissionsinsomelaboratorystudies,butthesmallnumberoffieldtrialssofarreportedhavedocumentednosignificanteffectsunderfieldconditions(e.g.,Scheeretal.,2011).

Itistooearlytoknowifpromisingresultsfromlaboratoryandshort‐termfieldexperimentscanbegeneralizedtolong‐termfieldconditions.Biocharsoiladditionsmaybeafuturesourceofcarboncreditsforpyrolysiswasteiflong‐termfieldexperimentsconfirmresultsfromshortertermstudies.Theclimateadvantageofaddingbiochartosoilislessclear,however,relativetootherpotentialusesofplantbiomass.Lifecycleanalyses(e.g.,Robertsetal.,2010)suggestthatbiocharmay

increaseordecreasenetemissionsdependingonalternativeusesoftheoriginalbiomassandlifecyclesystemboundaries.Furthermore,ifthebiomass(orbiochar)wasburneddirectlyforenergythenthesourceofdisplacedenergymustalsobeconsidered(Robertsetal.,2010).Nevertheless,boththesequestrationandN2Osuppressionpotentialofbiocharmeritfurtherstudy.

3.2.2 ManagementInfluencingGHGEmissionsinFloodedCroppingSystems

ThereareavarietyoffloodedcroppingsystemsintheUnitedStates,includingsystemsforrice,wildrice,cranberries,andtaro.Apartfromrice,thesesystemsarerelativelyminor(specialtycrops)andthereislittletonoresearchorinformationontheirGHGemissions.RicesystemsemitbothCH4andN2O;however,manyreportsshowaninverserelationshipbetweenCH4andN2Oduringthericecroppingseason,withCH4occurringunderanaerobicconditionsandN2Oemissionsoccurringunderaerobicconditions(Zouetal.,2005).Therefore,toaccuratelydetermineamitigationstrategyoneneedstoconsiderthenetcumulativeeffectofGHGemissionsbyevaluatingbothCH4andN2O.WaterandresiduemanagementhavereceivedthemostattentionintermsofofferingpossibilitiesformitigatingCH4emissions.Othermitigationoptionshavealsobeenexaminedandshowpromise(e.g.,Fengetal.,2013;Linquistetal.,2012;Majumdar,2003;WassmannandPathak,2007;Yagietal.,1997)andfurtherresearchisrequiredinmanyareasbeforetheseoptionscanbescaledup.TheintenthereisnottoprovideareviewoftheliteraturebuttoprovideabriefoverviewofsomefactorsaffectingGHGemissionsfromfloodedricesystems.

3.2.2.1 WaterManagementinFloodedRice

IntheUnitedStates,riceisplantedinoneoftwoways:(1)waterseeded,whereseedsaresownbyairplaneinfloodedfields;or(2)dry‐seeded,whereseedsaredrilledorbroadcast(thenincorporated)intodryfields.WaterseedingisthepredominantpracticeinCaliforniaandpartsofLouisiana,whiledryseedingispredominantinmuchofthesouthernUnitedStates(e.g.,Arkansas,Mississippi,Missouri,andTexas).Watermanagementvariesbetweenthesetwoestablishedpractices.Inwater‐seededrice,thefieldsaretypicallyfloodedfortheentireseason.However,inLouisiana,thefieldmaybedrainedwithapinpointfloodsystem(threetofivedays)orwithadelayedflood(upto20days)afterseeding.Indry‐seededrice,rainfallorflushirrigationeventsarerelieduponduringthefirstthreetofiveweeksofestablishmentandthenfloodedfortherestoftheseason.Inallcases,fieldsaretypicallydrainedafewweeksbeforeharvesttoallowthesoiltodryoutenoughtosupportharvestequipment.FurtherdetailsofU.S.riceproductionsystemscanbefoundinSnyderandSlaton(2001)andStreetandBollich(2003).

MidseasondrainorintermittentirrigationisastrategytomitigateCH4emissions.Thispracticeresultsinaerobicconditionsthatareunfavorableformethanogens.However,suchconditionsarefavorableforN2Oemissions(e.g.,Zouetal.,2005).MoststudiesreportthatmidseasondrainssignificantlydecreaseCH4emissionsbutincreaseN2Oemissionsrelativetocontinuousflooding.Regardless,netGHGemissionsinricesystemsareusuallydecreasedwithmidseasondraindespitetheincreaseinN2O.Wassmanetal.(2000)reportedthatCH4emissionreductionsrangedfromsevenpercentto80percent.ThereductioninCH4emissionsdependsonthenumberofdrainageeventsduringthecroppingseasonandonothermanagementfactorsandsoilproperties.Yanetal.(2005)reportedthatCH4fluxesfromricefieldswithsingleandmultipledrainageeventswerereducedby60percentand52percentcomparedtocontinuouslyfloodedricefields.ThispracticehasnotbeenwidelyevaluatedintheUnitedStates,anditmaybedifficulttodrainandre‐floodthelargerelativelyflatparcelsoflandthatarecommonlyusedforriceproductionintheUnitedStates.Furthermore,suchpracticescanleadtoincreasedweedanddiseasepressurealongwithloweryieldsandgrainquality.

Soilcarbonstocksarealsoinfluencedbywatermanagement.Forexample,carbonstocksinChinesericesystemsarehigherthaninuplandcrops,presumablyduetotheaccumulationofcarbonunderthefloodedconditions(Panetal.,2010;Wu,2011).ItremainsunknownifeffortstomitigateCH4emissionsintheUnitedStatesusingintermittentfloodingwillleadtoareductioninsoilcarbonstocks.

TheuseofmidseasondrainagehasbeenshowntodelayharvestinCalifornia.Therefore,inclimateswithashortgrowingseason,theuseofamidseasondrainwillincreaseriskofcropfailure,andthereforewillbealessappealingalternativetogrowers.

3.2.2.2 ResidueManagement

StrawmanagementhasalargeimpactonCH4production.Strawadditions,particularlythosewithahighcarbontonitrogenratio,increaseCH4emissionsbuthavethepotentialtoreduceN2Oemissions(e.g.,Zouetal.,2005).ThisreductioninN2OmaybeduetoincreasednitrogenimmobilizationormoreeffectiveconversiontoN2.LowcarbontonitrogenorganicmaterialstendtoincreaseN2Oemissions(Kaewpraditetal.,2008).Yanetal.(2005)reportedthatthetimingofstrawapplicationisalsoanimportantfactor.Forexample,applyingricestrawbeforetransplantingincreasedCH4emissionsby2.1times,whileapplyingricestrawinthepreviousseasonincreasedCH4emissionsby0.8times.SeveralstudieshavedemonstratedthatcompostingricestrawpriortoincorporationreducesCH4emissions(Wassmannetal.,2000);however,thisrequiresadditionalenergytocollectthestrawandthenspreaditbackonthefieldaftercomposting.

IncontrasttothepotentialforreducingCH4emissionswithremovalofricestraw,thereisalsothepotentialtoreducesoilcarbonstocksduetolesscarboninputtosoils.Othernutrients(particularlyK)areremovedinlargeamountswithresidues,andtheseneedtobereplacedtomaintaintheproductivityofthesystem.

3.2.2.3 OrganicAmendments

Variousorganicamendmentscanbeappliedtoricefields,includingfarmyardmanurespecialtymixesoforganicfertilizers,andgreenmanures(e.g.,covercrops).Basedonameta‐analysisbyLinquistetal.(2012),livestockmanureincreasesCH4emissionsby26percentandgreenmanuresincreasedCH4by192percent.NeithermanuresourcehadasignificanteffectonN2Oemissions.FewstudieshaveevaluatedtheinfluenceofdifferentmanurestorageandprocessingtechniquesonCH4emissions.OneexampleisastudybyWassmanetal.(2000),whofoundthatfermentationoffarmyardmanurepriortoapplicationcanreduceCH4emissions.FarmyardmanurewillalsoinfluencesoilcarbonstockandsoilN2Oemissions.

3.2.2.4 Varieties,RatoonCropping,andFallowManagement

SeasonalCH4(Lindauetal.,1995)andN2O(Chen‐Ching,1996)emissionsareaffectedbyricevariety.Thecauseofvarietaldifferencesvarybutmaybeduetogastransportthrougharenchymacells,differentrootingstructures,ordifferencesamongvarietiesintermsofrootexudates(WassmannandAulakh,2000).IdentifyingthemechanismsforvarietaldifferencesmayenablebreedingprogramstoselectvarietiesthathavelowerCH4emissions.

InsomeStates,theclimateallowsre‐sproutingofasecond,orratooncrop,thatgrowsfromthestubbleofthefirstcropafterharvesting.Ratooncropyieldsaresmallerthanthefirstcrop,butcanaddsubstantiallytotheoverallannualyield,therebyreducingcostsofproductionperunit.Inaddition,ittakesfewerresourcesandlesstimetogrowaratooncropthantogrowthefirstcrop.However,ratooninghashigherCH4emissionrates(abouttwotothreetimeshigher)thanthefirstcrop,becausethestrawfromthefirstcropremainsinthefieldunderanaerobicconditionsduringtheratoonperiodratherthanthefieldbeingdrainedsothatthestubblecandecayaerobically

(Lindauetal.,1995).Therefore,theamountofCH4producingorganicmaterial(i.e.,materialavailableforanaerobicdecomposition)isconsiderablyhigherthanwiththeprimarycrop.

ManagementofricefieldsduringthewinterhasasignificanteffectonannualGHGemissions.Forexample,inCalifornia,legislationinthe1990shaslimitedtheburningofricestrawtoamaximumof25percentofanarea,althoughinrealityonlyabout10percentofriceproductionfieldsareburned.Currently,ricestrawisincorporatedafterharvestonabout85percentofthericeproductionfieldsinCalifornia,andinthesefieldsabouthalfareintentionallyfloodedtofacilitatestrawdecomposition,althoughthisvaluecanvarywidelyfromyeartoyear.WinterfloodinghasincreasedannualCH4emissions(Devitoetal.,2000),butithasalsoincreasedthequalityofhabitatforoverwinteringwaterfowlinthePacificFlyway.Ricestrawisbaledandremovedonaboutfivepercentofthearea.

3.2.2.5 NitrificationandUreaseInhibitorsinFloodedRice

NitrificationinhibitorspreventorslowtheconversionofNH4+toNO3‐andthusreduceN2Oemissionsfromnitrificationandsubsequentdenitrification.Inameta‐analysisoftheseproducts,Akiyamaetal.(2010)foundthatinricesystemstheuseofnitrificationinhibitorsonaveragereducedN2Oemissionsby30percent,althoughsomeproductsweremoreeffectivethanothers.Certainnitrificationinhibitors(i.e.,dicyandiamide,thiosulfate,andencapsulatedcalciumcarbide)canmitigatebothCH4andN2Oemissions.ReducedCH4emissionsusingdicyandiamidewasattributedtoahigherredoxpotential,lowerpH,lowerFe2+,andlowerreadilymineralizablecarboncontent(Bharatietal.,2000).

Ureaseinhibitors,suchashydroquinone,slowthemicrobialconversionofureatoNH4+,thusreducingtheamountofnitrogenavailablefornitrificationanddentrification.BothCH4andN2Oemissionswerereducedwiththeuseofhydroquinone(Boeckxetal.,2005).ItissuggestedthatureaseinhibitorsmitigateCH4emissionbyinhibitingthemethanogenicfermentationofacetate(Wangetal.,1991).Furthermore,acombinationofaureaseinhibitor(hydroquinone)andanitrificationinhibitor(dicyandiamide)wasshowntoresultinlowerGHGemissionscomparedwithusingonlyoneoftheproducts(Boeckxetal.,2005).SeeSection3.2.1.1formoreinformationonnitrificationandureaseinhibitors.

3.2.2.6 FertilizerPlacementinFloodedRice

Incorporating/injectingorplacingfertilizerdeepintothesoilhasbeenshowninsomestudiestoreducebothCH4(Wassmannetal.,2000)andN2O(Keerthisingheetal.,1995)emissions.Whilemuchofafloodedricefield’ssoilisanaerobic,thefloodwaterandtopfewcentimetersofsoiltypicallyremainaerobicwhilesoilbelowfivecentimetersexistsinananaerobic,reducedstate(KeeneyandSahrawat,1986).Thusmineralnitrogeninthetopfewcentimetersofsoilmayundergonitrificationanddenitrification,whichcanleadtoN2Oemissions;butmineralnitrogeninlowersoildepthswillremainasammonium.Incontrast,nitrogenfertilizerthatisappliedtothesoilsurface(eitherpreseasonormidseason)tendsbemoresusceptibletolosseseitherfromammoniavolatilizationormorerapidnitrification‐denitrificationprocesses(Griggsetal.,2007).Byplacingnitrogenintoanaerobicsoillayers,itisbetterprotectedfromlossesandremainsavailableforcropnitrogenuptake(Linquistetal.,2009).TheeffectofdeepfertilizerplacementonCH4reductionremainsuncertain.SeeSection3.2.1.1formoreinformationonfertilizerplacement.

3.2.2.7 SulfurProducts

Sulfur‐containingfertilizers(i.e.,ammoniumsulfate,calciumsulfate,phosphogypsum,andsinglesuperphosphate)reduceCH4emissions(Lindauetal.,1998).ThemagnitudeofCH4reductionisdependentonfertilizationratewithaveragesbetween208and992kgSha‐1,reducingCH4

emissionsby28percentand53percent,respectively(Linquistetal.,2012).Atlowlevelsofsulfurfertilization,whicharecommoninrecommendedrates,theeffectonCH4emissionswillbelimited(Linquistetal.,2012).SulfurmitigatesCH4emissionsintwoways.First,SO4additionstosoiladdelectronacceptors,thusslowingsoilreduction(Majumdar,2003).Second,theproductofSO4reduction(H2S)mayinhibitmethanogenicbacteriaandthusdepressCH4production.Unfortunately,moststudieshavenotexaminedtheeffectonN2Oemissions.

3.2.3 Land‐UseChangetoCropland

Conversionfromoneland‐usecategory(e.g.,forestland,wetlands)tocroplandcanhavesignificanteffectsontheGHGemissionsandremovalsassociatedwiththelandunderconversion.Whenlandisconvertedtocropland,thereisoftenalossofcarbon,anincreaseinN2OandCH4emissions,areductioninCH4oxidation,andifbiomassisburned,anincreaseinnon‐CO2GHGemissions.Anumberofvariablesinfluencethedirectionandmagnitudeoftheemissionsandsinksincludingpriorlanduse,climate,andmanagement.Theinfluenceofland‐usechangeoncarbon,nitrogen,methane,andnon‐CO2GHGsarediscussedbelow.

3.2.3.1 InfluenceonCarbonStocks

Land‐useconversiontocroplandcanhavesignificanteffectsonbiomass,litter,andsoilcarbon(IPCC,2000).Houghtonetal.(1999)estimatedthatlandclearanceintheUnitedStateshasledtoalossof27PgCtotheatmospheresincethe1700s,althoughrecentlysomecarbonhasbeenrestoredwithconversionofcroplandbacktootherusesandalsoimprovedsoilmanagement(U.S.EPA,2010).ClearingforestleadstoalargelossofabovegroundandbelowgroundbiomassandlitterC;grasslandconversioncanalsoreducetheamountofcarboninthesepools,buttoalesserextentthanforestconversionbecausegrasslandshavelessbiomass.Soilcarbonlossescanbesignificantwithconversiontocultivatedcropmanagement(DavidsonandAckerman,1993),withrelativelossesintemperateregionsfrom20to30percentonaverage(Ogleetal.,2005).

Ultimately,thenetinfluenceoflandconversionwilldependonthepreviouslanduse,vegetationcomposition,andmanagement,andtheresultingcroplandsystemanditsassociatedvegetationcompositionandmanagement.Forexample,conversionofgrasslandtotreecrops,suchasorchards,mayleadtogainsincarbonrelativetothegrasslandduetoaccumulationofcarboninwoodybiomass.

3.2.3.2 InfluenceonSoilNitrousOxide

Theconversionoflandtocroplandgenerallyacceleratesnitrogencycling,withsubsequenteffectsonN2OandCH4fluxes.SoilnitrogenavailabilityisthefactorthatmostoftenlimitssoilN2Oemissions(seeSection3.2.1.1),soanypracticethatincreasestheconcentrationofinorganicnitrogeninsoilislikelytoalsoaccelerateN2Oemissions.Asnotedabove,land‐usechangetypicallyresultsinfastersoilorganicmatterturnoverandassociatednitrogenmineralization,whichmeansthatevenintheabsenceofnitrogenfertilizer,soilN2Ofluxeswillbehigheronconvertedland.Additionalnitrogenfromfertilizers,whethersyntheticororganic,orfromplantedlegumeswillfurtherenhanceN2Ofluxes,aswilltillage—insofarastillagestimulatesnitrogenmineralization.

TheconversionofunmanagedlandtocellulosicbiofuelproductionmayavoidadditionalGHGloadingifcareistakentoavoidsoilcarbonoxidationandexcesssoilnitrogenavailability(Robertsonetal.,2011).Thismightoccur,forexample,ifexistingperennialvegetationwereharvestedforfeedstockorwhennewperennialgrassesweredirect‐seededintoanotherwiseundisturbedsoilprofile,andwhennoorminimalnitrogeninputsareused.Althoughthecurrentmarketforcellulosicbiomassisnascentatbest,asitdevelopsinresponsetolegislativemandatesandenergydemandtherewillbepressuretoconvertlandsnowunmanagedintobiofuelcropping

systems.MinimizingtheGHGimpactoftheseconversionswillbecrucialforavoidinglong‐termcarbondebtthatwillotherwiseleadtocarbonsourcesratherthancarbonsinks,irrespectiveoftheircapacitytogeneratefossilfueloffsetcredits(Fargioneetal.,2008;Gelfandetal.,2011;Pineiroetal.,2009).

3.2.3.3 InfluenceonMethanotrophicActivity

Methanotrophicbacteriacapableofconsuming(oxidizing)atmosphericCH4arefoundinmostaerobicsoils.CH4uptakeinsoilsisgloballyimportant;thesizeofthesoilsinkisthesamemagnitudeastheatmosphericincreaseinCH4(IPCC,2001),suggestingthatsignificantchangesinthestrengthofthesoilsinkcouldsignificantlyaffectatmosphericCH4concentrationsifuptakedeclinesduetolanduseandmanagement.Inunmanageduplandecosystems,CH4uptakeiscontrolledlargelybytherateatwhichitdiffusestothesoilmicrositesinhabitedbyactivemethanotrophs.Diffusionisregulatedbyphysicalfactors—principallymoisturebutalsotemperature,soilstructure,andtheconcentrationofCH4inthesoil.

AgriculturalmanagementtypicallydiminishessoilCH4oxidationby70percentormore(Mosieretal.,1991;Robertsonetal.,2000;Smithetal.,2000)foratleastaslongasthesoilisfarmed.Themechanismforthissuppressionisnotwellunderstood;likelyitisrelatedtonitrogenavailabilityasaffectedbyenhancednitrogenmineralization,fertilizer,andothernitrogeninputs(Steudleretal.,1989;SuwanwareeandRobertson,2005).NH4+isknowntocompetitivelyinhibitmethanemonooxygenase,theprincipalenzymeresponsibleforoxidationatatmosphericconcentrations.Microbialdiversityalsoseemstoplayanimportantrole(Levineetal.,2011).

TherearenoknownagronomicpracticesthatpromotesoilCH4oxidation;althoughabetterunderstandingofthemechanismsresponsibleforitssuppressionmayeventuallysuggestmitigationopportunities.Todate,recoveryofsignificantCH4oxidationcapacityfollowingagriculturalmanagementhasonlybeendocumenteddecadesafterconversiontoforestorgrassland;completerecoveryappearstotakeacenturyorlonger(Robertsonetal.,2000;Smithetal.,2000).

3.2.3.4 Non‐CO2GHGEmissionsfromBurning

Burningcanbeconductedonlandsinpreparationforcultivationtofacilitateaccessforequipment,removestandingaccumulatedbiomass,andprovideorganicmaterial(ash)forincorporationintosoils.Burningofthebiomasscanbeanimportantsourceofnon‐CO2GHGs(N2O,CH4)aswellasprecursorstoGHGformation(CO,NOx)followingadditionalchemicalreactionsintheatmosphereorsoils.MoreinformationonburningofgrazinglandsvegetationcanbefoundinSection3.3.1.5,andburningoftheremainingbiomasswithclearingofforestcanbefoundinSection6.4.1.9.

3.3 GrazingLandManagement

Rangelandsaredefinedaslandonwhichtheclimaxorpotentialplantcoveriscomposedprincipallyofnativegrasses,grass‐likeplants,forbsorshrubssuitableforgrazingandbrowsing,andintroducedforagespeciesmanagedforgrazingandbrowsing.Conversely,pasturelandsrepresentlandmanagedprimarilyfortheproductionofintroducedforageplantsforlivestockgrazing,withmanagementconsistingoffertilization,weedcontrol,irrigation,reseedingorrenovation,andcontrolofgrazing(USDA,2009).HowgrazinglandsaremanagedinfluencesthepotentialforcarbonsequestrationorGHGemissions.TheparagraphsbelowhighlightsomeofthekeymanagementpracticesandtheirassociatedGHGemissionsandremovalssummarizingthecurrentstateofthescience.

3.3.1 ManagementActivityInfluencingGHGEmissions

Soilorganiccarbondominatestheterrestrialcarbonpoolingrazinglands.Abovegroundcarbonis<fivepercentofthetotalecosystemcarbonpoolinmostnon‐woodyplantdominatedecosystems,butupto25percentingrassland‐shrublandecosystems.Grazinglandscanbecarbonsinks,withratesofsoilorganiccarbonsequestrationupto0.5MgCha‐1year‐1forrangelands(DernerandSchuman,2007;Liebigetal.,2010)and1.4MgCha‐1year‐1forpastures(Franzluebbers,2005;2010a).Actualratesareoftenlessthantheseapparentmaximalratesofsoilorganiccarbonsequestrationduetomanagement,climate,weather,andotherenvironmentalconstraints.Potentiallyhighratesofsoilorganiccarbonaccumulationarepredictedinnewlyestablishedpasturesandrestorationofdegradedrangelands,whileimpropermanagementanddroughtcanresultinsignificantcarbonreleases.Duetothelargelandarea,themovementofcarbonintoandoutofthesoilreservoiringrazinglandcanbeanimportantcomponentoftheglobalcarboncycle.InadditiontosoilorganicC,alargepoolofsoilinorganiccarbonoccursascarbonatesinsemi‐aridandaridrangelandsoilsthatcanleadtoeithersequestrationorreleaseofCO2(Emmerich,2003).However,thedirectionandmagnitudeofsoilinorganiccarbonstocksarecurrentlypoorlyunderstood(Follettetal.,2001;Liebigetal.,2006;Svejcaretal.,2008).

Twoimportantmanagementfactorsthatcontrolthefateofsoilorganiccarboningrazinglandsarelong‐termchangesinproductionandqualityofabovegroundandbelowgroundbiomassthatcanalterthequantityofnitrogenavailableandtheC‐to‐Nratioofsoilorganicmatter(Pineiroetal.,2010),andgrazing‐inducedeffectsonvegetationcomposition,whichcanbeasimportantasthedirectimpactofgrazing(e.g.,grazingintensity)onsoilorganiccarbonsequestration(DernerandSchuman,2007).Therateofsoilorganiccarbonsequestrationcanbelinearfordecades(Franzluebbersetal.,2012),buteventuallydiminishestoasteady‐statelevelwithnofurtherchangeinthestockfollowingseveraldecadesofamanagementpractice(DernerandSchuman,2007).Additionalpositivechangesinmanagementorinputsareoftenneededtosequesteradditionalsoilorganiccarbon(Conantetal.,2001),butnegativechangesinmanagementcausinglossofsoilstructureandsurfacelittercovercanleadtoerosionandlossofproductivityresultinginadeclineinsoilorganiccarbon(Pineiroetal.,2010).

MethanefluxfromgrazinglandsiscontrolledbythebalanceofentericandmanureemissionsfromruminantanimalsanduptakeofCH4bysoil.(EmissionsandmethodsforestimatingCH4emissionsfromruminantsarediscussedfurtherinSection5.3).InthewesternUnitedStates,grasslandshavegreaterCH4uptakebysoilthandoneighboringcroplands(Liebigetal.,2005),probablyduetogreatersurfacesoilorganicmatterthatpromotesthegrowthofmethanotrophicbacteria.InanassessmentofGHGemissionsfromthreegrazinglandsystemsinNorthDakota,entericemissionsofCH4fromgrazingcattlewerethreetoninetimesgreater(onaCO2equivalentbasis)thanCH4uptakebysoil(Liebigetal.,2010).WithCH4emissionsdirectlytiedtonumberofcattle,fertilizedgrasslandsareoftenanetcarbonsourceduetoenhancedCH4emissionfromcattleandpotentiallygreaterN2Oemissions,whileunfertilizedgrasslandsareoftenanetcarbonsink(Luoetal.,2010;Tunneyetal.,2010).

3.3.1.1 LivestockGrazingPractices

Livestockgrazingpractices(i.e.,stockingrateandgrazingmethod)aresummarizedbelowalongwithdataontheinfluencethesepracticeshaveonGHGemissionsandremovals.

StockingRate:Stockingrateisthenumberofanimalspermanagementunitutilizedoveraspecifiedtimeperiod,e.g.,numberofsteersperacrepermonth.Basedonpublishedstudies,responsesofsoilorganiccarbontostockingrateandgrazingintensityhavebeenvariable,despitegrazingeithercausinganincreaseorhavinglittleeffectonthemorecommonlymeasuredpropertyofsoilbulkdensity(GreenwoodandMcKenzie,2001;Schumanetal.,1999).Innorthernmixed‐grassprairie,

soilorganiccarbonhasincreasedingrazedcomparedwithungrazedareas,partlyresultingfromincreasingdominanceofshallow‐rooted,grazing‐resistantspecies,suchasbluegrama(Boutelouagracilis),whichincorporatesalargeramountofrootmassintheuppersoilprofilethanthemid‐grassspeciesthatitreplacesduringgrazing(Derneretal.,2006).Furtherresearchisneededtodeterminetheextentofdifferentrootdistributionsontotalcarbonstorageinanentiresoilprofile.Increasingstockingratebeyondanoptimumforachievingmaximumlivestockproductionperunitlandarea(Bement,1969;Dunnetal.,2010)wouldbeexpectedtoresultinalossofsoilorganiccarbonduetoreducedplantvigorandrootdistributioninthesoilprofile.Withsuboptimalstockingrate,vigorofpastureforagesmaydeclineasplantresiduesdevelopathicklitterlayeratthesoilsurface.However,insemi‐aridregions,thehighUVlightintensitymaysignificantlyreducelitteronthesoilsurfacethroughphotochemicaldecompositionprocesses,regardlessofgrazingintensity(Brandtetal.,2010).Vegetationcompositionshiftsthatchangethequantityandqualityofplantmaterialproducedcaninfluencetheamountofcarboninputstosoils.Inmanagedpastures,ithasbeenshownthatsoilorganiccarboncanbeoptimizedwithamoderatestockingratecomparedwithnograzingorheavy,continuousgrazing(Franzluebbers,2010b).Anoptimizedstockingrateforaparticularregion(climaticconditions),vegetationcomposition,andsoiltypeisthoughttomaximizetheamountofsoilorganiccarbonsequestered.

Limitedevidenceshowsthatgrazingatmoderatelevelscanfurtherincreaseenvironmentalbenefitsoverthoseofgrasslandestablishmentalone,inadditiontoprovidinganimportanteconomicreturntoproducers.Ifsoilorganiccarbonweretodeclinewithovergrazing,therewouldalsobeadeclineinanimalproductivityduetolackofforage.Therefore,anegativerelationshipbetweensoilorganiccarbonstorageandanimalproductivityislikelywhengrazingintensityexceedsamoderatelevel.ThisresponseislikelymodifiedundermoderategrazingpressureduetothefactthatgreateranimalproductperheadcanbeachievedwithlowerGHGemissions.Limitingtheeffectofhighstockingrateonsoilorganiccarbonlevelsmaybeachievablewithhighnitrogenfertilizerinputs,anoutcomewithanuncertaincarbonfootprintrelativetoGHGintensity.StockingrateandfertilizernitrogeninputinteractionsneedtobequantifiedtoaccuratelyassesstotalGHGintensity.SomeevidenceinthehumidUnitedStatessuggeststhatovergrazingcanleadtoincreasedsoilerosionandareductioninsoilquality.Literaturefromotherregionshasalsoshownincreasingsoilerosionanddecliningsoilqualitywithexcessivestockingrates.Whileevidenceislacking,anassumptionisthatsoilorganiccarbonfollowsthissamepositiveresponsetomoderategrazingandnegativeresponsetoovergrazing.

EmissionsofN2Ofromgrazinglandsareaffectedbygrazing,butnetfluxcanbeincreasedordecreased,dependingonstockingrate,grazingsystem,andseason(Allardetal.,2007).StockingratehadlittleinfluenceonN2Oemissionsfrommixed‐grassprairieinNorthDakota(Liebigetal.,2010).WhileelevatedN2Oemissionsmaybeexpectedunderincreasedstockingrate,Wolfetal.(2010)suggestedthatgrazingcancounteractpotentialN‐inducedemissionsonrangelandsbyreducingsurfacebiomass,resultinginmoreextremesoiltemperatures,lowersoilmoisture,andcorrespondinginhibitionofmicrobialactivityresponsibleforN2Oemissions.Ifgrazingintensityonpastureswereviewedasafertilizereffectwithincreasinganimalmanuredeposition,thenN2Ofluxfromagrazingeffectdoesnotbehaveinthesamemannerasmanufacturednitrogenfertilizerinputs.Interactionsbetweenstockingrateandnitrogenfertilizerinputshavenotbeenquantified,despitesuchdiversityinmanagementlikelyoccursamongproducers.StockingrateandmanureandfertilizernitrogeninputsareareasrequiringfurtherresearchtobetterunderstandthecomplexsetofcontrollingfactorsinadditiontosoiltextureandenvironmentalconditionsonN2Oemissionsingrazinglands.Onrangelands,theabundanceofN‐fixinglegumesintheplantcommunitybecomesmorecriticalforincreasingSOC,particularlysincefertilizeradditionsandmanurearenotassignificantforreturningnitrogentothesoilcomparedtopasturesystems.ThisisanarearequiringfurtherresearchtobetterunderstandthecontrollingfactorsonN2Oemissions.

GrazingMethod:Grazingmethodsvarybasedonproducergoalsandthetypeofforageavailable(Scheafferetal.,2009).Twodistinctgrazingmethods,continuousandrotationalgrazing,representtheprevalentmethodsemployedongrazinglandsintheUnitedStatestomanagethelivestock.Continuousgrazingallowsanimalstofreelymoveandhavefullaccesstoagrazingarea,whereasrotationalgrazingismorecontrolled,involvingmovementofanimalsbasedonmonitoringforagecondition,suchasplantheight,betweentwoormorepaddockssubdividedfromalargergrazingarea.Rotationalgrazingterminologyhasbeenconfusedwithtermssuchasholisticgrazing,plannedgrazing,prescribedgrazing,andmanagement‐intensivegrazing,whichcontinuetobeusedwithmultipleandambiguousmeaningsdespiteattemptstostandardizedefinitions(SRM,1998).Termstodefineintentionsofrotationalgrazingsystemsincluderest‐rotation,deferred‐rotation,high‐intensity‐short‐duration,andseason‐longgrazing(Briskeetal.,2008;Briskeetal.,2011).Herewedefinerotationalgrazingasthemovementoflivestockbetweentwoormoresubunitsofgrazinglandsuchthatalternatingperiodsofgrazingandnograzing(‘rest’)occurwithinasinglegrowingseason(HeitschmidtandTaylor,1991).

Rotationalgrazinglimitsplantsfromreachingreproductivestagesinwhichforagequalityrapidlydeclines.Thiscontrastswithcontinuousgrazinginwhichthereismoreselectivegrazingofthehighestqualityforages.Assuch,foragequalitymaybemaintainedatahighlevellongerintothegrowingseason.Therefore,rotationalstockinginthehumidUnitedStatescouldprovidemoreuniformforageconsumptionacrosspasturesandallowsufficientresttoforagespeciesbetweengrazingeventstopromotegreaterproduction.Pastureswithgreaterplantproductionviaanimprovedstockingmethodwouldbeexpectedtohavelowersoilerosionandgreatersoilorganiccarbonstorage.Althoughtheseexpectationsseemintuitive,therearelimiteddatainthescientificliteraturetosupportthem.Twostudieshavesuggestedanincreaseinsoilorganiccarbonwithrotationalgrazingcomparedwithcontinualseason‐longgrazing(Conantetal.,2003;Teagueetal.,2010),andanotherstudyfoundnodifferencebetweensystems(Manleyetal.,1995).Sincerotationalgrazingdataaremostlyavailableforrangelandandfewstudiesconductedonpastures,thereisnotenoughevidencetoevaluatehowrotationalgrazingmightaffectsoilorganiccarboninpastures.Giventhatthepreponderanceofevidencesuggeststhatrotationalgrazingdoesnotinfluencevegetationproductioninrangelands(Briskeetal.,2008),changesinsoilorganiccarbonwithrotationalgrazingwouldbeexpectedonlyifsubstantialvegetationchangeoccurredindependentlyfromstockingrate.Rangelandstypicallyhaveamuchhigherdiversityandmultiplegrowthpatternsofforbs,cool‐seasonandwarm‐seasongrasses,whichwouldresultinasmallerinfluenceofstockingmethodonvegetationphenology(i.e.,keepingforageinavegetativeratherthanareproductivestate)thanwouldoccurinmonocultureorsimplemixturesofforagesinpastures.Muchmoreresearchongrazingmethodisneeded,duetothehighadoptionrateandpromotionofthebenefitsofimprovedgrazingmethodsforsoilorganiccarbonsequestrationbyproducersandagriculturaladvisors(BeetzandRhinehart,2010).

3.3.1.2 ForageOptions

Cool‐andwarm‐seasonforageshavegrowthactivityatdifferenttimesoftheyear,therebyaffectingwhenrootandlittercarboninputsaresuppliedtosoil.Dependingonenvironmentalgrowingconditions(i.e.,relativelyshort,cool,andwetsummerwithlong,coldwinterversuslong,hot,anddrysummerwithmild,wetwinter),theperformanceofcool‐versuswarm‐seasonforageswillvaryacrossregions.InthesoutheasternUnitedStates,perennialcool‐seasonforages(e.g.,tallfescue)haveproducedgreatersoilorganiccarbonthanwarm‐seasonforage(e.g.,bermudagrass)ingrazinglandsystems,despitethemorevigorousgrowinghabitofbermudagrass(Franzluebbersetal.,2000).Thisresultislikelyduetotheopportunitiesofforagesforgrowthandthebalanceofwaterinsoilthatremainsformicrobialdecompositionoforganicmatter.

Timingofforagegrazingcanaffectplantproductivity,wildlifehabitat,andcompactionofsoil.Eachoftheseeffectscan,inturn,affectsoilorganiccarbonsequestrationandGHGemissions.Thecapacityofsoiltowithstandcompactionforcesofanimaltreading,resultinginsignificantdeformation,destabilization,lossofinfiltrationcapacity,andsoilorganiccarbonsequestration,canbeexceeded—especiallyunderwetconditions(Bilottaetal.,2007).Soilsaturationduringwinterandspringleadtosevereeffectsfromanimaltrampling.InnorthernlatitudesandrangelandsofthewesternUnitedStatessubjecttofreeze‐thawcycles,sandyandloamysoilsarelesslikelytobeaffectedbythenegativeimpactsofcompaction.Intuitively,deferringgrazingtoperiodsoflimitedactiveforagegrowth(e.g.,winterandspring)mightcontributetoincreasedsoilcompaction.However,allowingforagetoaccumulatetofullcanopypriortograzingmightbebeneficialtocontrollingerosionbyprovidingalongerperiodofforageandresiduecover.Grazingofwintercovercropsmayalsobeaneffectivefarm‐diversitystrategy,buttheeffectsonsoilerosioncontrolandsoilconditionneedtobequantified.Wildlifemanagementguidelinesonrangelandsuggestlonger‐term(>oneyear)resttoaccumulatevegetationstructureforcertainbirdsneedinghabitat.Timingofgrazingcouldbeacriticalfactorincontrollingcompaction,susceptibilitytoerosion,andsoilorganiccarbonsequestration,sothesequenceofwhenpasturesaregrazedshouldberotatedamongyearstoensurethatplantcommunitiesarenotalwaysgrazedatthesametimetoensuregreatercommunitysustainability.

Organicmatter‐richsurfacesoilabsorbscompactiveforcesofgrazingmuchlikeasponge,inwhichsoiloftenreboundsinvolumeonceforcesareremoved.However,effectsofwintergrazingofdeferredgrowthmaybedifferentincolderthaninwarmerregions:frozensoilmayavoidcompaction,butnutrientrunoffmaybecomemoreimportant(Clarketal.,2004).InthesouthernUnitedStates,perennialcool‐seasongrassesareoftengrazedduringlatewinterandthroughoutspringduringtypicallywetconditions,butduetoactiveforagegrowth,soilcanalsodryquicklyandtramplingmaynotalwayscausedamage.InGeorgia,soilorganiccarbonwasgreaterunderlong‐termstandsofcool‐seasontallfescue(typicallygrazedinspringandautumn)thanunderwarm‐seasonbermudagrass(typicallygrazedinsummer)(Franzluebbersetal.,2000).

InthesoutheasternUnitedStates,annualcool‐seasonforagesareoftenplantedasacovercropfollowingsummercropsorsod‐seededintoperennialgrasspastures.ThispracticecanenhanceforageproductionandshouldincreasesoilorganicC,althoughlimiteddataareavailabletosupportthisconclusion.Inanintegratedcrop/livestocksysteminthesoutheasternUnitedStates,therewasalimitedeffectofgrazingannualcovercropsonsoilorganicC,eitherinthesummerorwintercomparedwithungrazedcovercrops(FranzluebbersandStuedemann,2009).

3.3.1.3 Irrigation

WaterisalimitingfactorintheabilityofplantstofixcarbonandsubsequentlyproducethecarboninputnecessarytoaccumulatesoilorganicC.ItisalsoafactorlimitingdecompositionofsoilorganicC.Whiletheextentofirrigationingrazinglandsislimited,whereitoccursthereareconsequencesforsoilorganiccarbonstorage.Forexample,someproductivemeadowsinthewesternUnitedStatesareirrigated.Howirrigationaffectssoilorganiccarbonwilldependonthequantity,frequency,andtimingofirrigationevents.Irrigationonlyatpeakplantgrowthstageswilllikelycauseamuchgreaterpositiveimpactonforagecarbonfixationthananegativeimpactonsoilorganiccarbondecomposition.Inthesamemanner,irrigationquantity,frequency,andtimingwilllikelyaffectN2OandCH4emissions,althoughpulsedresponsesoftheseGHGscouldlikelybemuchmoredramatic.Unfortunately,thereareonlylimitedstudiesonthesepotentialimpacts.SeeSection3.2.1.4formoreinformationonirrigationmethods.

Inacomparisonofagriculturalsystemswithsurroundingaridandsemi‐aridnaturalvegetation,Entryetal.(2002)foundthatsoilorganiccarbonwasgreaterinirrigatedagriculturalsystemsdue

toenhancedproductivity.EmissionofN2Ofromirrigatedsystemsoccursfollowingcloselytimedirrigationandnitrogenfertilizerapplicationsincroplandconditions,andthiswouldbeexpectedundergrazinglandsaswell,buttherearefewdataavailable(Liebigetal.,2006;Liebigetal.,2012).

3.3.1.4 NutrientManagement(SyntheticandOrganic)

Fertilizersareoftenappliedtopastures,duetothehighyieldresponsewithadequateprecipitation,butarelesscommoninwesternrangelandsduetoinconsistentyieldresponseandriskycost‐effectivenesswithlimitedandvariableprecipitation.NitrogenavailabilityinsoildeterminestoalargeextenttheemissionsofN2O.Grazinglandstypicallyhavelowernitrogenavailabilityinsoilthancroplands,andthereforehavelowerN2Oemissions(Liebigetal.,2005).However,applicationoffertilizernitrogentorangelandhasbeenfoundtoconsistentlystimulateN2Oemissions(Flechardetal.,2007).Liebigetal.(2010)observedtwo‐foldgreaterN2Oemissionsfromfertilizedcrestedwheatgrasscomparedwithunfertilizedmixed‐grassprairie.AdditionoffertilizernitrogentopastureinMichiganhadanegligibleeffectonN2Oemissions(AmbusandRobertson,2006),whereasapplicationofpoultrymanureonabermudagrasspastureinArkansasincreasedN2Oemissionsby45percentcomparedwithpasturewithoutmanure;N2Ofluxandsoilnitratedynamicswerepositivelyassociated(Saueretal.,2009).Astrategytoreducesoilnitratebyinterseedingannualryegrassonmanure‐amendedsoildecreasedN2Oemissionsby50percent.Similartocropland,reducingsoilnitratetolowlevelsduringperiodsoflowrootactivityandhighlevelsduringperiodsofhighrootactivitywillgenerallyenhanceplantnitrogenuptakeandreduceN2Oemissions.ApplicationofcompostedgreenwastecouldsequesterC,butthisresearchtopichasnotbeenfullyevaluated.AsignificantincreaseinsoilorganiccarbonhasonlybeendemonstratedatoneoftwositesinCalifornia(Ryalsetal.,2014).Frommodelsimulations,compostapplicationhasbeenshowntoreducetheoverallGHGemissiononCO2equivalentbasis,bysequesteringcarbonandreducingN2Oemissions,whilemanureslurryandinorganicfertilizerapplicationsledtonetGHGemissionsonCO2equivalentbasis(DeLongeetal.,2013).Formoreinformationonmanagementoptionsassociatedwithfertilizationpractices,seeSection3.2.1.1.

3.3.1.5 PrescribedFires

Burninghasthepotentialtoaltersoilorganiccarbonthrougheffectsonphotosynthesis,soil,andcanopyrespiration,andthroughspecieschanges,inadditiontostabilizingorincreasinglivestockgains,improvinghabitatdiversity,andreducingfuelloads(Bouttonetal.,2009;Toombsetal.,2010).Althoughcarbonlossfromburninggrazinglandsisaminorcomponentoftheannualcarbonemissions,burningrangelandswithasignificantwoodyabovegroundplantbiomasscanresultinsubstantialimmediateecosystemcarbonloss(BremerandHam,2010;Rauetal.,2010).However,prescribedburningofgrazinglandscouldalsoaffectlong‐livedcharthataccumulatesinsoil,andthereforewouldinfluencesoilcarbonstocks.Burningalsoleadstonon‐CO2GHGemissions,whichcanbesignificantduetothehigherglobalwarmingpotentialofthesegasescomparedwithCO2(IPCC,2006).Formoreinformationonnon‐CO2GHGemissionsfromburning,seeSection3.2.3.4.

3.3.1.6 ErosionControl

Riparianbufferscanbeasignificantsinkforexcessnutrientsrunningoffneighboringgrazinglands.Thefateofnutrientsisdependentontheflowcharacteristicsandtypeofvegetation.ExcessnitrateinsaturatedsoilofriparianareascanleadtosignificantN2Oemissions—althoughtheseemissionsaretypicallytreatedasindirect,withtheemissionsassociatedwiththefieldorlivestockfacilitythatiscontributingtheexcessnutrients(SeeSection3.2.1.1).TransportofsolublecarbonintoriparianareascouldalsoenhanceCH4emissionsfromsaturatedsoil.

3.3.1.7 ManagementofDrainedWetlands

DrainageofwetlandorhydricsoilsthatareusedforgrazinghasimplicationsforsoilorganiccarbonandGHGemissions,similartodrainageforcropproduction.Thewaterregimeandplantcommunitiesaresignificantlyalteredandsoilsaremodifiedfromanaerobictoaerobicconditions.Increasingoxygeninsoilwillcauseorganicmattertodecomposemorerapidlythanundersaturatedconditions,resultinginreleaseofCO2(Eagleetal.,2010;FranzluebbersandSteiner,2002;IPCC,2006;Liebigetal.,2012).LargeemissionsofCO2resultfromdrainageofwetlands(Allen,2007;2012),anddrainagecanalsoincreasenitrogenmineralizationandenhanceN2Oemissionsdirectly(IPCC,2006).EmissionsofCH4arereducedconsiderablywithdrainage,butthisimpactisoftennotconsideredinestimationofGHGemissions(IPCC,2006).Alargeproportionofgrasslandwetlandshavebeendirectlydrainedormodifiedtoenhanceagriculturalproduction(DahlandJohnson,1991),andmanyotherwetlandsareindirectlyaffectedbysubsurfacetiledrainsandagriculturalpracticesinsurroundingcatchments.SeeSection3.2.1.6formoreinformationaboutmanagementofdrainedsoils.

3.3.1.8 LimeAmendments

LimeamendmentsareneededwhensoilpHislow(e.g.,pH<5)toenhanceproductivityandsupportbalancednutrientlevelsingrazinglandsoils.Typicallimingmaterialsingrazinglandsarecalciticlimestone(CaCO3),dolomiticlimestone(CaMg(CO3)2),andconfinedlivestockmanure,particularlypoultrylitter,whichhaslimingactivityfromlimeadditivetothefeedration.Whencarbonatelimeisappliedtosoilitdissolvesinsolutionovertime,withthecationandcarbonatedissociating.ThereispotentialforreleasingCO2totheatmospheredependingonwhetherthelimereactswithcarbonicornitricacidinthesoilsolution.Theenhancedplantnutrientofferedbylimingcanhaveanetpositiveeffectonthecarbonbalanceforanextendedperiodoftime.SeeSection3.2.1.7formoreinformationonlimeandtheconsequencesforGHGemissions.

3.3.1.9 WoodyPlantEncroachment

Woodyplantencroachment3leadstocarbonaccumulationinabove‐groundandrootbiomassandmayincreaseoverallecosystemcarbonstorage,butcandegradeagriculturalproductivityofgrazingland(McClaranetal.,2008).Overthepastcenturyinwesternrangelands,soilorganiccarbonhasincreasedinnear‐surfacesoilswithwoodyplantencroachment(Bouttonetal.,2009;Creameretal.,2011;Liaoetal.,2006;Liebigetal.,2012).Removalofwoodyplantsbyfireorothermechanismsdepletestheseshallow,relativelysusceptiblesoilorganiccarbonstoresassociatedwithencroachment(Neffetal.,2009;Rauetal.,2010);butdoesnothaveaneffectonSOCortotalnitrogenstocksatdepthsof>20cm(Daietal.,2006).Regardless,removalofthewoodyplantswillcauseadeclineinabovegroundbiomasscarbonstocks(Rauetal.,2010).

InasummaryofresearchonCH4emissionsfromgrazinglands,Liebigetal.(2012)reportedCH4uptakeundermesquite,butnetCH4productionundergrasslandanddeadmesquitestumps.Methaneuptakeundermesquitewasassociatedwithreducedsoilbulkdensityandincreasedsoilmoisture(McLainandMartens,2006),aswellasgreaternitrogenaccrual/accumulationassociatedintheareaaroundmesquiteplants(10meters)(BouttonandLiao,2010;Liaoetal.,2006;Liuetal.,2010).Methaneuptakeundermesquitewasalsoassociatedwithalteredsoilmicrobialcommunities(Hollisteretal.,2010;LiaoandBoutton,2008),whichcanaffectNOxandN2Orates,whileCH4productionfromgrasslandandwoodydetrituswaslikelycausedbytermiteactivity.The

3Woodyencroachmentwilleventuallyleadtoatransitionfromgrazinglandtoaforest.SeeChapter7:LandUseChangefordefinitionofforestlandtodeterminewhenwoodyencroachmentwillleadtoatransitiontoforestland.

roleofmesquitetofixN,therebyalteringnitrogendynamics,resultedinN2Oemissionsundermesquitecanopyfour‐foldgreaterthanundergrassesorwoodydetritus(McLainetal.,2008).

3.3.2 Land‐UseChangetoGrazingLands

Land‐useconversiontograzinglandsinfluencesthecarbonstocksandGHGemissionsofaparcel.Priorlanduse,climate,soiltype,andmanagementpracticesarejustafewofthefactorsinfluencingthemagnitudeanddirectionofGHGemissionsandremovalsresultingfromaland‐useconversiontograzinglands.Theparagraphsbelowsummarizethecurrentstateofthescienceontheinfluenceofaland‐useconversiononcarbonstocks,soilN2O,CH4,andnon‐CO2GHGsresultingfrombiomassburning.

3.3.2.1 InfluenceonCarbonStocks

Establishmentofpasturesonpreviouscroplandhelpsreducesoilerosionandimprovessoilquality(Singeretal.,2009).Thereissubstantialevidencethatestablishmentofpasturesleadstosignificantsoilorganiccarbonsequestration.Therateofaccumulationacrossanumberofstudiesaveraged0.84MgCha‐1year‐1(Franzluebbers,2010a).Literatureisinadequatetodeterminewhetherforagecompositionorsoiltypehaveadiscernibleinfluenceonsoilorganiccarbonstock(seeSection3.3.1.2).Thequantityofforageproducedandthequantityofresiduesfromsurfacelitterandrootbiomassarelikelykeydeterminantsofsoilorganiccarbonaccumulation.Thesequantitiescanbeinfluencedbyfactorssuchasforagemixture,climaticconditions,soiltype,inherentsoilfertility,fertilizerapplication,andliming.

3.3.2.2 InfluenceonSoilNitrousOxide

Dependinguponpreviouslanduse,grasslandestablishmentmayormaynotaffectnetN2Oemissionsduringland‐usechange.Ingeneral,emissionsofN2Oarecontrolledbysoilnitrogenavailabilitywithadditionalinfluenceofsoiloxygenandsolublecarbonavailability.Ifthepreviouslandusewasforexample,anutrient‐limitedforest,convertedsubsequentlytohigh‐fertilitypasture,thenN2Oemissionswouldlikelyincrease.Ifthepreviouslandusewasnutrient‐richcroplandconvertedtopasture,thenN2OemissionswouldlikelydeclineduetogreateropportunityforperennialforagespeciestoassimilateavailablesoilnitrogenandthusreduceopportunitiesforsoilnitrogentransformationstoN2O.Thisisanarearequiringfurtherresearchtoobtainquantitativeresponses,however.

3.3.2.3 InfluenceonMethanotrophicActivity

Land‐usechangetograzingland,particularlyfromforestland,mayinvolvefertilizationtoenhanceforageproduction.NitrogenfertilizationcausesareductionofmethanotrophicactivityinsoilsandthereforereducestheuptakeofCH4fromtheatmosphere(AmbusandRobertson,2006).SeeSection3.2.3.3formoreinformationontheimpactofland‐usechangeonmethanotrophicactivity.

3.3.2.4 Non‐CO2GHGEmissionsfromBurning

BiomassburningingrazinglandcanbeanimportantsourceofGHGs(CO2,N2O,CH4)(Aaldeetal.,2006;AndreaeandMerlet,2001;Badarinathetal.,2009;IPCC,2006).Whileconversionofcroplandtograzinglandrarelyinvolvesburning,conversionofforesttograzinglandcaninvolveburningofthewoodand/orslashleftfromlandclearing.TheeffectonGHGemissionsfrombiomassburningisdiscussedfurtherinthecroplandsection(Section3.2.3.4)andintheforestlandsection(Section6.4.1.9).

3.4 Agroforestry

AgroforestryrepresentsauniquecasewithinGHGaccounting,encompassingbothforestandagriculturalcomponents,alongwithmanycombinationsoftheirrespectivemanagementactivities(Table3‐1andTable3‐2).AgroforestryisdefinedwithintheUnitedStatesasan“intensiveland‐usemanagementthatoptimizesthebenefits(physical,biological,ecological,economic,andsocial)frombiophysicalinteractionscreatedwhentreesand/orshrubsaredeliberatelycombinedwithcropsand/orlivestock”(GoldandGarrett,2009).Anotherwayoflookingatagroforestryisasasetoftree‐based4conservation/productionpracticescombinedintobiggeragriculturaloperations,providingforest‐derivedfunctionsandinteractingwithagriculture‐derivedfunctionsinsupportofagriculturallanduse.Whileprovidingmanyotherservices(seeTable3‐3),agroforestrycancontributetocarbonsequestration,GHGmitigation,andadaptationtoshiftingclimate(CAST,2011;IPCC,2000;Morganetal.,2010;Verchotetal.,2007).

Table3‐3:SixCategoriesofAgroforestryPracticesPracticedintheUnitedStates

Practice Descriptiona Benefitsb

Alleycropping

Treesorshrubsplantedinsetsofsingleormultiplerowswithagronomic,horticulturalcrops,orforagesproducedinthealleysbetweenthesetsofwoodyplantsthatproduceadditionalproducts

Produceannualandhigher‐valuebutlonger‐termcropsfordiversificationofincome

Enhancemicroclimateconditionstoimprovecroporforagequalityandquantity

Reducesurfacewaterrunoffanderosion Improvesoilqualitybyincreasingutilizationandcyclingofnutrients

Altersubsurfacewaterquantityorwatertabledepths

Enhancewildlifeandbeneficialinsecthabitat Decreaseoffsitemovementofnutrientsorchemicals Increasecarbonstorageinplantbiomassandsoils Improveairquality

Forestfarming(alsocalledmulti‐storycropping)

Existingorplantedstandsoftreesorshrubsthataremanagedasanoverstorywithanunderstoryofwoodyand/ornon‐woodyplantsthataregrownforavarietyforproducts

Improvecropdiversitybygrowingmixedbutcompatiblecropshavingdifferentheightsonthesamearea

Improvesoilqualitybyincreasingutilizationandcyclingofnutrientandmaintainingorincreasingsoilorganicmatter

Increasenetcarbonstorageinplantbiomassandsoil

Riparianforestbuffersc(combinesNaturalResourcesConservationServicePracticeStandards:RiparianForestBufferandFilterStrip)

Acombinationoftrees,shrubs,andgrassesestablishedonthebanksofstreams,rivers,wetlands,andlakes

Decreaseoffsitemovementofnutrientsorchemicals Stabilizestreambanks Enhanceaquaticandterrestrialhabitats Provideeconomicdiversificationeitherthroughplantproductionorrecreationalfees

Increasecarbonstorageinplantbiomassandsoils

4Alsoreferredtoastrees‐outside‐forests,theterm“tree”hereincludesbothtreeandshrubs(Bellefontaineetal.,2002).

Practice Descriptiona Benefitsb

SilvopastureTreescombinedwithpastureandlivestockproduction

Providediversificationofcropsintimeandspace Produceannualandhigher‐valuebutlonger‐termcrops

Decreaseoffsitemovementofnutrientsorchemicals

Windbreaks(alsoreferredtoasshelterbelts)

Linearplantingsoftreesandshrubstoformbarrierstoreducewindspeed(maybespecificallyreferredtoascroporfieldwindbreak,livestockwindbreak,livingsnowfence,orfarmsteadwindbreak,dependingontheprimaryuse)

Controlwinderosion Protectwind‐sensitivecrops Enhancecropyields Reduceanimalstressandmortality Serveasabarriertodust,odor,andpesticidedrift Conserveenergy Providesnowmanagementbenefitstokeeproadsopenorharvestmoisture

Specialapplications

Useofagroforestrytechnologiestohelpsolvespecialconcerns,suchasdisposalofanimalwastesorfilteringirrigationtailwater,whileproducingashort‐orlong‐rotationwoodycrop

Treatmunicipalandagriculturalwastes Treatstormwater Useincenterpivotcornerplantings Producebiofeedstock Reduceimpactsofflooding Decreaseoffsitemovementofnutrientsorchemicals

Source:USDANaturalResourcesConservationService(2012).aDescriptionsfollowUSDANaturalResourcesConservationServiceConservationPracticesStandards.bAllagroforestryplantingsaddincreaseddiversitywithintheagriculturallandscape.Assuch,theywillimprovewildlifehabitatandgenerallyaredesignedormanagedwiththisasasecondarybenefit.cRiparianforestbufferreferstotheplantedpractice.Thiscategorydoesnotincludenaturallyestablishedriparianforests.

IntheUnitedStates,fivemaincategoriesofagroforestrypracticesarerecognized:alleycropping,forestfarming,riparianforestbuffers,silvopasture,andwindbreaks.Thereisanemergingsixthcategoryofspecialapplicationsoradaptationsofthesepractices(Table3‐3).Thesepracticesaretreatedwithinthecroplandandgrazinglandsystemsectionwiththeexceptionofforestfarming.Forestfarming(alsoreferredtoasmulti‐storycroppingwithinUSDANaturalResourcesConservationServicePracticeStandards)involvesthemanipulationofexistingforestcanopycoverinordertoproducehigh‐valuenon‐timber(i.e.,food,floral,medicinal,andcraft)productsintheunderstory,thusmaintaininglanduseasforest.Assuch,GHGaccountinginforestfarmingpracticeswillneedtobetreatedwithinthemethodsandapproachespresentedinSection6.2andSection6.4.

Themanyservicesderivedfromagroforestrypracticescanextendwellbeyondthesmallparceloramountoflandtheyphysicallyoccupywithintheagriculturallandscape(Bellefontaineetal.,2002;Garrett,2009).Theuseofagroforestrytechnologiesareimportantcomponentsattherural/communityinterface,aswellaswithinurbansettingstoaddressemergingneedssuchasstormwatertreatment,recreationorgreenspace,andfeedstockproduction(Schoenebergeretal.,2001).Althoughagroforestryiscategorizedintothesepractices,eachagroforestryplanting,evenwithinapractice,potentiallyrepresentsauniquecaseofspeciesselection,arrangement,placementwithinotherpracticesandthelargerlandscape,anduseofmanagementactivities,dependingonlandownerobjectives.Agroforestryplantingsarethereforemoreofa“designerlandscapefeature”thanastandardizedandeasilydescribedpractice(Mizeetal.,2008)withinGHGaccountingactivities.

Silvopastureprovidesagoodillustrationofthiscomplexityinagroforestrysystems.Silvopastureisthedeliberatecombinationofthreecomponents—trees,forage,andlivestock—alongwiththerangeoftheirrespectivemanagementactivities.Studiesdemonstrateahighercarbonsequestrationpotentialinsilvopasturecomparedwithforestorpasturealone(Haileetal.,2010;Nairetal.,2007;

SharrowandIsmail,2004).Muchofthisnewcarbonisinthewoodybiomass,butsoilcarbonalsohasthepotentialtoincreaseasaconsequenceofcarboninputsfromthetrees,whichovertimeextendfurtherintotheforagecomponent(Peichletal.,2006),aswellasmanagementoftheforageandofthelivestock(seeFranzluebbersandStuedemann,2009;Karkietal.,2009).Managementactivitieswithinasilvopasturemayincludefertilization,liming,cultivation,andharvestingoftheforagecrop(insomeyears);periodicharvestingofpineneedlesforpinestraw;incorporationofprunedwoodymaterialintotheforagecomponent;anddifferentgrazingintensitiesandrotations.Thefrequencyandintensityofmanagementactivitiesandinputsfromallthreecomponentscanvarysignificantlyfromyeartoyear,whichmakesaccountingforthesequesteredcarboninasilvopastureoperationchallenging.

RatesandamountsofGHGemissionswithineachagroforestryplantingwillvarydependingonpriorlandmanagementandcurrentconditions(i.e.,site,climate),aswellasbystanddevelopment.Theseratesandamountswillalsobedependentonlandowners’decisionsthatdetermineplantingdesign,aswellasmanagementactivities—agricultural,forestry,andgrazing—usedoverthelifetimeofanagroforestrysystem(Table3‐4).

Table3‐4:ManagementActivities5andOtherFactorsWithinAgroforestryPracticesThatMayAlterCarbonSequestrationandGHGEmissionAmounts

Practice ManagementActivities

Windbreaks

Establishmentdisturbancetosoilduringsitepreparation Depositionofwind‐andwater‐transportedsediments,nutrients,andotheragriculturalchemicalsintotheplanting Windbreakrenovation(removalofdeadanddyingtreesovertime)

Riparianforestbuffers

Establishmentdisturbancetosoilduringsitepreparation Depositionofwind‐andwater‐transportedsediments,nutrients,andotheragriculturalchemicalsintotheplanting HarvestingofherbaceousmaterialsplantedinZone3(zoneclosesttocrop/grazingsystem)andofwoodymaterialsplantedinZone2(middlezone)

Alleycropping

Establishmentdisturbancetosoilduringsitepreparation Weedcontrol(mechanicalorchemical) Pruning,thinning,andharvestingofwoodymaterial(amountandfrequencyvarygreatlydependingonshort‐andlong‐termobjectiveofpractice) Fertilizationforalleycropandoccasionallyneededfortreesinrows Tillageinalleys(frequencyandintensity) Cropspeciesusedinalleyproduction Complexharvestingschedulesstratifiedinspaceandtime

Silvopasture

Establishmentdisturbancetosoilduringsitepreparation Weedcontrol(mechanicalorchemical) Pruning,thinning,andharvestingofwoodymaterial(amountandfrequencyvarygreatlydependingonshort‐andlong‐termobjectiveofpractice) Fertilizationofforagecomponent Tillageinforagecomponent(frequencyandintensity) Cropspeciesusedinforagecomponent Grazingmanagement(timing,intensity,frequency) Complexharvestingschedulesstratifiedinspaceandtime

3.4.1 CarbonStocks

Agroforestry’spotentialforsequesteringlargeamountsofcarbonperunitareaiswellrecognized(Dixonetal.,1994;KumarandNair,2011;Nairetal.,2010),withsequestrationratesbeinggreater

5ForestFarmingisnotincludedintheseconsiderations.

thanmanyoftheotheragriculturaloptions(IPCC,2000).Carbonissequestereddirectlyintothewoodybiomassandsoil.Indirectly,agroforestrypracticescanaltercarboncyclingbyenhancingcropandforageproduction(upto15H—heightoftrees—distancefromthewindbreak)andtrappingwind‐blownandrunofferosion(Brandleetal.,2009).Lackofdatalimitsaccountingoftheseothercarbonfluxesimpactedbytheadditionoftreesandisbeyondthescopeofthiseffort.

WoodyBiomass:Themajorityofnewcarboncontributedtoasitebyagroforestrywillbefromtheproductionofwoodybiomass,withthelargercontributionbeingfromtheabovegroundwoodybiomass,asgenerallyobservedinforestestablishmentplantings(NuiandDuiker,2006).Themoreopenenvironmentcreatedinagroforestryplantingsresultsinthetreeshavingdifferentgrowthformsthanencounteredunderforestconditions—e.g.,greaterbranchproduction(Zhou,1999)andspecificgravity(Zhouetal.,2011)—whichwillneedtobetakenintoaccountwhenestimatingtheabovegroundwoodybiomass.

Thebelowgroundbiomasspoolinagroforestryplantingswillalsobeasignificantportionofnewcarbonaddedtothesite.However,measuring,estimating,and/orverifyingthiscomponentisverydifficultandexpensive.Thecontributionsfromrootbiomasscanbeestimatedusingvariousapproachesthatrelyonknowingtheabovegroundportion.

ForestProductsandOtherRemovedMaterials:Windbreaksandriparianforestbuffersareplantedforpurposesthatrequirethetreestobeinplaceforthetargetedfunction(s)(i.e.,alterationofmicroclimate;interceptionofsediments,nutrients,andchemicals).Windbreakrenovation(removalofdeadtreesandreplanting)isrecommendedtomaintainmicroclimatebenefits(Brandleetal.,2009).Periodicharvestingofplantmaterialsintheherbaceouszone(adjacenttocropfield)andmiddlewoodyzoneisalsorecommendedinriparianforestbufferstomaintainhigherratesofnutrientuptakeandthereforewaterqualityservices(Dosskeyetal.,2010).Moreinnovativeanddiversifiedplantingdesignsthatincorporatebioenergyfeedstocksarebeingconsideredforbothofthesepractices,whichwouldincreaselevelsofharvestingwithinthesesystems.Inthecaseofriparianforestbuffers,harvestingoftheherbaceousandwoodymiddlezoneforbioenergyfeedstockswouldservetoreplenishahighernutrientuptakerateandthuswaterqualityservices,aswellasprovideanadditionalincomestream(Schoenebergeretal.,2008).Manyalleycroppingandsilvopasturesystemsaremanagedforhigh‐valueveneerandsaw‐timber.Thesetrees,alongwithsomespecialapplicationsofagroforestrytechnologies,arealsobeinginvestigatedfortheiruseinproducingbioenergyfeedstocks.Fortheseplantings,removalorharvestingofabovegroundwoodymaterialcanoccurasearlyasthreeyearsto75yearsormore,dependingontheproduct.Harvestedmaterialscanalsoincludestem‐pruning,generallyupto15feetoverseveralyearstoattainacleanbole,toperiodicthinninginordertomaintainacanopycoverthatisoptimalforthegrowthofthetreeaswellasthecropbeinggrowninthealleys.Thematerialmaybeleftonsitetocreatewildlifehabitat,choppedandincorporatedintothesoil,ortakenoff‐siteandburned.

Soil:StudieshavedocumentedthatU.S.agroforestrypracticesgenerallyhavegreatersoilcarbonstocks(underthewholepractice,whichmayvaryfromjustunderawindbreaktounderthewholetree/cropsystem,suchasalleycropping)whencomparedwiththatinconventionalagriculturalandgrazingpractices(Nairetal.,2010).However,estimatingchangeorfluxinsoilcarbonstocksinagroforestryplantingsischallengingduetoitsinherentlyhighspatialandtemporalvariability.Forinstance,SharrowandIsmail(2004)foundvariabilityofsoilcarbontobetwotothreetimesgreaterinanon‐grazedsilvopasturesystemthanintheadjacentforestorpasturealone.

Soilcarboncanincreaseinagroforestrysystemsduetoaddedcarboninputsfromthetrees,theeliminationofcarbonlossduetoannualcroppingactivities(i.e.,conservationtillage),andpotentiallytheadditionofcarbonthroughotheragriculturalmanagementactivities,suchasincorporationofdifferentcrops,covercrops,residuemanagement,andfertilizationregimes.

ChangesinsoilcarbonstockshavebeenestimatedinanumberofforestestablishmentplotsfromtheMidwest,andwerefoundtovaryfrom‐0.07to0.58MgCha‐1year‐1and‐0.85to0.56MgCha‐1year‐1indeciduousandconiferousplots,respectively.Pauletal.(2003)attributedthevariationtotheimpactandvariablerecoveryfromtreeplanting,butalsomentionedthepossibilitythatvariationmaybeduetotheuseofpresent‐daycroppingfieldsasthecarbonbaselineforcomparison.Manyagroforestrystudiesarereportingcomparableratesofsoilsequestration(seeNairetal.,2010).Resultsfromtemperateagroforestrystudiesindicate,especiallyforalleysreceivinghighleveloforganicmatterinputfromthetrees,thatitmaybeseveralyearsbeforesignificantlymeasurablecarbondifferencesaredetectablebetweentheagroforestryplantingandtraditionalsolecroppingsystem(Peichletal.,2006;Udawattaetal.,2009).Theamountanddurationofsoilorganicmatteraccumulationinagriculturalsoilswithagroforestrymanagementwilldependonthedegreetowhichpriorsoilcarbonstocksaredepleted.Inaddition,itwilldependonthesoilsingeneral,climate,placementwithinalandscape,typeofvegetation,andmostimportantly,bytheadditionalmanagementactivitiesemployedinthemixedtree/agriculturalsystem(Table3‐4).

NotethatcarbonincreasesfromnitrogeninputsmaybeoffsetthroughenhancedN2Oemissions,dependingonanumberoffactors(seeSection6.4.1.6).Manyagroforestryplantings,suchaswindbreaksandriparianforestbuffers,arepurposefullydesignedtointerceptsoilinwinderosionandsurfacerunoff,whichisanotheradditionofcarbontothispool(Saueretal.,2007).DepositionofsedimentwillinfluencecyclingofbothelementsandthereforenetGHGvalues(McCartyandRitchie,2002;SudmeyerandScott,2002).Wecurrentlylacktheunderstandinganddataneededforadequatelymodelingandthereforepredictingtheseintra‐andinter‐soilcarbontransfersfromerosionanddeposition.

3.4.2 NitrousOxide

DataondirectN2Oemissionsinagroforestryplantingsaresparse.Thefewstudiesto‐datefoundreducedN2Oemissionsinafforestedplotsthatwereolderthanfiveyears(Allenetal.,2009),underwindbreaks(RyskowskiandKedziora,2007)andriparianforestbuffers(Kim,2008).AlleycroppingsystemsreducedN2Oemissionsby0.7kgha‐1year‐1comparedwiththeannualcroppingsystemswithnotreecover(ThevathasanandGordon,2004).Thesestudiessuggestthetreescanactasa“nitrogen‐safetynet”inthesystem,takingupthe“extra”nitrogenthatmightotherwiseresultinN2Oemissions.Inaddition,reducednitrogenleachinghasbeendocumentedwithinagroforestryplantingscomparedwiththeannualcroppingsystemwithnotreecover(Allenetal.,2004;Lopez‐Diazetal.,2011;Nairetal.,2007).ThereducedleachingimpliesthatlessnitrogenisavailableforindirectsoilN2Oemissions,whichcouldbebeneficialinthoseagroforestryplantingsrequiringfertilization(i.e.,alleycroppingandsilvopasturesystems)orthatreceivelargeinputsofnitrogenthroughsurfaceandsubsurfacerunoff(i.e.,riparianforestbuffers).Asmanyagroforestryplantingsarepurposefullydesignedandplantedtoprovidetighternutrientcyclingcapabilitiesasameanstoprotectwaterquality(Olsonetal.,2000),thecapabilityandcapacityofthesesystemstoreduceN2OemissionsinagriculturalsystemswarrantsfurtherstudytodeterminewhetherandhowitshouldbeaccountedforinGHGaccountingmethods.

3.4.3 Methane

VerylittleresearchhasbeendonetodeterminewhethertheestablishmentofagroforestryplantingscanleadtoanychangeinCH4sinksorsourcesinsoilsduetochangesinmethanotrophyormethanogenesis,respectively.Kimetal.(2010)didnotfindanyevidenceinestablishedriparianforestbuffersinIowa(sevento17yearsold)thatCH4fluxdifferedfromneighboringcropfields.RiparianforestbufferscouldpotentiallyserveasaCH4emittergiventheperiodicfloodingthatmayoccurwithintheseplantings.However,riparianforestbuffersestablishedonagriculturallandsmay

notbesignificantemittersofCH4becausethehydrologicalconnectionswithintheselandscapeshavebeendecoupled.Thisindicatesuseofriparianforest(naturallyoccurring)deriveddatamayresultinoverestimatingsink/sourcecapacityofriparianforestbuffers.Ingeneral,thereisinsufficientdatatomodelandpredictmethanefluxesinagroforestryatthistime.

3.4.4 ManagementInteractions

Agroforestrypracticescanindirectlyaltercarboncyclingbyenhancingcropandforageproductionandtrappingwindblownandsurfacerunoffsediments.ExaminingthecarbonpotentialofwindbreaksintheGreatPlains,Brandleetal.(1992)estimatedindirectcarbonbenefitscouldpotentiallybedoubletheamountofthecarbonsequesteredinthewood.Althoughprojectstoexamineindirectcarbonbenefitsfromseveraloftheagroforestrypracticesareongoing,wecurrentlylacktheabilitytomodelorpredicttheseimpacts.

3.5 EstimationMethods

ThissectionprovidesmethodsforestimatingGHGemissionsfromcroplandandgrazinglandsystemsonanentity’sland.Themethodsareappliedforbothlandremainingincroplandorgrazinglands,aswellasland‐usechangetocroplandorgrazinglands.Themethodsprovidedareforestimatingtheemissionlevelsforagivenyearonaparcelofland.Aparcelisafieldintheentity’soperationwithuniformmanagement.Ifmanagementvariesacrossthefield,thenthefieldshouldbesubdividedintoseparateparcelsforestimatingtheemissions.

Trendsacrossyearsorcomparisonstobaselinescanbemadeusingtheannualemissionestimates.Guidanceisnotgivenhereonhowtodevelopbaselinesorsubsequenttrendsforemissionestimation.Thelevelofemissionsforcarbonstocksisbasedonestimatingthechangeinstockfromthebeginningandendoftheyear,whilethelevelofemissionsforN2OandCH4arebasedonestimatingthetotalannualemissions.Methodsarealsoprovidedforestimatingtotalemissionsofprecursorgasesemittedduringbiomassburning,aswellasnitrogencompoundsthatarevolatilizedorsubjecttoleachingandrunofffromanentity’scroplandorgrazinglandthatarelaterconvertedintoGHGs.

Themethodsrangeincomplexityforthedifferentemissionsourcecategoriesaccordingtothestateofthescienceandpriormethoddevelopment.Simplemethodsareselectedforseveraloftheemissionorcarbonstockchangesourcecategories;becausethemorecomplexmethodsarenotfullydevelopedforoperationalaccountingofemissionsorthesimplemethodsprovideareasonablyaccurateandpreciseresult.Althoughsimplicitymaybepreferredfortransparencyinestimation,someofthemethodsusemorecomplexapproaches,suchasprocess‐basedsimulationmodels,becausethesemethodsgreatlyimprovetheaccuracyand/orprecisionoftheresult.

3.5.1 BiomassCarbonStockChanges

3.5.1.1 RationaleforSelectedMethod

BothIPCC(2006)andtheU.S.EnvironmentalProtectionAgency(2011)considerherbaceousbiomasscarbonstockstobeephemeral,andrecognizethattherearenonetemissionstotheatmospherefollowingcropgrowthandsenescenceduringoneannualcropcycle(Westetal.,2011).However,withrespecttochangesinlanduse(e.g.,foresttocropland),theIPCC(Lascoetal.,2006)recommendsthatcroplandbiomassbecountedintheyearthatlandconversionoccurs,andthesameassumptionalsoappliesforgrassland(Verchotetal.,2006).AccordingtotheIPCC,accountingfortheherbaceousbiomasscarbonstockduringchangesinlanduseisnecessarytoaccountfortheinfluenceofherbaceousplantsonCO2uptakefromtheatmosphereandstorageintheterrestrialbiosphere.However,thismethoddoesnotrecognizechangesinherbaceousbiomassthatoccurwithchangesincroprotations,nordoesitrecognizelong‐termincreasesinannualcropyields.ThemethodisaconsideredaTier2methodasdefinedbytheIPCCbecauseitincorporatesfactorsthatarebasedonU.S.specificdata.

Agroforestry,alongwithotherwoodyvegetationincroplands,suchasorchardsandvineyards,sequestersignificantamountsofnewcarbonwithinlong‐livedbiomassovertimewithtreegrowth.MethodsforestimatingtheabovegroundwoodyandwholetreebiomassfortreesgrowingunderforestconditionsaredescribedintheForestrySectionofthisreport.However,thesemethods,developedfromforest‐derived(i.e.,greatercanopyclosure)conditions,donotaccuratelyreflectconditionsencounteredinagroforestryorwoodycrops.Treesgrowingunderwindbreakandotherlinear‐typeplantingshavebeendocumentedtodifferfromforest‐growntreesintermsofarchitectureandproperties,suchascrown:trunkallocation(Zhou,1999),specificgravity(Zhouetal.,2011),andtaper(Zhouetal.,inreview).Moreover,theForestInventoryandAnalysisprogramoftheUSDAForestServiceandNationalResourceInventoryoftheUSDANaturalResourcesConservationServicedonotcollectagroforestryorwoodycropdatathroughtheirsurveys(Perryetal.,2005).Therefore,aTier3methodusingprocess‐basedmodelsisaviablealternativeforestimatingthecarbonstockchangesassociatedwithagroforestryandwoodycropswithoutdirectmeasurementthroughasurvey.Specifically,theDAYCENTmodelhasbeenparameterizedtosimulatetreegrowthandhasbeenadoptedforestimatingwoodybiomasscarbonforagroforestryandwoodycrops.

MethodforEstimatingBiomassCarbonStockChanges

AmodifiedversionofthemethodologydevelopedbytheIPCC(Lascoetal.,2006;Verchotetal.,2006)hasbeenadoptedforentity‐scaleestimationofherbaceousandwoodybiomassstockchangesassociatedwithlanduse.

TheDAYCENTprocess‐basedsimulationmodelorthetraditionalforestinventoryapproachesareusedtoestimatecarbonforabovegroundbiomassforagroforestry.

U.S.specificdefaultvalues(Westetal.,2010)areusedforestimatingbiomasscarbonforannualcropsandgrazinglands.TheIPCCdefaultisusedforestimatingthecarbonfractionvalue.YieldinunitsofdrymattercanbeestimatedbytheentityoraveragevaluesfromUSDA‐NationalAgriculturalStatisticsServicestatisticscanbeused.

Thismethodwaschosenbecauseitcapturestheinfluenceofland‐usechangeoncroporforagespeciesonbiomasscarbonstocksbyusingU.S.specificdefaultvalueswhereentityspecificactivitydataarenotavailableandaprocess‐basedsimulationmodelforagroforestrysystems.

3.5.1.2 DescriptionofMethod

AmodifiedversionofthemethodologydevelopedbytheIPCC(Lascoetal.,2006;Verchotetal.,2006)hasbeenadoptedforentity‐scalereportingintheUnitedStatesofherbaceousandwoodybiomassstockchangesassociatedwithlandusechange.Themethodconsistsofestimatingthemeanannualbiomassstockforacroplandorgrazinglandsfollowingalandusechange,whichcanbeaveragedacrossyearsforacroporrotation.Thismethodonlyaddressesachangeintheherbaceousbiomasscarbonstocksintheyearfollowingaland‐usechange,consistentwiththeIPCCmethods(Lascoetal.,2006;Verchotetal.,2006).Incontrast,carbonstockchangeinwoodybiomassisestimatedeveryyear.

UseEquation3‐1toestimatethetotalbiomasscarbonstockchangeforalandparceloverayear:

HerbaceousBiomass:Estimatethemeanannualherbaceousbiomassstockinalandparcelforcroplandorgrazinglandfollowingalandusechangewiththefollowingequation:

Themeanannualbiomassstockisintendedtorepresentthetimeperiodfollowingharvestwherenocropexistsandbothlitterandrootsaredecomposingquickly(Gilletal.,2002),andthetimeperiodduringthegrowingseasonwherebiomasscontinuestogrowuntilitreachespeakannualbiomass.Theaverageofzerobiomassandpeakbiomass(e.g.,peakbiomassdividedbytwo)isconsideredrepresentativeofthemeanannualcarbonstock(i.e.,Yf=0.5).

Equation3‐3isusedtoestimatethepeakabovegroundbiomassinalandparcelfromharvestyielddataincroplandsorpeakforageyieldsingrazinglands.

Equation3‐1:TotalBiomassCarbon StockChange

ΔCBiomass=(Ht+Wt)–(Ht‐1+Wt‐1)

Where:

ΔCBiomass=Totalchangeinbiomasscarbonstock(metrictonsCO2‐eqyear‐1)

H =Meanannualherbaceousbiomass(metrictonsCO2‐eqyear‐1)

W =Meanannualwoodybiomass(metrictonsCO2‐eqyear‐1)

t =Currentyearstocks

t‐1 =Previousyear’sstocks

Equation3‐2:MeanAnnualHerbaceousBiomassCarbon Stock

H=[Hpeak+(Hpeak×R:S)]×A×CO2MW/Yf

Where:

H =Meanannualherbaceousbiomasscarbonstock(metrictonsCO2‐eqyear‐1)

Hpeak =Annualpeakabovegroundbiomass(metrictonsCha‐1year‐1)

R:S =Root‐shootratio(unitless)

A =Areaoflandparcel(ha)

CO2MW=RatioofmolecularweightofCO2tocarbon=44/12(metrictonsCO2(metrictonsC)‐1)

Yf =Approximatefractionofcalendaryearrepresentingthegrowingseason(unitless)

Thismethodcapturestheinfluenceofland‐usechangeandchangesincroporforagespeciesonbiomasscarbonstocks.Therefore,cropharvestorpeakforageyieldsshouldbeaveragedacrossyearsaslongasthesameforagespecies,croporrotationofcropsaregrown.Theharvestindexissettooneforgrazinglands.

Peakforageestimatesforgrazinglandscanbeestimatedusingthebiomassclippingmethod.6Thismethodisdestructivewiththeremovalofforagesamplesfromthefield.Non‐destructivemethodscanalsobeusedincludingthecomparativeyieldmethodforrangelands7,ortherobelpolemethodonrangelandsorpastures(Harmoneyetal.,1997;Vermeireetal.,2002).Anysamplingthatisdone,whetherdestructiveornon‐destructive,shouldoccuratlocationsthatarerepresentativeofthelandparcel.Ifsamplingtheforageisnotfeasible,defaultforageproductionvaluesareprovidedbytheNaturalResourcesConservationServiceinEcologicalSiteDescriptions(ESDs).8AfteridentifyingtheappropriateESD,theentitywouldselecttheplantcommunitythatisrepresentativeoftheparcel.ThesevaluesrepresenttotalproductionforthesitesoYfinEquation3‐2wouldbesetto1iftheabovegroundforageproductionisobtainedfromanESD.

WoodyBiomass:Thelargestamountofcarboncapturedbyagroforestrysystemsisinwoodybiomass,withthemajorityoccurringintheabovegroundbiomass.Woodycropsalsogaincarbonastheygrow.Thismethodalsoaddressescarbonremovalsthroughharvestorothereventsthatremovetreebiomass.

Themethodstoestimatebiomasscarboninalandparcelforthemore‐opengrowthofagroforestrysystemsandwoodycrops(WtandWt‐1inEquation3‐1)arebasedonDAYCENTmodelsimulationsandgrowthfunctionsforagroforestry.AgroforestrypracticesarebasedontheNaturalResourcesConservationServiceagroforestrypracticestandards,whichareprovidedinapicklist.Forwoodycrops,theDAYCENTmodelsimulatestheinfluenceofcommonmanagementpracticesonbiomassstocks,includingirrigation,fertilization,organicmatteramendments,groundcovermanagement,

6Seesection15,“StandingBiomass”http://www.nrisurvey.org/nrcs/Grazingland/2011/instructions/instruction.htm7Seesection13,“DryWeightRank”http://www.nrisurvey.org/nrcs/Grazingland/2011/instructions/instruction.htm8SeeESDshttps://esis.sc.egov.usda.gov/

Equation3‐3:AbovegroundHerbaceousBiomassCarbonStock

Hpeak=(Ydm/HI)×C

Where:

Hpeak=Annualpeakabovegroundherbaceousbiomasscarbonstock (metrictonsCha‐1year‐1)

Ydm =Cropharvestorforageyield,correctedfordrymattercontent (metrictonsbiomassha‐1year‐1)

Y =Cropharvestorforageyield(metrictonsbiomassha‐1year‐1)

DM =Drymattercontentofharvestedcropbiomassorforage(dimensionless)

HI =HarvestIndex(dimensionless)

C =Carbonfractionofabovegroundbiomass(dimensionless)

pruningofbranches,thinningofyoungfruit,andharvestandremovalofmaturefruit.Giventhepractice,DAYCENTsimulateschangesinwoodybiomasscarbonstocksforthereportingperiod.

Foragroforestrysystemswheretheentityhasmeasuredtreeparameters,anempiricalmodelisprovidedtomorepreciselyestimatewoodybiomasscarbongrowthincrementfortheyear(MerwinandTownsend,2007;Merwinetal.,2009).TheempiricalmodelusesanindividualtreegrowthequationsbasedonLessard(2000)andLessardetal.(2001).Carbonpoolsarethenderivedfromdiameter‐basedallometricequationsthatpredicttotalabovegroundbiomasscomponentsfor10broadspeciesgroupsintheUnitedStates.(Jenkinsetal.,2003;2004).BothpublishedandunpublisheddatafortheU.S.ForestServiceForestInventoryandAnalysisprogramwereusedtodevelopthegrowthincrementmodel.

Inaddition,harvestedwoodyproductsassociatedwithagroforestryareestimatedusingtheapproachesdescribedintheForestryChapter(Section6.5).Woodyproductsmaybeharvestedfromsilvopasture,alleycropping,andotheragroforestrypractices,providingavarietyofproductssuchasveneer,sawtimber,andbioenergyfeedstocks.

3.5.1.3 ActivityData

Activityandrelateddataneededtoestimatebiomasscarbonforannualcropsandgrazinglands(asapplicable)include:

Croptype,croplandarea,andharvestindices; Typeofforage,grazingarea,andpeakforageyielddata; Totalabovegroundyieldofcroporpeakforageyieldforgrazinglands(metrictonsbiomass

perha); Root:shootratios; Carbonfractions;and Drymattercontentofforageandharvestedcropbiomasstoestimatedrymattercontent.

Iftheentitydoesnotprovidevalues,defaultvaluesformoisturecontent,residue‐yieldratios,androot:shootratiosareprovidedinTable3‐5.Ageneraldefaultvalueforcropcarbonfractionis0.45.Insomeyears,theentitymaynotharvestthecropduetodrought,pestoutbreaksorotherreasonsforcropfailure.Inthosecases,theentityshouldprovidetheaverageyieldthattheyhaveharvestedinthepast,andanapproximatepercentageofaveragecropgrowththatoccurredintheyear.Theyieldisestimatedbasedonmultiplyingtheaveragecropyieldbythepercentageofcropgrowthobtainedpriortocroploss.Peakforageyieldswillvaryfromyeartoyear,butcanbebasedonafive‐yearaverage.

Table3‐5:RepresentativeDryMatterContentofHarvestedCropBiomass,HarvestIndex,andRoot:ShootRatiosforVariousCrops.a

CropDryMatterContent

HarvestIndexRoot:ShootRatio

FoodcropsBarley 0.865(3.8%) 0.46(18.7%) 0.11(90.7%)Beans 0.84(3.3%) 0.46(18.7%) 0.08(89.7%)Corngrain 0.86(1.9%) 0.53(15.0%) 0.18(97.3%)Cornsilage 0.74(1.9%) 0.95(3.3%) 0.18(97.1%)Cotton 0.92(1.4%) 0.40(20.0%) 0.17(44.0%)Millet 0.90(1.9%) 0.46(17.6%) 0.25(91.1%)Oats 0.865(1.9%) 0.52(18.7%) 0.40(90.9%)Peanuts 0.91(1.9%) 0.40(16.6%) 0.07(12.4%)Potatoes 0.20(9.3%) 0.50(20.0%) 0.07(44.1%)

CropDryMatterContent

HarvestIndexRoot:ShootRatio

Rice 0.91(1.6%) 0.42(28.1%) 0.22(13.2%)Rye 0.90(1.9%) 0.50(18.7%) 0.14(90.1%)Sorghumgrain 0.86(1.9%) 0.44(14.8%) 0.18(97.2%)Sorghumsilage 0.74(1.9%) 0.95(3.3%) 0.18(97.2%)Soybean 0.875(1.7%) 0.42(16.7%) 0.19(89.8%)Sugarbeets 0.15(12.4%) 0.40(24.1%) 0.43(43.9%)Sugarcane 0.258(11.6%) 0.75(6.4%) 0.18(37.4%)Sunflower 0.91(1.9%) 0.27(11.1%) 0.06(44.0%)Tobacco 0.80(1.9%) 0.60(3.3%) 0.80(44.0%)Wheat 0.865(3.8%) 0.39(17.7%) 0.20(86.2%)

ForageandFoddercrops

Alfalfahay 0.87(1.8%) 0.95(3.3%) 0.87(21.8%)Non‐legumehay 0.87(1.8%) 0.95(3.3%) 0.87(21.8%)Nitrogen‐fixingforages 0.35(3.3%) 0.95(3.3%) 1.1(21.2%)

Non‐nitrogen‐fixingforages 0.35(3.3%) 0.95(3.3%) 1.5(21.2%)

Perennialgrasses 0.35(3.3%) 0.95(3.3%) 1.5(21.2%)Grass‐clovermixtures 0.35(3.3%) 0.95(3.3%) 1.5(21.2%)

Source:RevisedfromWestetal.(2010).aUncertaintyisexpressedonapercentagebasisashalfofthe95%confidenceinterval.

Activitydataforestimatingcarboninabovegroundbiomassforagroforestrywillentailthecollectionofsomelevelofinventoryoftreesassociatedwiththeagroforestrypractice.SimplifiedinventoryapproachesrequiringaminimumofworkbythelandownerhavebeendevelopedbytheUSDANaturalResourcesConservationServiceandtheColoradoStateUniversityNaturalResourceEcologicalLaboratory(USDA,2012),whicharelargelybasedonmethodsdescribedintheNaturalResourcesConservationServiceNationalForestHandbook(USDANRCS,2004).Thespecificactivitydatarequirementsinclude:

Speciesoftreesandnumberbyageofdiameterclassforeachagroforestrypractice;and Diameteratbreastheightforasubsampleoftreesusingoneofthreesamplingmethodsthat

capturethespacingarrangementsanddensitieswithinthedifferentpractices(i.e.,rowtypeplantings,woodlot‐likeplantings,andriparianforestbuffers).

3.5.1.4 AncillaryData

Noancillarydataareneededforthismethod.

3.5.1.5 ModelOutput

Modeloutputisgeneratedforthechangeinbiomasscarbonstocks.Thischangeisdeterminedbasedonsubtractingthetotalbiomasscarbonstockinthepreviousyearfromthetotalstockinthecurrentyear,whichwillincludebothherbaceousandwoodybiomass.Theherbaceousstockswillrepresentmeanestimatesoveryearsifthesameforages,crop,orrotationofcropsaregrown,andisonlyestimatedforalandusechange.TheapproachforestimatingbiomasscarbonforwetlandsandforestlandsaredescribedinSections4.3.1and6.2.1,respectively.

Emissionsintensityisalsoestimatedbasedontheamountofemissionsperunitofyieldforcropsincroplandsystems,orofanimalproductsingrazingsystems.Notethatthebiomasschangeisbasedsolelyonwoodyplantgrowthexceptinayearfollowingaland‐usechange.

Theemissionsintensityisestimatedwiththefollowingequation:

3.5.1.6 LimitationsandUncertainty

Uncertaintyinherbaceouscarbonstockchangeswillresultfromlackofprecisionincroporforageyields,residue‐yieldratios,root‐shootratios,andcarbonfractions,aswellastheuncertaintiesassociatedwithestimatingthebiomasscarbonstocksfortheotherlanduses.Emissionsintensitywillalsoincludeuncertaintyinthetotalyieldforthecrop,meat,ormilkproduct.Thisherbaceousbiomassmethodisbasedontheassumptionthathalfofthecropharvestyieldsorpeakforageamountsprovideanaccurateestimateofthemeanannualcarbonstockincroplandorgrazinglands.Thisassumptionwarrantsfurtherstudy,andthemethodmayneedtoberefinedinthefuture.

UncertaintiesinmodelparametersarecombinedusingaMonteCarlosimulationapproach.Uncertaintyisassumedtobeminorforthemanagementactivitydataprovidedbytheentity.Table3‐6providestherelativeuncertaintyfortheDAYCENTmodelandthecarbonfractionofbiomass.

Table3‐6:AvailableUncertaintyDataforBiomassCarbonStockChanges

Parameter Mean UnitsRelativeUncertainty

Distribution DataSourceLow(%) High(%)

DAYCENT(empiricaluncertainty)

NS Various NS NS NormalOgleetal.(2007);EPA(2013)

Carbonfractionofabovegroundbiomass

0.45 Fraction 11 11 Normal IPCC(1997)

NS=NotShown.Dataarenotshownforparametersthathave100’sto1000’sofvalues(denotedasNS).

Theuncertaintydifferswhetheritisherbaceousbiomassortrees.Uncertaintyassociatedwithestimatingcarboninlivetreesisinfluencedbyanumberoffactors,includingsamplingandmeasurementerroranderrorassociatedwithregressionmodels(seeMelsonetal.2011;furtherdiscussioninForestrySection).Estimatingcarboninagroforestrytrees,especiallyforyoungseedlingsandsaplings(upto10yearsorsodependingonspeciesandgrowingconditions)remainshighlyuncertainparticularlysincetraditionalforestry‐derivedequationshavebeenshowntounderestimatewhole‐treebiomassinagroforestrysystemsandrequiresadditionalfieldworktofurtherdocumentbiomasscarbonallocationdifferences.Melsonetal.(2011)notedintheirforest‐basedworkthatestimationoflive‐treecarbonwassensitivetomodelselection(withmodel‐selectionerrorofpotentially20to40percent),andthatmodelselectioncouldbeimprovedbymatchingtreeformtoexistingequationsforuseinthemodels.On‐goingworkcomparingagroforestry‐derivedequationswithavarietyofforest‐derivedequationsintheGreatPlainsregionindicateuncertaintycouldbereducedthroughuseofacorrectionfactor.Currentlybelowgroundbiomass/Cestimatesarecalculatedusingtwoapproaches:root:shootratios(seeBirdsey,1992),

Equation3‐4:EmissionsIntensityofBiomassCarbon StockChange

EIBiomassC=ΔCBiomass/Y

Where:

EIBiomassC =Emissionsintensity(metrictonsCO2permetrictondrymattercropyield,metrictonsCO2perkgcarcassyield,ormetrictonsCO2perkgfluidmilkyield)

ΔCBiomass =ChangeinbiomassstockinCO2equivalents(metrictonsCO2‐eqyear‐1)

Y =Totalyieldofcrop(metrictonsdrymattercropyield),meat(kgcarcassyield)ormilkproduction(kgfluidmilkyield)

andabovegrounddensityallometry(Cairnsetal.,1997),bothwithlargeuncertaintiesduetolackofdata.Thefullsetofprobabilitydistributionshavenotbeendevelopedfortheagroforestrymethod,andsowillrequirefurtherresearchbeforeuncertaintycanbeestimated.SeeChapter6,Forestry,forfurtherdiscussionofuncertaintyoftreevolumeandbiomassequations.

3.5.2 LitterCarbonStockChanges

Litterinherbaceousbiomassdecomposesmostlyoveraone‐yearperiod.HowevertheinfluenceoflittercarbonstocksonatmosphericCO2isassumedtobeinsignificantafteraddressingthechangesinbiomassandsubsequentinfluenceonsoilcarbonstocks.Furthermethodsdevelopmentmaybepossibleinthefuture,giventhispotentiallimitationtothemethodsinthisreport.Forcroplandorgrazinglandsystemswithtrees,coarsewoodydebrisandlittercarbonshouldbeestimatedbasedonforestmethods(SeeSection6.2.2.4and6.2.2.5).ThelossoflitterandcoarsewoodydebriswithconversionfromforestlandtocroplandandgrazinglandisalsoaddressedinSection6.3.

3.5.3 SoilCarbonStockChanges

SOCstocksareinfluencedbylanduseandmanagementincroplandandgrazinglandsystems,aswellasconversionfromotherlandusesintothesesystems(Aaldeetal.,2006).SOCpoolscanbemodifiedduetochangesincarboninputsandoutputs(Paustianetal.,1997).Carboninputswillchangeovertimeduetointerannualvariabilityandlongertermtrendsinnetprimaryproduction,aswellasdifferencesincarbonremovalsfromharvestingandresiduemanagementpractices.ExternalcarboninputswillalsohaveaninfluenceontheSOCstocks,suchasmanure,compost,sewagesludge,woodchips,andbiocharamendments.Carbonoutputswillchangeduetointerannualvariabilityandlongertermtrendsinmicrobialdecompositionrates.Inaddition,erosionanddepositioncontributetochangesinSOCstocksassociatedwithcropandgrazinglandsoils.Recentstudies(Hardenetal.,2008;VanOostetal.,2007)provideevidencethatthemajorityofcarboninerodedsoilsisdynamicallyreplaced,compensatingforthelosses,andatleastsomeofthecarbontransportedfromthesiteisdepositedattheedgeoffields,downslope,orinrivers.Inallcases,SOCismovedfromonelocationtoanotherundertheassumptionthatonlyaportionofthe

MethodforEstimatingSoilCarbonStockChanges

Mineralsoils: TheDAYCENTprocess‐basedsimulationmodelestimatesthesoilorganiccarbon(SOC)at

thebeginningandendoftheyear.TheseinputsareenteredintotheIPCCequationtoestimatecarbonstockchangesinmineralsoilsdevelopedbyLascoetal.(2006),andVerchotetal.(2006).

ThismethodwaschosenbecausetheDAYCENTmodelhasbeendemonstratedtorepresentthedynamicsofsoilorganiccarbonandestimatesoilorganiccarbonstockchangeinU.S.croplandandgrasslands(Partonetal.,1993),anduncertaintieshavebeenquantified(Ogleetal.(2007).Themodelcapturessoilmoisturedynamics,plantproduction,andthermalcontrolsonnetprimaryproductionanddecompositionwithatimestepofamonthorless.

OrganicSoils: IPCCequationdevelopedbyAaldeetal.(2006;USDA,2011)usingregionspecific

emissionfactorsfromOgleetal.(2003). Thismethodwaschosenbecauseitistheonlyreadilyavailablemodelforestimatingsoil

carbonstockchangesfromorganicsoils.

carbonintransportislosttotheatmosphere.Thisassumptionmayhavesignificantvariationduetothediversityofenvironmentalconditionsinwhicherodedcarbonistransportedandsubsequentlyresides.Otherenvironmentaldriverswillalsoinfluencecarbondynamicsinsoils,particularlyweatherandsoilcharacteristics.

Process‐basedmodels,whichareconsideredanIPCCTier3methodology,havebeendevelopedandsufficientlyevaluatedforapplicationinanoperationaltooltoestimateSOCstockchangesinmineralsoils.TheDAYCENTprocess‐basedmodel(Partonetal.,1987;Parton,1998)hasbeenselectedbecauseitiswell‐testedforestimatingsoilcarbondynamicsincroplandandgrazinglandsystems(Partonetal.,1993)andisalsousedintheU.S.nationalGHGinventory(Ogleetal.,2010;U.S.EPA,2011).DelGrossoetal.(2011)demonstratedthereductioninuncertaintyassociatedwiththemoreadvancedapproachusingtheDAYCENTmodelcomparedtothelowertiermethods.TheDAYCENTmodelsimulatesplantproductionbyrepresentinglong‐termeffectsoflanduseandmanagementonnetprimaryproduction(NPP),asinfluencedbyselectionofcropsandforagegrasses.TheinfluenceofmanagementpracticesonNPParealsosimulated,includingmineralfertilization,organicamendments,irrigationandfertigation,liming,greenmanuresandcovercrops,croppingintensity,hayorpastureinrotationwithannualcrops,grazingintensityandstockingrate,andbarefallow.Nutrientandmoisturedynamicsareinfluencedbysoilcharacteristics,suchassoiltexture.ThemethodaddressesinterannualvariabilityduetoannualchangesinmanagementandtheeffectofweatheronNPP.

IntheDAYCENTmodel,threesoilorganiccarbonpoolsareincludedrepresentingactive,slow,andpassivesoilorganicmatter,whichhavedifferentturnovertimes.Itisgenerallyconsideredthattheactivecarbonpoolismicrobialbiomassandassociatedmetaboliteshavingarapidturnover(monthstoyears),theslowcarbonpoolhasintermediatestabilityandturnovertimes(decades),andthepassivecarbonpoolrepresentshighlyprocessedandhumifieddecompositionproductswithlongerturnovertimes(centuries).However,thesepoolsarekineticallydefinedanddonotnecessarilyrepresentexplicitfractionsofsoilorganiccarbonthatcanbeisolated.Soiltexture,temperature,moistureavailability,aeration,burning,andotherfactorsarerepresentedinthesimulationsthatinfluencethedecompositionandlossofcarbonfromthesepools.

Themodelsimulatesmanagementpracticesinfluencingsoilorganiccarbonpools.Thesepracticesincludeadditionofcarboninmanureandotherorganicamendments,suchascompost,woodchips,andbiochar;tillageintensity;residuemanagement(retentionofresiduesinfieldwithoutincorporation,retentioninthefieldwithincorporation,andremovalwithharvest,burning,orgrazing).Theinfluenceofbareandvegetatedfallowsisrepresented,inadditiontoirrigationeffectsondecompositionincroplandandgrazinglandsystems.Themodelcanalsosimulatesetting‐asidecroplandfromproduction;theinfluenceoffireonoxidationofsoilorganicmatter;andwoodyplantencroachment,agroforestry,andsilvopastureeffectsoncarboninputsandoutputs.

Awater/soilmoisturesubmodel(e.g.,Partonetal.,1987)isusedtorepresenttheinfluenceofweather,irrigation,croptype,andmanagementonsoilmoisturedynamics.Thisimpactisparticularlyimportantbecausemoisturetendstobeamoreproximalfactorcontrollingsoilorganiccarbondynamics,which,inturn,isinfluencedbylanduseandmanagementactivity.Forexample,irrigationinfluencesplantproductionandcarboninputsbecauseofthemodificationtothemoistureregime.

ThemodeledestimatesfromDAYCENTarecombinedwithmeasurementdatafromamonitoringnetworktoformallyevaluateuncertainty.Thisapproachleveragesthescalabilityofthemodelwhileprovidinganunderlyingmeasurement‐basisforthemethod(Conantetal.,2011;Ogleetal.,2007).

Erosionanddepositioninfluencesoilorganiccarbonstocks(Izaurraldeetal.,2007)andthereforearerepresentedinthemethod,althoughthereisuncertaintyintheneteffectonCO2exchangebetweenthebiosphereandatmosphere.Moreover,thereisalsosomeriskofdouble‐countingcarbonasitistransferredacrossownershipboundaries,intermsofwhoreceivescreditfortheerodedcarbonintheiraccounting.Regardless,erosionclearlyhasanimpactoncarbonstocksinafield,whichcanbeestimatedwithreasonableaccuracyusingerosioncalculators,suchastheRevisedUniversalSoilLossEquation,Version2(RUSLE2)forwatererosion(USDA,2003)andWindErosionPredictionSystem(WEPS)forwinderosion(USDA,2004).Therefore,thecurrentmethodwillincludeanestimateoferosion‐relatedcarbonlossfromafield,butneitherthefateoferodedC,northedepositionofcarbonfromotherareasontoalandparcel,willbeestimated.Asmorestudiesareconducted,carbontransportanddepositioncanbeincorporatedinfutureversionsofthemethod.

Drainageoforganicsoilsforcropproductionleadstonetannualemissionsduetoincreaseddecompositionoftheorganicmatterafterloweringthewatertableandcreatingaerobicconditionsintheupperlayersofthesoil(Allen,2012;ArmentanoandMenges,1986).Therehasbeenlessevaluationofprocess‐basedmodelsfororganicsoils,particularlythesimulationofwatertabledynamicsthroughouttheyear,whichwillinfluencetheemissionrate.Consequently,theapproachisbasedonmoresimplisticemissionfactorapproachdevelopedbytheIPCC(Aaldeetal.,2006).ThemethodincorporatesU.S.emissionratesassociatedwithregion‐specificdrainagepatterns(Ogleetal.,2003),soitisaTier2methodasdefinedbytheIPCC.

ThemethodrepresentingtheinfluenceoflanduseandmanagementonSOCandassociatedCO2fluxtotheatmosphereisestimatedwithacarbonstockchangeapproach(Aaldeetal.,2006).Formineralsoils,themethodwillrequireestimatesofcarbonstocksatthebeginningandendoftheyearinordertoestimatetheannualchangeusingtheequationbelow.Incontrast,carbonstockchangesinorganicsoils(i.e.,Histosols)willaddressonlytheemissionsoccurringwithdrainage,whichisthetypicalsituationincropland.Emissionsoccurinorganicsoilsfollowingdrainageduetotheconversionofananaerobicenvironmentwithahighwatertabletoaerobicconditions(ArmentanoandMenges,1986),resultinginasignificantlossofcarbontotheatmosphere(Ogleetal.,2003).Recentdataonsubsidencewereusedtoderivetheseestimates(e.g.,Shihetal.,1998).

MineralSoils:ThemodeltoestimatechangesinSOCstocksformineralsoilshasbeenadaptedfromthemethoddevelopedbyIPCC(Aaldeetal.,2006).Theannualchangeinstockstoa30centimeterdepthforalandparcelisestimatedusingthefollowingequation:

Equation3‐5:ChangeinSoilOrganicCarbon StocksforMineralSoils

ΔCMineral=[(SOCt‐SOCt‐1)/t]×A×CO2MW

Where:

ΔCMineral =Annualchangeinmineralsoilorganiccarbonstock(metrictonsCO2‐eqyear‐1)

SOCt =Soilorganiccarbonstockattheendoftheyear(metrictonsCha‐1)

SOCt‐1 =Soilorganiccarbonstockatthebeginningoftheyear(metrictonsCha‐1)

t =1year

A =Areaofparcel(ha)

CO2MW =RatioofmolecularweightofCO2tocarbon =44/12(metrictonsCO2(metrictonsC)‐1)

TheDAYCENTmodelisusedtosimulatetheSOCstocksatthebeginningandendofeachyearforEquation3‐5basedonrecentmanagementpracticesforalandparcel.InitialvaluesforDAYCENTareneededfortheSOCt‐1andarebasedonasimulationofhistoricalmanagementtoprovideaccuratestocksanddistributionoforganiccarbonamongthepoolsrepresentedinthemodel(active,slow,andpassivesoilorganicmatterpools).Eachpoolhasadifferentturnoverrate(representingtheheterogeneousnatureofsoilorganicmatter),andtheamountofcarbonineachpoolatanypointintimeinfluencestheforwardtrajectoryofthetotalsoilorganiccarbonstorage(Partonetal.,1987).Bysimulatingthehistoricallanduse,thedistributionsofcarboninactive,slow,andpassivepoolsareestimatedinanunbiasedway.

Threestepsarerequiredtoestimatetheinitialvalues.Thefirststepinvolvesrunningthemodeltoasteady‐statecondition(e.g.,equilibrium)undernativevegetation,historicalclimatedata,andthesoilphysicalattributesforthelandparcel.Thesecondstepistosimulateperiodoftimefromthe1800’sto1980and1980to2000.Theentityisprovidedalistofoptionsforselectingthepracticesthatbestmatchthelandmanagementfortheparcel.From2000totheinitialyearforreporting,theentityentersmorespecificdataoncropsplanted,tillagepractices,fertilizationpractices,irrigation,andothermanagementactivity(SeeSection3.5.3.3formoreinformation).Thesimulatedcarbonstockattheendofthesimulationprovidestheinitialbaselinevalue(SOCt‐1).

Thestockattheendofayear(SOCt)isestimatedbytheDAYCENTmodelbasedonsimulatingmanagementactivityduringthespecificyear.Theentityprovidesthemanagementactivityforthelandparcel,includingcropsplanted,tillagepractices,fertilizationpractices,irrigationandothermanagementactivitydata(SeeSection3.5.3.3formoreinformation).ThechangeinSOCstocksareestimatedforadditionalyearsbyusingtheendingstockfromthepreviousyearastheinitialSOCstock(SOCt‐1)andthensimulatingthemanagementforanotheryeartoproducethestockattheendofthenextyear(SOCt).

ErodedcarbonisestimatedwiththeRUSLE2forwatererosion(USDA,2003)andWEPSforwinderosion(USDA,2004).NeitherthedepositionofcarbononthesitenorthefateoferodedcarbonisinthisversionoftheUSDAmethods.TheerodedcarbonestimateisreportedseparatelytoaccountforuncertaintyassociatedwiththepotentialeffectoferosiononSOCstocks,andmaybeusedasadiscountfortheSOCstockchangesestimatewithEquation3‐5.

TheDAYCENTmodelisnotabletoestimatesoilorganiccarbonstocksinmineralsoilsforallcrops.IninstanceswhereacropisnotestimatedbytheDAYCENTmodel,themethoddevelopedbytheIPCC(2006)(i.e.,aTier1methodology)maybeused(SeeAppendix3‐B).

OrganicSoils:ThemethodologyforestimatingsoilcarbonstockchangesindrainedorganicsoilshasbeenadoptedfromIPCC(Aaldeetal.,2006).ThemethodappliestoHistosolsandsoilsthathavehighorganicmattercontentanddevelopedundersaturated,anaerobicconditionsforatleastpartoftheyear,whichincludesHistels,Historthels,andHistoturbels.Thefollowingequationisusedtoestimateemissionsfromalandparcel:

EmissionfactorshavebeenadoptedfromOgleetal.(2003)andareregion‐specific,basedontypicaldrainagepatternsandclimaticcontrolsondecompositionrates;theseratesarealsousedintheU.S.nationalGHGinventory(U.S.EPA,2011).Drainedcroplandsoilslosecarbonatarateof11±2.5metrictonsCha‐1year‐1incooltemperateregions,14±2.5metrictonsCha‐1year‐1inwarmtemperateregions,and14±3.3metrictonsCha‐1year‐1insubtropicalclimateregions.Organicsoilsingrazinglandsaretypicallynotdrainedtothedepthofcroplandsystems,andthereforetheemissionfactorsareonly25percentofthecroplandvalues(Ogleetal.,2003).

Theactivitydatarequirementsvarybetweenmineralsoilsandorganicsoils.Mineralsoilsrequirethefollowingactivitydataforcroplands:

Areaoflandparcel(i.e.,field); Cropselectionandrotationsequence; Plantingandharvestingdates; Residuemanagement,includingamountharvested,burned,grazed,orleftinthefield; Irrigationmethod,applicationrate,andtimingofwaterapplications; Mineralfertilizertype,applicationrate,andtimingofapplication(s); Limeamendmenttype,applicationrate,andtimingofapplication(s); Organicamendmenttype,applicationrate,andtimingofapplication(s); Tillageimplements,datesofoperation,andnumberofpassesineachoperation(whichcan

beusedtodeterminetillageintensitywiththeSTIRModel(USDANRCS,2008)); Useofdrainagepracticesanddepthofdrainage(commonlyinhydricsoils);and Covercroptypes,planting,andharvestingdates(ifapplicable).

Themethodforgrazinglandonmineralsoilsrequiresthefollowingmanagementactivitydata:

Areaoflandparcel(i.e.,field); Plantspeciescomposition; Periodsofgrazingduringtheyear; Animaltype,class,andsizeusedforgrazing; Stockingratesandmethods; Irrigationmethod,applicationrate,andtimingofwaterapplications; Mineralfertilizertype,applicationrate,andtimingofapplication(s); Limeamendmenttype,applicationrate,andtimingofapplication(s); Organicamendmenttype,applicationrate,andtimingofapplication(s); Pasture/Range/Paddock(PRP)Nexcreteddirectlyontolandbylivestock(i.e.,manurethat

isnotmanaged);

Equation3‐6:ChangeinSoilOrganicCarbon StocksforOrganicSoils

ΔCOrganic=A×EF×CO2MW

Where:

ΔCOrganic=AnnualCO2emissionsfromdrainedorganicsoilsincropandgrazinglands (metrictonsCO2‐eqyear‐1)

A =Areaofdrainedorganicsoils(ha)

EF =Emissionfactor(metrictonsCha‐1year‐1)

CO2MW=RatioofmolecularweightofCO2toC(=44/12)(metrictonsCO2(metrictonsC)‐1)

Useofdrainagepracticesanddepthofdrainage(commonlyinhydricsoils); Levelofwoodyplantencroachment;and Totalyieldofcrop(metrictonsdrymattercropyieldyear‐1),meat(kgcarcassyieldyear‐1)

ormilk(kgfluidmilkyear‐1).

Longer‐termhistoryofsitemanagementwillbeusedtosimulateinitialsoilorganiccarbonstocksforthecroporgrazingsystem.Inordertoestimatetheinitialvalues,theentitywillneedtoprovidemanagementactivitydataforthepastthreedecades.Alistofmanagementsystemswillbeprovided.Theentitywillalsoprovidethepreviouslanduseandyearofconversionifaland‐usechangeoccurredduringthepastthreedecades.Historicaldataforactivityfrommorethanthreedecadesinthepastwillberepresentedbasedonnationalagriculturalstatisticsusingenterprisebudgetsandcensusdataforvariousregionsinthecountry.However,anentitycanprovidethelongertermhistoryifitisknown.Dataonthecarbonandnitrogencontentoforganicamendmentswillalsobeneededfromtheentity,althoughdefaultsareprovidedbelowiftheentitydoesnothavethisinformation.Pasture/Range/Paddock(PRP)manureNinputistheNexcreteddirectlyontolandbylivestock,andthemanureisnotcollectedormanaged(deKleinetal.,2006).TheamountofPRPmanureNisestimatedwiththelivestockmethods(SeeChapter5,Section5.3.2EntericFermentationandHousingEmissionsfromBeefProductionSystems)andassumedtobesplitwith50%oftheNinurineandtheother50%oftheNinsolids.

Table3‐7:NitrogenandCarbonFractionsofCommonOrganicFertilizers–MidpointandRange(PercentbyWeight)

OrganicFertilizer %Na %CPoultrymanure 2.25%(1.5‐3) 8.75%(7‐10.5)b

Pig,horse,cowmanure 0.45%(0.3‐0.6) 5.1%(3.4–6.8)c

Greenmanure 3.25%(1.5‐5) 42%d

Compost 1.25%(0.5‐2) 16%(12‐20)e

Seaweedmeal 2.5%(2‐3) 27%f

Sewagesludge 3%(1‐5) 11.7%(3.9‐19.5)b

Fishwaste 7%(4‐10) 24.3%(14.6‐34)g

Blood 11%(10‐12) 35.2%(32‐38.4)h

Humanurine/nightsoil 1.25%(1‐1.5) 9.5%(9‐10)iaHue,N.V.OrganicFertilizersinSustainableAgricultureRetrievedfromhttp://www.ctahr.hawaii.edu/huen/hue_organic.htm.bUSDA.1992.AgriculturalWasteCharacteristics.Chapter4.InAnimalWasteManagementFieldHandbook:NaturalResourcesConservationService,UnitedStatesDepartmentofAgriculture.cEPA,2013.InventoryofU.S.GreenhouseGasEmissionsandSinks:1990‐2011.WeightedU.S.averagecarbon:nitrogenratioformanureavailableforapplication.dAssumesdrymatteris42%carbon.eA1Organics.CompostClassification,SpecificationandResourceManual.http://www.a1organics.com/CLSP/CLASS%20MANUAL%20‐%20COLORADO.pdffhttp://www.naorganics.com/en/science_analysis.asp.NorthAtlanticOrganics.gHartz,T.K.andP.R.Johnstone.2006.Nitrogenavailablefromhigh‐nitrogen‐containingorganicfertilizers.HortTechnology16:39‐42.hSonon,D,etal.2012.Mineralizationofhigh‐Norganicfertilizers.ClemsonUniversity.iPolprasert,C.2007.OrganicWasteRecycling:TechnologyandManagement.IWAPublishing.

Themethodfororganicsoilsrequiresthefollowingactivitydataforcroplandsandgrazinglands:

Areaoflandparcel(i.e.,field);and Totalyieldofcrop(metrictonsdrymattercropyieldyear‐1),meat(kgcarcassyieldyear‐1)

ormilk(kgfluidmilkyear‐1).

Ancillarydataforthemineralsoilmethodincludehistoricalweatherpatternsandsoilcharacteristics.WeatherdatamaybebasedonnationaldatasetssuchastheParameter‐ElevationRegressionsonIndependentSlopesModel(PRISM)data(Dalyetal.,2008).SoilcharacteristicsmayalsobebasedonnationaldatasetssuchastheSoilSurveyGeographicDatabase(SSURGO)(SoilSurveyStaff,2011).However,therewillalsobeanoptionforentitiestosubstitutesoilsdatacollectedfromthespecificfield.Theerosionmodelwillalsorequireancillarydataontopography(i.e.,slope),lengthoffieldandroworientation,cropcanopyheight,diversions,surfaceresiduecover,andsoiltexture.

Noancillarydataareneededforthemethodtoestimateemissionsfromdrainageoforganicsoils.

3.5.3.5 ModelOutput

Modeloutputisgeneratedforthequantityofemissionsandemissionsintensity.Thechangeinmineralsoilorganiccarbonstocksisestimatedbasedonstockchangesoverfive‐yeartimeperiodsinordertomanageuncertainty.Uncertaintiesinthemodel‐basedestimatesareaboutthreetimeslargerforannualestimatesinchangeratecomparedwithfive‐yearblocks(CompareU.S.EnvironmentalProtectionAgency(2009)and(2010)).Uncertaintiesarelargeratthefinertimescalebecausethereislargevariabilityinmeasurementsofsoilcarbonstockchangesatannualtimescales,andthisvariabilityisincorporatedintothemodeluncertaintyusingtheempiricallybasedmethod(Ogleetal.,2007).Inaddition,trendsinsoilorganiccarbonwillbeestimatedforthe30previousyearsofhistoryandthereportingperiod.

Emissionsintensityisbasedontheamountofemissionsperunitofyieldforcropsincroplandsystemsoranimalproductsingrazingsystems.Theemissionsintensityisestimatedwiththefollowingequation:

Uncertaintiesinthemineralsoilmethodsincludeimprecisionandbiasintheprocess‐basedmodelparametersandalgorithms,inadditiontouncertaintiesintheactivityandancillarydata.Uncertaintyintheparameterizationandalgorithmswillbequantifiedwithanempiricallybasedapproach,asusedintheU.S.nationalGHGinventory(Ogleetal.,2007;U.S.EPA,2011).ThemethodcombinesmodelingandmeasurementstoprovideanestimateofSOCstockchangesforentityscalereporting(Conantetal.,2011).Measurementsofcarbonstockchangesareexpectedtobebasedon

Equation3‐7:EmissionsIntensityofSoilOrganicCarbon StockChange

EISoilC=(ΔCMineral+ΔCOrganic)/Y

Where:

EISoilC =Emissionsintensity(metrictonsCO2permetrictondrymattercropyield,metrictonsCO2perkgcarcassyield,metrictonsCO2perkgfluidmilkyield)

ΔCMineral =AnnualCO2equivalentemissionsfromsoilorganiccarbonchangeinmineralsoils(metrictonsCO2‐eqyear‐1)

ΔCOrganic =AnnualCO2equivalentemissionsfromsoilorganiccarbonchangeinorganicsoils,Histosols(metrictonsCO2‐eqyear‐1)

Y =Totalyieldofcrop(metrictonsdrymattercropyieldyear‐1),meat(kgcarcassyieldyear‐1)ormilkproduction(kgfluidmilkyieldyear‐1)

anationalsoilmonitoringnetwork(Spenceretal.,2011).Thenetworkshouldincludesamplesfromdifferentregionsofthecountryandsoiltypesthatareusedforcropproductionorgrazing,andarandomsamplingofthemanagementsystemsineachoftheregions.ThesamplingplotswillneedtobedesignedforresamplingovertimeinordertoevaluatethemodeledchangesinSOCstocks(Conantetal.,2003).Uncertaintiesinnationaldatasetsforweatherwillbebasedoninformationincludedwiththedataset,whileuncertaintiesintheSSURGOshouldbequantifiedusingtheunderlyingfielddatathatformthebasisforthemappingexercise,oranindependentaccuracyassessmentofthemapproduct.Otherinputdataisassumedtobeknownbytheentity,suchasthecropplants,yields,tillage,andresiduemanagementpractices.

Thelimitationsofthemineralsoilcarbonmethodincludenoassessmentoftheeffectoflanduseandmanagementinsub‐surfacelayersofthesoilprofile(below30centimeters),noassessmentofthelocationoftransportanddepositionoferodedC,andlimiteddatatoassessuncertaintyintheparametersandalgorithmsusingtheempiricallybasedmethod.Foragroforestry,theDAYCENTmodelhasbeenusedintheCOMET‐Farmvoluntarycarbonreportingtooltosimulatesoilorganiccarbonstockchanges.However,thereareseveralunknownswiththeuseoftheDAYCENTmodelforestimatingsoilorganiccarbonstockchangesinagroforestry,includingwhetherthemodelisabletotakeintoaccounttheinteractionsoccurringbetweenwoodyandherbaceousvegetationandrespectivemanagementactivities.OelbermannandVoroney(2011)evaluatedtheuseoftheCenturymodel,themonthlytime‐stepversionoftheDAYCENTmodel,topredictsoilorganiccarbonintemperateandtropicalalleycroppingsystemsthatwere13and19yearsold,respectively.Theyfoundthatthemodelunderestimatedthelevelsofsoilorganiccarboncomparedwithmeasuredvalues.Withmoretesting,themethodsmayberevisedinthefuturetousetheDAYCENTmodelforthepurposesofestimatingsoilorganiccarbonstockchangesinagroforestrysystems.

Biocharresearchhasbeenanareaofrapiddevelopmentoverthepastfewyears,buttherearestilluncertainties.Biocharisaproductofcombustedbiomassthathasavarietyofchemicalstructuresdependingonthebiomassandpyrolysismethod,andthevariationhasimplicationsforthestabilityofthecarboninthesoil(Spokas,2010).BiocharcanhaveconcomitantimpactsonemissionsofotherGHGssuchasCH4andN2O(Cayuelaetal.,2010;Malghanietal.,2013;Yuetal.,2013),althoughsomestudieshaveshownnoeffect(Caseetal.,2013;Cloughetal.,2010).SoilamendmentswithbiocharmayalsoprimethedecompositionofthenativesoilorganicmatteralthoughtheCO2emissionsfromprimingappeartobeconsiderablysmallerthanthecarbonaddedinthebiochar(Stewartetal.,2013;WoolfandLehmann,2012).Otherresearchsuggeststhattheremayevenbe“negative”primingleadingtoareductioninheterotrophicrespiration(Caseetal.,2013).Furthermore,thetemporaldurationoftheGHGmitigationpotentialofbiocharisalsouncertainbutappearstobeofashorttermnature(Spokas,2013).TheinfluenceofbiocharonemissionsandprimingneedsmoreresearchbeforethefulleffectofbiocharoncarbonsequestrationandGHGemissionscanbeincorporatedintomodelsandGHGreportingframeworks.Microbialdegradationofbiocharcanoccurovertimescalesrangingfromaslittleasafewdecadesto1000sofyears(Spokas,2010).Inthetechnicalmethods,biocharistreatedasahighcarbontolownitrogenamendmentintheDAYCENTmodelframework,butwithaconservativeresidencetimeofthecarbonfromdecadestoacentury.Thesemethodscanbefurtherrefinedinthefutureasthedifferenttypesandresidencetimesofbiochararefurtherresolved.

Themethodfororganicsoilsalsohaslimitations,particularlytheinabilitytoestimatetheeffectofmitigationmeasuressuchaswatertablemanagementbecauseemissionfactorsaresetforeachclimateregion(i.e.,currentlyscalingfactorsarenotavailabletorevisetheemissionfactorsforwatertablemanagement).Onlycompleterestorationofthewetlandwithnofurtherdrainagecanbeaddressedwiththemethod(i.e.,assumesnofurtheremissionsofCO2).However,ifcrop

productionismaintainedonthelandparcel,themostpracticalmethodforreducingemissionsistoraisethewatertabletoneartherootingdepthofthecropduringthegrowingseasonandthennotdrainingthesoilduringthenon‐growingseason(Jongedyketal.,1950;Shihetal.,1998),orpossiblymanagingthesystemwithperiodicflooding(Morrisetal.,2004).

Forallsystemsthereisadditionaluncertaintyassociatedwithclimatechange.Modeledoutputforanygivenlocationassumestemperatureandprecipitationsimilartothatofthepast30years,theperiodforwhichhistoricalweatherisusedtosimulatesoilorganiccarbondynamics.Expectedchangesintemperature,precipitation,andextremeeventssuchasdroughts,floods,andheatwaveswilladdfurtheruncertaintytoestimatesofsoilorganiccarbonstockchange.

WhilethereisconsiderableevidenceandmechanisticunderstandingabouttheinfluenceoflanduseandmanagementonSOC,thereislessknownabouttheeffectonsoilinorganicC.Consequently,thereisuncertaintyassociatedwithlanduseandmanagementimpactsonsoilinorganiccarbonstocks,whichcannotbequantified.CurrentmethodsdonotincludeimpactsoninorganicC,butthismaybeaddedinthefutureasmorestudiesareconductedandmethodsaredeveloped.

UncertaintiesinmodelparametersandstructurearecombinedusingaMonteCarlosimulationapproach.Uncertaintyisassumedtobeminorforthemanagementactivitydataprovidedbytheentity.Table3‐8providestheprobabilitydistributionfunctionsassociatedwiththemineralandorganicsoilsmethods.

Table3‐8:AvailableUncertaintyDataforSoilOrganicCarbonStockChange

Parameter Mean Units

RelativeUncertainty

Distribution DataSourceLow(%)

High(%)

DAYCENT(empiricaluncertainty) NS Various NS NS Normal

Ogleetal.(2007);EPA(2013)

Emissionfactorforcroplandincooltemperateregions 11

metrictonsCha‐1year‐1 45 45 Normal

Ogleetal.(2003)

Emissionfactorforcroplandinwarmtemperateregions 14

Ogleetal.(2003)

Emissionfactorforcroplandinsubtropicalregions 14

Ogleetal.(2003)

Emissionfactorforgrazinglandincooltemperateregions

2.8metrictonsCha‐1year‐1

45 45 NormalOgleetal.(2003)

Emissionfactorforgrazinglandinwarmtemperateregions

Emissionfactorforgrazinglandinsubtropicalregions

NS=NotShown.Dataarenotshownforparametersthathave100’sto1000’sofvalues(denotedasNS).

3.5.4 SoilNitrousOxide

N2Ofluxesarenotoriouslydifficulttomeasurebecauseofthelaborrequiredtosampleemissions,combinedwithhighspatialandtemporalvariability.AgronomicpracticesthataffectN2Ofluxesinonesoil,climate,orsite‐yearmayhavelittleornomeasurableeffectinothers.Consequently,considerablecareisrequiredtoensurethatmethodstoestimatechangesinemissionsforaparticularcroppingpracticeareaccurateandrobustforthegeographicregionforwhichtheyareproposed,oraresufficientlygeneralizabletobeaccurateinaggregate.

DeKleinetal.(2006)providethreeestimationstrategiesfordirectN2Oemissionsfromcropland.Twoarebasedonemissionfactors,theproportionofnitrogenaddedtoacropthatbecomesN2O.Tier1isbasedonanear‐universalemissionfactor,applicablegloballywithoutregardtogeography,croppingpractice,orfertilizerplacement,timing,orformulation.Tier2methodsutilize

MethodforEstimatingSoilDirectN2OEmissions

MineralSoils

Themethodisbasedonusingresultsfromprocess‐basedmodelsandmeasuredN2OemissionsincombinationwithscalingfactorsbasedonU.S.specificempiricaldataonaseasonaltimescale.

Process‐basedmodeling(anensembleapproachusingDAYCENTandDNDC)combinedwithfielddataanalysisareusedtoderivebaseemissionratesforthemajorcroppingsystemsanddominantsoiltextureclassesineachUSDALandResourceRegion.Incaseswherethereareinsufficientempiricaldatatoderiveabaseemissionrate,thebaseemissionrateisbasedontheIPCCdefaultfactor.Thebaseemissionfactorsareadjustedbyscalingfactorsrelatedtospecificcropmanagementpracticesthatarederivedfromexperimentaldata.

OrganicSoils

DirectN2OemissionsfromdrainageoforganicsoilsusestheIPCCequationsdevelopedindeKleinetal.,(2006).Themethodfororganicsoilsassumesthatthereisstillasignificantorganichorizoninthesoil,andtherefore,therearesubstantialinputsofnitrogenfromoxidationoforganicmatter.

TheemissionratefordrainedorganicsoilsisbasedonIPCCTier1emissionfactor(0.008metrictonsN2O‐Nha‐1year‐1).

Thismethodreliesonentityspecificactivitydataasinputintotheequations.

MethodforEstimatingSoilIndirectN2OEmissions

ThismethodusestheIPCCequationforindirectsoilN2O(deKleinetal.,2006). IPCCdefaultsareusedforestimatingtheproportionofnitrogenthatissubjectto

leaching,runoff,andvolatilization.Inlandparcelswheretheprecipitationplusirrigationwaterinputislessthan80percentofthepotentialevapotranspiration,nitrogenleachingandrunoffareconsiderednegligibleandnoindirectN2Oemissionsareestimatedfromleachingandrunoff.

Thismethodusesentityspecificseasonaldataonnitrogenmanagementpractices.

geographic,crop,orpractice‐specificemissionfactorswherefieldtestsshowthatafactordifferentfromtheonepercentTier1factoriswarranted.AtpresentthereisonlyoneTier2exampleintheprimaryliteraturethatisspecifictoconditionsintheUnitedStates,anditisforcornintheNorthCentralregion(Millaretal.,2010).ThismethodhasbeenincorporatedintoseveralN2Oreductionprotocols(VerifiedCarbonStandard,AmericanCarbonRegistry,andClimateActionReserve).ThethirdoptionforestimatingdirectN2Oemissions,orTier3,isameasurementorprocess‐basedmodelingapproach.Inthiscase,emissionsaremonitoredspecificallyfortheentity’sfieldbydeployinginstrumentsinameasurementsystemorbygatheringtheinformationspecifictothefieldconditionstosimulateN2Oemissionswithaprocess‐basedmodel.Thisthirdoptionisthemostprecise,butrequiresmoreresourcesandsufficienttestingpriortoimplementation.

InSection3.2.1.1,severalpracticesarediscussedthathavebeenshowntoreduceN2Oemissionsinfieldexperiments.However,manyoftheexperimentshavebeenconductedforalimitednumberofspecificcroppingsystemsandregions.Consequently,therearenomitigationpracticesforwhichemissionreductionshavebeenquantifiedunderallconditionsintheUnitedStates.Nevertheless,formanypracticesthereissufficientknowledgeatthecroppingsystemandregionallevelstoestablishthatadoptionwillreducesoilN2Oemissions.

Process‐basedsimulationmodelsuseknowledgeofC,N,andwaterprocesses(amongothers)topredictecosystemresponsestoclimateandotherenvironmentalfactors,includingcropandgrazinglandmanagement(seesoilcarbonmethodologyinSection3.5.3).N2Ofluxescanbepredictedusingsimulationmodels(Chenetal.,2008;DelGrossoetal.,2010).Akeyadvantageofsimulationmodelsisthattheyaregeneralizabletoawidevarietyofsoils,climates,andcroppingsystems,allowingfactorstointeractincomplexwaysthatmaybedifficulttopredictwithlesssophisticatedapproaches.However,adisadvantageisthatcomplexitycanlimittheirtransparency,andatpresenttherearestillsubstantialdatagapsthatlimitourabilitytofullytestavailablemodelsfortheirsensitivitytodifferentmanagementpracticesacrossvariousregionsandcropsintheUnitedStates.

Toovercomethesechallenges,ahybridapproachthatutilizesprocess‐basedsimulationmodelsandfielddatawasdevelopedtoestimateN2Oemissions.Themethodusesabaseemissionrateassociatedwiththetypicalamountofnitrogenapplied,andthenadjustmentsareappliedviascalingfactorstoaccountformanagementpracticesthataffectN2Oemissions.ThisapproachisaTier3methodasdefinedbytheIPCC.

Baseemissionratesareestimatedforeachdominantcropandthreesoiltextureclasses(coarse,medium,fine)withinaclimaticregionusingprocess‐basedsimulationmodeling.ThefactorsaredevelopedatthescaleofUSDALandResourceRegions(LRR).FielddataindicatethatN2Oemissionsgenerallyincreaseastheamountofappliednitrogenincreases,especiallywhennitrogenapplicationratesexceedcropuptakerates(Hobenetal.,2011;Kimetal.,2013;McSwineyandRobertson,2005;Shcherbaketal.,inpress)Researchdatafromfieldexperimentswerecompiledandusedtoadjusttheemissionratesfornitrogenfertilizerapplicationratesthatexceededthetypicalnitrogenapplicationrateforthecropinalandresourceregion.Forcropswheresufficientdataarenotavailabletosimulatethebaseemissionratewithaprocess‐basedmodel,thestandardIPCCTier1emissionfactorisapplied.Inaddition,forlandparcelsthathaveamixofcropswhereonlysomecanbesimulated,thestandardIPCCTier1approachshouldalsobeapplied.

Emissionsareaffectedbyspecificfarmmanagementpracticessuchasreducingtillageintensity;addingnitrificationinhibitors,orchanginghow,whenandwherenitrogenfertilizersareapplied.ToaccountfortheeffectofmanagementpracticesonN2Oemission,scalingfactorsweredevelopedtoadjustthebaseemissionrates.Thescalingfactorswereestimatedfromavailableresearchdata(SeeAppendix3‐Aformoreinformation).Managementpracticesotherthanthoseincludedinthe

equationmayalsomitigateN2Oemissions,buttherearenotcurrentlysufficientdatatocreategeneralizedscalingfactors.Additionaldatamayleadtotheirinclusioninfutureupdatestothemethod.

ThismethodincorporatesmoreinformationthanamethodbasedsolelyontheIPCCmodel.Itprovidesatransparentandscience‐basedmeansofestimatingannualizedN2Oemissionsfromcropandgrazinglands,anditfacilitatestheestimationofuncertainty.ForN2Oemissionsfromcropandgrazinglands,anIPCCTier1approachisonlysensitivetonitrogenapplicationrate,andthereforedoesnotreflectthefullsuiteoffactorsthatareknowntoinfluenceN2Oemissionsincludingclimate,soils,crops,andmanagementpracticesthatrangefromtillagetocovercropstofertilizertiming,placement,formulation,andadditives.DynamicprocessmodelsasembodiedintheIPCCTier3approachcan,inconcept,accountformostofthesefactorsbuttodatehavenotbeensufficientlyevaluatedformanyU.S.locations,crops,andmanagementpractices.Thisreporttakesahybridapproachthatrepresentsthebestavailablescienceatthetimeofpublication:dynamicprocessmodelstoestimatebaselineN2Oemissionsforthosecropsandlocationssufficientlyevaluated,thenscaledbymanagementpracticestotheextentsupportedbyavailableresearchresults.InitialtestingindicatesthatthismethodismoresensitivetoU.S.nutrientmanagementpracticesthantheIPCCTier1approach.Theauthorsanticipatepublicationofanaddendumthatwillprovidetestresultsandsuggestfurthertuningofthemethod.Overtime,asdynamicprocessmodelsarefurtherdevelopedandtested.ThemethodwilllikelymigratetowardsanexclusiveTier3approachtobetteraccountformanagementeffectsgiventhelocalvariablesandconditions.Intheinterim,inadditiontoprovidingbest‐availableandreliableestimatesofN2Oemissionsfromcropandgrazinglands,themethodoutlinedhereisexpectedtosetaresearchagendathatprovidesforbroaderevaluationofenvironmentalconditionsandmanagementpracticesinfluencingN2Oemissionsaswellasfurtherdevelopmentofmodelstomoreaccuratelyestimateemissions.

OffsiteorindirectN2Oemissions,whichoccurwhenreactivenitrogenescapestodownwindordownstreamecosystemswherefavorableconditionsforN2Oproductionexist,areevenmoredifficulttoestimatethandirectemissionsbecausethereisuncertaintyinboththeamountofreactivenitrogenthatescapesandtheportionofthisnitrogenthatisconvertedtoN2O.Ideally,fluxesofvolatileandsolublereactivenitrogenleavingtheentity’sparceloflandwouldbecombinedwithatmospherictransportandhydrologicmodelstosimulatethefateofreactiveN.Atpresenttherearenolinkedmodelingapproachessufficientlytestedtobeusedinanoperationalframework.Consequently,theindirectN2OemissionsarebasedontheIPCCTier1method(deKleinetal.,2006).

Similarly,directN2OemissionsfromdrainageoforganicsoilsarebasedontheIPCCTier1methods(deKleinetal.,2006).AlthoughresearchisongoingtoprovideimprovedemissionfactorsandmethodsforestimatingN2Oemissionsfromdrainageoforganicsoils(Allen,2012),moretestingwillbeneededbeforeincorporatingthemintoanoperationalmethod.Futurerevisionstothesemethodswillneedtoconsideradvancementsandrevisethemethodsaccordingly.

N2Oisemittedfromcroplandbothdirectlyandindirectly.Directemissionsarefluxesfromcroplandorgrazinglandswheretherearenitrogenadditionsornitrogenmineralizedfromsoilorganicmatter.IndirectemissionsoccurwhenreactivenitrogenisvolatilizedasNH3orNOxortransportedviasurfacerunofforleachinginsolubleformsfromcroplandorgrazinglands,leadingtoN2Oemissionsinanotherlocation.

DirectN2OEmissions

MineralSoils:TotaldirectN2OemissionsfrommineralsoilsareestimatedforalandparcelusingEquation3‐8.

Thepractice‐scaledemissionratefortheparcelofland(ERp)isestimatedusingEquation3‐9.

Equation3‐8:DirectSoilN2OEmissionsfromMineralSoils

N2ODirect=ERp×A×N2OMW×N2OGWP

Where:

N2ODirect=TotaldirectsoilN2Oemissionforparcelofland(metrictonsCO2‐eqyear‐1)

ERp =Practice‐scaledemissionrateforlandparcel(metrictonsN2O‐Nha‐1year‐1)

A =Areaofparcelofland(ha)

N2OMW =RatioofmolecularweightsofN2OtoN2O‐N =44/28(metrictonsN2O(metrictonsN2O‐N)‐1)

N2OGWP =GlobalwarmingpotentialforN2O(metrictonsCO2‐eq(metrictonsN2O)‐1)

aAdifferencearisesintheERbestimationofPRPmanureNinputandtheactualPRPmanureNinputbecauseatypicalrateofNinputwasassumedintheDAYCENTandDNDCsimulationsfortheERbcalculation(SeeTextbox3‐1andAppendix3‐A).bEmissionfactorsfromdeKleinetal.(2006).

Inthisequation,thebaseemissionrate(ERb)variesbytheamountofnitrogeninputtothesoil.TheratemayalsovaryfordifferentcropandgrazinglandsystemsbyLRRtocapturevariationinclimate,andbytextureclassinordertorepresenttheinfluenceofsoilheterogeneityonN2Oemissions.MoreinformationaboutbaseemissionratesisgiveninTextbox3‐1.

Practice‐basedemissionscalingfactors(0to1)areusedtoadjusttheportionoftheemissionrateassociatedwithslowreleasefertilizers(Ssr),nitrificationinhibitors(Sinh),andpasture/range/paddock(PRP)manurenitrogenadditions(Sprp,cps).Theslow‐releasefertilizer,

Equation3‐9:Practice‐Scaled SoilN2OEmissionRateforMineralSoils

ERp=[ERb+(ΔNprp*EFprp)]x{1+[Ssrx(Nsr/Ni)]}x{1+[Sinhx(Ninb/Ni)]}x(1+Still)x{1–[Nresidr/(Ni+Nresidr)]}

Where:

ERp =Practice‐scaledemissionrateforlandparcel(metrictonsN2O‐Nha‐1year‐1)

ERb =Baseemissionrateforcroporgrazinglandthatvariesbasedonnitrogeninputratefrommineralfertilizer,organicamendments,residues,andadditionalmineralizationwithland‐usechangeortillagechange

(metrictonsN2O‐Nha‐1year‐1)

ΔNprp =DifferenceinPRPmanureNexcretionabetweenthePRPmanureNexcretionbasedonentityactivitydata(NPRPe)andPRPmanureNexcretionforthebaseemissionrate(NPRPb)(metrictonsN)

=NPRPe‐NPRPb

EFprp =EmissionfactorforPRPmanureNinputtosoils,0.02metrictonsN2O‐Nha‐1year‐1(metrictonsN)‐1forcattle,poultryandswine,and0.01metrictonsN2O‐N(metrictonsN)‐1forotherlivestockb

Ni =Nitrogeninputs,includingmineralfertilizer,organicamendments,PRPmanureN,residues,andSOMmineralization(SeeEquation3‐11)

(metrictonsNha‐1year‐1)Ssr =Scalingfactorforslow‐releasefertilizers,0wherenoeffect(dimensionless)

Nsr =Nitrogeninslow‐releasenitrogenfertilizerappliedtotheparcelofland (metrictonsNha‐1year‐1)

Sinh =Scalingfactorfornitrificationinhibitors,0wherenoeffect(dimensionless)

Ninh =Nitrogeninnitrogenfertilizerwithinhibitorappliedtotheparcelofland (metrictonsNha‐1year‐1)

Still =Scalingfactorforno‐tillage,0exceptforNT(dimensionless)

Nresidr =Nremovedthroughcollection,grazing,harvestingorburningofabovegroundresidues(metrictonsNha‐1year‐1).EstimateusingEquation3‐10forresultsgeneratedwithDAYCENTandDNDCmodelswiththeexceptionofhaycrops.NocalculationisneededforresultsgeneratedbytheIPCCmethodorforresultsassociatedwithhaycropsgeneratedbyDAYCENTandDNDC(setvalueto0).

nitrificationinhibitorandPRPmanurescalingfactorsareweightedsothattheireffectisonlyontheamountofnitrogeninfluencedbythesepracticesrelativetotheentirepoolofnitrogen(i.e.,theamountofslow‐releasefertilizer,fertilizerwithnitrificationinhibitororPRPmanurenitrogenaddedtothesoil).Incontrast,scalingfactorsfortillage(Still)areusedtoscaletheentireemissionrateundertheassumptionthatthispracticeinfluencestheentirepoolofmineralnitrogeninputs(i.e.,Ni).

Table3‐9:ScalingFactorsforNitrogenManagementPractices

ManagementPracticeNitrogen Management

FactorFactor(ProportionalChangeinEmissions) Source

Slow‐releasefertilizeruse Ssr ‐0.21(‐0.12to‐0.30) SeeAppendix3‐AManurenitrogendirectlydepositedonpasture/range/paddock

Sprp,cps +0.5(0.33to0.67) IPCC(2006)

Nitrificationinhibitoruse Sinh–semiarid/aridclimate ‐0.38(‐0.21to‐0.51) SeeAppendix3‐ANitrificationinhibitoruse Sinh–mesic/wetclimate ‐0.40(‐0.24to‐0.52) SeeAppendix3‐A

TillageStill–semiarid/aridclimate(<10yearsfollowingno‐till

adoption)0.38(0.04to0.72)

vanKesseletal.(2012),Sixetal.(2004)

TillageStill–semiarid/aridclimate(≥10yearsfollowingno‐till

adoption)‐0.33(‐0.16to‐0.5)

Equation3‐10:AbovegroundResidueN Removal

ForCrops:

Nresidr=[((Ydm/HI)–Ydm)xRr)xNa]

ForGrazingForage:

Nresidr=[Ydmx(Fr+Rr)xNa]

Where:

Nresidr=Nremovedthroughcollection,grazing,harvestingorburningofabovegroundresidues(metrictonsNha‐1year‐1)

Ydm =Cropharvestorforageyield,correctedformoisturecontent (metrictonsbiomassha‐1year‐1) =YxDM

Y =Cropharvestortotalforageyield(metrictonsbiomassha‐1year‐1)

DM =Drymattercontentofharvestedbiomass(dimensionless)

Fr =Proportionofliveforageremovedbygrazinganimals(dimensionless)

Rr =Proportionofcrop/forageresidueremovedduetoharvest,burningorgrazing(dimensionless)

Na =Nitrogenfractionofabovegroundresiduebiomassforthecroporforage (metrictonsN(metrictonsbiomass)‐1)

ManagementPracticeNitrogen Management

FactorFactor(ProportionalChangeinEmissions)

Source

TillageStill–mesic/wetclimate

(<10yearsfollowingno‐tilladoption)

‐0.015(‐0.16to0.16)vanKesseletal.(2012),

Sixetal.(2004)

TillageStill–mesic/wetclimate

(≥10yearsfollowingno‐tilladoption)

‐0.09(‐0.19to0.01) vanKesseletal.(2012),Sixetal.(2004)

Note:SeeAppendix3‐AforfurtherexplanationonthepracticesincludedinthesoilN2Omethodandthesourcesofdatathatwereusedtoderivethebaseemissionratesandscalingfactorsforthemanagementpractices.

Textbox3‐1:BaseEmissionRateforDirectSoilN2OEmissionsfromMineralSoils

Thebaseemissionrateisacroporgrazinglandspecificestimatethatvariesbasedonthetotalmineralnitrogeninputtothesoil.Therearetwomethodsusedtoestimatethebaseemissionrate.Thefirstmethodusesacombinationofprocess‐basedmodelingandmeasurementdatatoestimatesN2Obaseemissionratesbylandresourceregion,majorcroptype,andsoiltextureclass.ThesecondmethodusesthedefaultIPCCemissionfactorofonepercent(deKleinetal.,2006),multiplyingthisvaluebythetotalnitrogeninput(SeeEquation3‐11)toestimatethebaseemissionrate.Thesecondapproachisusedforcropsthatarenotincludedintheprocess‐basedmodelinganalysis.

Theremainderofthisboxdescribesthefirstmethod.Theequationforthefirstmethod,combiningthemodelingandmeasurementdata,isgivenbelow:

ERb=ER0+(EFtypical+(SEF×ΔNf))×Nf

ERb =Baseemissionrate(metrictonsN2O‐Nha‐1year‐1)

ER0 =Emissionratemodeledat0levelofnitrogeninput(Nt=0) (metrictonsN2O‐Nha‐1year‐1)

EFtypical =Emissionfactorforthetypicalfertilizationrate(metrictonsN2O‐N(metrictonsN)‐1) =(ERtypical–ER0)/Ntf

ERtypical=Emissionrateforthetypicalcasemodeled(metrictonsN2O‐Nha‐1year‐1)

SEF =BaseEFscalar; forΔNf>zero:SEF=0.0274forallnon‐grasslandcrops,

SEF=0.117forgrasslands; forΔNf<zero(lessthanorthesameastypicalfertilizerrates):SEF=0; ((metrictonsN2O‐N(metrictonsN)‐2)hayear)

ΔNf =Nf‐Ntf(metrictonsNha‐1year‐1)

Nf =Actualnitrogenfertilizerrate,includingsyntheticandorganic(metrictonsNha‐1year‐1)

Ntf =Typicalnitrogenfertilizerrate(metrictonsNha‐1year‐1)

Process‐basedmodelswereusedtosimulateN2OemissionsatthetypicalnitrogenfertilizationrateformajorcommoditycropsaccordingtotheUSDAAgriculturalResourceManagementSurveydata(ERtypical),inadditiontoazerorateapplication(ER0).TheN2Oemissionatthetypicalrateoffertilizationformajorcommoditycropsareproducedforcoarse,medium,andfinetexturedsoilsineachlandresourceregion.Theemissionfactor(EFtypical)forfertilizationratesgreaterthanthetypicalrateforthecroporgrassarescaledaccordingtothetrendinmeasuredsoilN2Odataacrossarangeoffertilizationratesbasedonexperimentaldata.ThechangeintheemissionfactorbetweenthetypicalnitrogenfertilizationrateandahigherratewasaveragedtoderiveanemissionfactorscalarorrateofchangeperunitofadditionalN.Thescalarismultipliedbytheadditionalnitrogentoderiveanadjustmenttotheemissionfactor(SEF×ΔNf)thatisthenaddedtotheemissionfactorderivedforthetypicalfertilizerrate(EFtypical).NoscalingisdoneforthecasewhereΔNf≤zero,i.e.,wherethefertilizationrateisequaltoorlessthanthetypicalrateofnitrogenapplication.InthiscaseSEF=0suchthatSEF×ΔNf=0.Theresultingemissionfactorismultipliedbytheactualfertilizerrate(Nf)andaddedtotheemissionrateatthe0levelofnitrogenfertilization(ER0)toderivethebaseemissionrate(ERb).

Nitrogeninputsareestimatedwiththefollowingequation:

aTheapproachforestimatingnitrogenmineralizationinputsisconsistentwiththeU.S.NationalInventory(U.S.EPA,2013).bPasture/Range/Paddock(PRP)manureNisatermutilizedbytheIPCC(deKleinetal.,2006)fortheNexcreteddirectlyontolandbylivestock,andthemanureisnotcollectedormanaged.ThetotalPRPmanureNisestimatedwithmethodsfromChapter5,andassumedtobesplitwith50%oftheNinurineand50%oftheNinsolids.

ThetotalNmineralizationisestimatedfromtheDAYCENTmineralsoilCmethodinaggregateformanureamendments(Nman),compost(Ncom),residues(Nres),soilorganicmatter(Nsmin)andsolidsassociatedwithPRPmanure,andisusedtoapproximatetheseNinputsinEquation3‐11.ThisapproachcreatesalinkagebetweenthemineralsoilCmethod(SeeSection3.5.3.2)andtheN2Omethod,ensuringconsistencyintreatmentofN.IninstanceswherecropscannotbeestimatedbytheDAYCENTmineralsoilCmethod,themethodfromtheIPCCguidelines(Aaldeetal.,2006)canbeusedtoestimatetheNinputsfrommineralizationwiththeexceptionofNsmin,whichissetto0(SeeAppendix3‐B).

OrganicSoils:ThemethodfororganicsoilsincludesHistosolsandsoilsthathavehighorganicmattercontentanddevelopedundersaturated,anaerobicconditionsforatleastpartoftheyear,whichincludesHistels,Historthels,Histoturbels.Themethodassumesthatthereisasignificantorganichorizoninthesoil,andtherefore,majorinputsofnitrogenarefromoxidationoforganicmatterratherthanexternalinputsfromsyntheticandorganicfertilizers.Iftheseassumptionsarenottrue,thentheentityshouldusethemineralsoilmethodtoestimatetheN2Oemissions.TotaldirectN2Oemissionsfromdrainedorganicsoilsareestimatedforindividualparcelsofland(i.e.,fields)withthefollowingequation:

Equation3‐11:NitrogenInputsa

Ni=Nsfert+Nman+Ncomp+Nresid+Nsmin+Nprp

Where:

Ni =Nitrogeninputs,includingmineralfertilizer,organicamendments,PRPmanureN,residues,andSOMmineralization

(metrictonsNha‐1year‐1)

Nsfert =Nitrogeninsyntheticfertilizerappliedtoaparcelofland (metrictonsNha‐1year‐1)

Nman =Nitrogenmineralizationfrommanureamendments(orsewagesludge)appliedtoaparcelofland(metrictonsNha‐1year‐1)

Ncomp =Nitrogenmineralizationfromcompostappliedtoaparcelofland (metrictonsNha‐1year‐1)

Nresid =Nitrogenmineralizationfromcropandcovercropresiduesaboveandbelowgroundthatareleftontheparceloflandfollowingsenescence(i.e.,notcollected,grazed,orburned)(metrictonsNha‐1year‐1)

Nsmin=NitrogeninputsfromsoilorganicmattermineralizationasestimatedbytheDAYCENTmineralsoilCmethod(SeeSection3.5.3.2)(metrictonsNha‐1year‐1).Valuesetto0forcropsthatarenotestimatedwiththeDAYCENTmineralsoilCmethod.

Nprp =Nitrogeninurineandmineralizationfromsolidsassociatedwithmanureinpasture/range/paddock(PRP)(metrictonsNha‐1year‐1)b

IndirectN2OEmissions:ThemethodtoestimateindirectN2OemissionsformineralsoilshasbeenadoptedfromtheapproachdevelopedbyIPCC(deKleinetal.,2006).ThefollowingequationisusedtoestimatethetotalindirectN2Oemissionsassociatedwithnitrogenvolatilizationandnitrogenleachingandrunofffromthelandparcel:

Thefollowingequationisusedtoestimatetheindirectemissionsassociatedwithnitrogenvolatilizationfromthelandparcel:

Equation3‐12:DirectN2OEmissionsfromDrainageofOrganicSoils(Histosols)

N2OORGANIC=AOS×EROS

Where:

N2OORGANIC =DirectsoilN2Oemissionfromdrainageoforganicsoils (metrictonsN2O‐Nyear‐1)

Aos =Areaoforganicsoilsdrainedonaparcelofland(ha)

EROS =EmissionrateforcroppedHistosols, IPCCTier1EROS=0.008metrictonsN2O‐Nha‐1year‐1

Equation3‐13:TotalIndirectSoilN2OEmissionsfromMineralSoils

N2OIndirect=(N2OVol+N2OLeach)×N2OMW×N2OGWP

Where:

N2OIndirect=IndirectsoilN2Oemission(metrictonsCO2‐eqyear‐1)

N2OVol =N2Oemittedbyecosystemreceivingvolatilizednitrogen (metrictonsN2O‐Nyear‐1)

N2OLeach =N2Oemittedbyecosystemreceivingleachedandrunoffnitrogen (metrictonsN2O‐Nyear‐1)

N2OMW =RatioofmolecularweightsofN2OtoN2O‐N=44/28 (metrictonsN2O(metrictonsN2O‐N)‐1)

N2OGWP =GlobalwarmingpotentialforN2O(metrictonsCO2‐eq(metrictonsN2O)‐1)

TheIPCCdefaultsareusedforFRSNandFRON.

ThefollowingequationisusedtoestimatetheindirectN2Oemissionsassociatedwithleachingoroverlandflowofreactivenitrogenthatistransportedfromthelandparcel(i.e.,field):

Thefractionofnitrogenthatisleachedfromaprofilewillvarydependingonthelevelofprecipitationandirrigationwaterappliedinthefield.Inlandparcels(i.e.,fields)wheretheprecipitationplusirrigationwaterinputislessthan80percentofthepotentialevapotranspiration,nitrogenleachingandrunoffareconsiderednegligibleandnoindirectN2Oemissionsareestimated(U.S.EPA,2011).IPCCdefaultfractionsareusedforEFleachandFRleachwherenocovercropsarepresent.Wherewintercovercropsprecedethecashcrop,FRleachisfurtheradjustedtoaccountforcovercropeffectsonnitrateleaching.Inameta‐analysisof36geographicallydistributedfield

Equation3‐14:IndirectSoilN2OEmissionsfromMineralSoils—Volatilization

N2OVol=[(FSN×FRSN)+(FON×FRON)]×EFVOL

Where:

N2OVol =IndirectsoilN2Oemittedbyecosystemreceivingvolatilizednitrogen (metrictonsN2O‐Nyear‐1)

FSN =Syntheticnitrogenfertilizerapplied(metrictonsNyear‐1)

FRSN =FractionofNSNthatvolatilizesasNH3andNOx.IPCCdefaultTier1=0.10 (metrictonsN(metrictonNsfert)‐1)

FON =Nitrogenfertilizerappliedoforganicoriginincludingmanure,sewagesludge,compostandotherorganicamendments(metrictonsNyear‐1)

FRON =FractionorproportionofFONthatvolatilizesasNH3andNOx.IPCCdefaultTier1=0.2(metrictonsN(metrictonNON)‐1)

EFVOL =EmissionfactorforvolatilizednitrogenorproportionofnitrogenvolatilizedasNH3andNOxthatistransformedtoN2Oinreceivingecosystem;IPCCTier1EF=0.01(metrictonsN2O‐N(metrictonN)‐1)

Equation3‐15:IndirectSoilN2OEmissionsfromMineralSoils—LeachingandRunoff

N2Oleach=(Ni×FRleach)×EFleach

Where:

N2Oleach =IndirectsoilN2Oemittedbyecosystemreceivingleachedandrunoffnitrogen(metrictonsN2O‐Nyear‐1)

Ni =Nitrogeninputs,includingmineralfertilizer,organicamendments,PRPmanureN,residues,andSOMmineralization(metrictonsNha‐1year‐1)(SeeEquation3‐11)

FRleach =FractionorproportionofNithatleachesorrunsoff.IPCCdefaultTier1=0.30excepta)whereirrigation+precipitationislessthan80%ofpotentialevapotranspiration(metrictonsN(metrictonN)‐1)FRleach=0;andb)croppingsystemswithleguminousornon‐leguminouswintercovercrops,forleguminouscovercrops,FRleach=0.18,andfornon‐leguminouscovercrops,FRleach=0.09.

EFleach =EmissionfactorforleachedandrunoffnitrogenorproportionofleachedandrunoffnitrogenthatistransformedtoN2Oinreceivingecosystem;IPCCTier1EF

=0.0075(metrictonsN2O‐N(metrictonN)‐1)

studies,Tonittoetal.(2006)founda40percentand70percentreductioninnitrateleachingwiththeuseoflegumeandnon‐legumecovercrops,respectively.Accordingly,FRleach,isreducedto0.18forlegumecovercrops(0.3×(1‐0.4);or18%oftotalnitrogeninputs)and0.09fornon‐legumecovercrops(0.3×(1‐0.7);orninepercentoftotalnitrogeninputs).

Calculatingemissionsrequiresthefollowingactivitydataforcroplands:

Areaoflandparcel(i.e.,field); Prior‐yearcroptype,drymatteryields,andresidue‐yieldratiostocalculatecropresidue

nitrogeninput,includingcovercrop(ifpresent); Residuemanagement,includingamountharvested,burned,grazed,orleftinthefield; Syntheticfertilizertype(chemicalformulation)andcoatings(ifpresent); Syntheticandorganicfertilizerapplicationrate,applicationmethod(broadcast,banded,or

injected,includingdepthofinjection),timingofapplication(s); Typeofnitrificationinhibitorapplications(ifused); Tillageimplements,datesofoperation,andnumberofpassesineachoperation(whichcan

beusedtodeterminetillageintensitywiththeSTIRModel),(USDANRCS,2008); Irrigationmethod,applicationrateandtimingofapplications; Totaldrymatteryieldofcrop(metrictonsdrymatteryear‐1),drymattercontentofyield,

andharvestindex;and Covercroptypes,planting,andharvestingdates(ifapplicable).

Themethodforgrazinglandrequiresthefollowingmanagementactivitydata:

Areaoflandparcel(i.e.,field); Prior‐yeargrasstypeanddrymatterproductiontocalculategrassnitrogeninput; Syntheticfertilizertype(chemicalformulation)andcoatings(ifpresent); Organicamendmenttypesandtiming; Syntheticandorganicamendmentapplicationrate,applicationmethod(broadcast,banded,

orinjected,includingdepthofinjection),timingofapplication(s); Pasture/range/paddock(PRP)Nexcreteddirectlyontolandbylivestock(i.e.,manurethat

isnotmanaged); Typeofnitrificationinhibitorapplications(ifused); Tillageimplements,datesofoperation,andnumberofpassesineachoperationwhichcan

beusedtodeterminetillageintensitywiththeSTIRModel,(USDANRCS,2008); Irrigationmethod,applicationrate,andtimingofapplications; Periodsofgrazingduringtheyear; Animaltype,class,andsizeusedforgrazing; Stockingratesandmethods;and Totalyieldofmeat(kgcarcassyieldyear‐1)ormilk(kgfluidmilkyear‐1).

Cropyieldsareprovidedbytheentityforthecropsystem,orpeakforageamountsforgrazingsystems.Insomeyears,theentitymaynotharvestthecropduetodrought,pestoutbreaks,orotherreasonsforcropfailure.Inthosecases,theentityshouldprovidetheaverageyieldthattheyhaveharvestedinthepastfiveyears,andanapproximatepercentageofcropgrowththatoccurredpriortocropfailure.Theyieldisestimatedbasedonmultiplyingtheaveragecropyieldbythepercentageofcropgrowthobtainedpriortofailure.

Tocalculatetheamountofsyntheticfertilizernitrogenappliedtosoils,thetypeoffertilizerappliedanditsnitrogencontentarerequired.Table3‐10providesnitrogencontentinformationforcommontypesofsyntheticfertilizers.

Pasture/range/paddock(PRP)manureNinputistheNexcreteddirectlyontolandbylivestock,andthemanureisnotcollectedormanaged(deKleinetal.,2006).TheamountofPRPmanureNisestimatedwiththelivestockmethods(SeeChapter5),andassumedtobesplitwith50%oftheNinurineandtheother50%oftheNinsolids.

AncillarydataforestimatingdirectsoilN2Oemissionsfrommineralsoilsincludelandresourceregion,soiltexture,andclimatevariables.Landresourceregioncanbeidentifiedbasedonthegeographiccoordinatesofthefield.SoildataareavailablefromnationaldatasetssuchasSSURGO(SoilSurveyStaff,2011),andaveragegrowingseasonprecipitationandevapotranspirationdataareavailablefromnationalweatherdatasetssuchasPRISM(Dalyetal.,2008).Thesedataareusedbythemodelstodeterminebaseemissionrates.

3.5.4.5 ModelOutput

N2Oemissionsareexpressedbothasthequantityofemissionsandasemissionsintensity—emissionsperunityield,e.g.,gN2OperMggrainoranimalproduct.Reducingtheemissionsintensitycanbeassumedtoavoidemissionsfromindirectland‐usechange.Incontrast,iftheemissionsintensityincreasesduetoalossofyield,thenthereispotentialforadditionallandtobeconvertedintoagriculturetomakeupforayieldloss.

TheprimarylimitationofN2Oestimationmodelsisthattheydependonsurrogatemeasuresthatwillnotallowfluxesforaparticularlocationortimetobepredictedprecisely.Nevertheless,whileitmaybedecades,ifever,beforeannualratesofN2Oemissionsfromaspecificfieldcanbemeasuredwithgreatcertaintyandforlowcost,averageestimatesforsimilarcroppingsystemsandlandscapeswillconvergeasestimatesaggregatetolargerareas.

Table3‐10:NitrogenFractionofCommonSyntheticFertilizers(percentbyweight)

SyntheticFertilizer %NAmmoniumnitrate(NH4NO3) 33.5%Ammoniumnitratelimestone 20.5%Ammoniumsulfate 20.75%Anhydrousammonia 82%Aquaammonia 22.5%Calciumcyanamide(CaCN2) 21%Calciumammonianitrate 27.0%Diammoniumphosphate 18%Monoammoniumphosphate 11%Potassiumnitrate(KNO3) 13%Sodiumnitrate(NaNO3) 16%UreaCO(NH2)2 45%Source:Fertilizer101(2011).

Equation3‐16:SoilN2OEmissionsIntensity

EIN2O=(N2ODirect+N2OIndirect)/Y

Where:

EIN2O =N2Oemissionsintensity (metrictonsCO2‐eqpermetrictondrymattercropyieldorkgcarcassorkgfluid

N2ODirect =TotaldirectsoilN2Oemission(metrictonsCO2‐eqyear‐1)(SeeEquation3‐8)

N2OIndirect=TotalindirectsoilN2Oemission(metrictonsCO2‐eqyear‐1)(SeeEquation3‐13)

Y =Totalyieldofcrop(metrictonsdrymattercropyieldyear‐1),meat(kgcarcassyieldyear‐1),ormilkproduction(kgfluidmilkyieldyear‐1)

Limitationsinthemethodalsooccurdueto:

Lackofknowledgeofhowdifferentpracticesaffectfluxesinsomeregionsandcroppingsystems.

LackofknowledgeabouthowsomeofthemanagementpracticesinteractwitheachotherandwithsoilandclimatefactorstoaffectthefundamentalprocessesdrivingN2Oemissions—e.g.,nitrification,denitrification,gasdiffusion,etc.—andincorporationoftheseeffectsintoprocessmodels.

Limitednumberofdatasetscurrentlyavailabletotesttheefficacyofpracticestomitigatefluxesandtoevaluateprocess‐basedmodels.

Limitednumberofdatasetswithmorethantwofertilizerratestoestimatethescalarsforemissionfactorsassociatedwiththebaseemissionrates,particularlythepossibilityfornon‐linearscalars.

ThemineralsoilsmethodassumesaonepercentemissionfactorforindirectN2Oemissionsfromvolatilizednitrogenand0.75percentemissionfactorforleachedNO3‐.However,thereisevidencethattheEFforNO3‐leachingvariesfrom0.75%,dependingonthetypeofwaterway(Beaulieuetal.,2011)anditisalsolikelythatthesoilN2Oemissionsfromatmosphericdepositionofnitrogenwillvarydependingonthenitrogenstatusofthereceivingecosystem.

Thefractionofnitrogenthatisvolatilized(assumedtobe10percentforinorganicnitrogensourcesand20percentfororganicnitrogensourcesinEquation3‐15)isveryuncertain.Likewise,thefractionofnitrogenthatisleachedfromaprofileorrunsoffishighlyuncertain(assumedtobe30percentofallnitrogensourcesexceptwhereprecipitationplusirrigationislessthan80percentofpotentialevapotranspiration;U.S.EnvironmentalProtectionAgency,2011).Experimentssuggestthatgrossgeneralizationsarenotvalidandthatmanypracticescanreducebothvolatilizednitrogenandthenitrogenthatislostbyleachingandrunoff.9

ClimatechangewillaffectmodeloutputinsofarasbaselineN2Oestimatesaresimulatedforanygivenlocationusingtemperatureandprecipitationdistributionsforthepast30years.Expectedchangesintemperature,precipitation,andextremeeventssuchasdroughts,floods,andheatwaveswilladdfurtheruncertaintytoestimatesofallN2Oemissionsandpotentiallyinteractwithscalingfactors.Cropnitrogenmanagementmayfurtherchangewithclimatechange(Robertson,2013).

UncertaintiesinmodelparametersarecombinedusingaMonteCarlosimulationapproach.UncertaintyisassumedtobeminorforthemanagementactivitydataprovidedbytheentityTable3‐11providestheprobabilitydistributionfunctionstoestimateuncertaintyinthedirectandindirectsoilN2Oemissions.DataarenotshownforDNDCandDAYCENToutputthataredelineatedbyLRR,soiltype,andclimate.

9TheIPCCfactorsassumethatthemaximumabovegroundnitrogenrecoverybycropsis50to60percent.However,ratesofnitrogenrecoverycanbesignificantlyhigherwithbestpractices.

Table3‐11:AvailableUncertaintyDataforDirectandIndirectN2OEmissions

ParameterEstimatedValue Units

EffectiveLowerLimit

EffectiveUpperLimit

Distribution DataSource

TypicaldirectN2Oemissionrateand0‐levelinputratefromprocess‐basedmodel

NS Various NS NSMultiple

distributionsDAYCENT,DNDC

Scalingfactorforslow‐releasefertilizers

‐0.21ProportionalChangeinEmissions

‐0.30 ‐0.12 Normal Appendix3‐A

ScalingfactorforPRPmanureN

+0.5ProportionalChangeinEmissions

0.33 0.67 NormalAppendix3‐

Scalingfactornitrificationinhibitors–semi‐arid/aridclimate

‐0.51 ‐0.21 Normal Appendix3‐A

Scalingfactornitrificationinhibitors–mesicclimate

‐0.52 ‐0.24 NormalAppendix3‐

Scalingfactorforno‐till,semi‐arid/aridclimate,<10years

0.38ProportionalChangeinEmissions

0.04 0.72 Normal

Scalingfactorforno‐till,semi‐arid/aridclimate,≥10years

‐0.5 ‐0.16 Normal

Scalingfactorforno‐till,mesic/wetclimate,<10years

‐0.015

ProportionalChangeinEmissions

‐0.16 0.16 Normal

Scalingfactorforno‐till,mesic/wetclimate,≥10years

‐0.19 0.01 Normal

BaseEFscalar–croplandfornon‐grasslandcrops

0.0274

(metrictonsN2O‐N(metrictonsN)‐2)ha

NormalAppendix3‐

BaseEFscalar–forgrasslands 0.117

(metrictonsN2O‐N(metrictonsN)‐2)ha

NormalAppendix3‐

EmissionrateforcroppedHistosols 0.008

metrictonsN2O‐Nha‐1year‐1

0.002 0.024 Uniform IPCC(2006)

Fractionofsyntheticnitrogen(NSN)thatvolatilizesasNH3andNOx

0.1metrictonsN(metrictonNsfert)‐1

0.03 0.3 Uniform IPCC(2006)

ParameterEstimatedValue Units

EffectiveLowerLimit

EffectiveUpperLimit

Fractionofnitrogeninorganicamendments(FON)thatvolatilizesasNH3andNOx

0.2metrictonsN(metrictonNON)‐1

0.05 0.5 Uniform IPCC(2006)

EmissionfactorforvolatilizednitrogenasNH3andNOxthatistransformedtoN2O.

0.01metrictonsN2O‐N(metric

tonN)‐10.002 0.05 Uniform IPCC(2006)

FractionofNtthatleachesorrunsoffexceptinsystemswithcovercrops

0.3metrictonsN(metrictonN)‐1 0.1 0.8 Uniform IPCC(2006)

FractionofNtthatleachesorrunsoffwithaleguminouscovercrop

0.18 metrictonsN(metrictonN)‐1

0.14 0.26 Log‐Normal Tonittoetal.(2006)

FractionofNtthatleachesorrunsoffwithnon‐leguminouscovercrop

0.09metrictonsN(metrictonN)‐1

0.06 0.15 Log‐NormalTonittoetal.(2006)

EmissionfactorforleachedandrunoffnitrogenthatistransformedtoN2O

0.0075metrictonsN(metrictonN)‐1 0.0005 0.025 Uniform IPCC(2006)

NS=NotShown.Dataarenotshownforparametersthathave100’sto1000’sofvalues(denotedasNS).Dataareprovidedinsupplementarymaterialavailableonline.

3.5.5 MethaneUptakebySoils

TherearenoagronomicpracticesknowntoenhanceCH4uptake(oxidation)incroplands,otherthaninwetlandsconvertedtofloodedrice(discussedinSection3.2.2).Agronomicactivityuniversallyreducesmethanotrophyinarablesoilsby70percentormore(Mosieretal.,1991;Robertsonetal.,2000;Smithetal.,2000).RecoveryofCH4oxidationuponabandonmentfromagricultureisslow,probablytaking50to100yearsforthedevelopmentofeven50percentofformer(original)rates(Levineetal.,2011).NorecoveryhasbeendocumentedforCRPgrasslandsorperennialbiofuelcropstodate.TherearecurrentlynomodelsforquantifyingCH4oxidationrecoveryotherthanrateofreversiontonaturalvegetation,sothisisaTier3methodasdefinedbytheIPCC.

Themodelisbasedonaveragevaluesformethaneoxidationinnaturalvegetation—whethergrassland,coniferousforest,ordeciduousforest—attenuatedbycurrentlandusepractices.AveragevaluesarefromthedatasetusedbyDelGrossoetal.(2000a),whoreportedaveragefluxes(±standarddeviation)fortemperateandtropicalgrasslandsoilsof3.2±1.9kgCH4ha‐1year‐1;forconiferousforestsoils,2.8±1.4kgCH4ha‐1year‐1;andfordeciduousforestsoils,11.8±5kgCH4ha‐1year‐1.Managementreducespotential(historic)oxidationto30percentoforiginalratesbasedonavailabledata(DelGrossoetal.,2000a;Mosieretal.,1991;Robertsonetal.,2000;Smithetal.,2000)asnotedinSections3.2.3.3and3.3.2.3.Recoveryofoxidationisassumedtooccurovertheperiodrequiredforecologicalsuccessiontorestoreoriginalvegetation(DelGrossoetal.,2000a;Mosieretal.,1991;Robertsonetal.,2000;Smithetal.,2000),whichisapproximatedat100yearsafterabandonmentfromagricultureorforestharvest.Recoveryisassumedtooccuratalinearrate(Smithetal.,2000)suchthatsuccessionalforestsandgrasslandswillconsumeCH4ataratethatisbetween30and100percentoftheoriginaloxidationcapacitybetweentheinitialyearofabandonmentuntilyear100.Thefollowingequationisusedtoestimatemethaneoxidationforalandparcel:

MethodforEstimatingMethaneUptakebySoil

Methaneuptakebysoilusesanequationbasedonaveragevaluesformethaneoxidationinnaturalvegetation—whethergrassland,coniferousforest,ordeciduousforest—attenuatedbycurrentlandusepractices.

AnnualaverageCH4oxidationfluxesarefromthedatasetusedbyDelGrossoetal.(2000a)whoreviewedaveragefluxesfromgrasslandandagriculturalsoils,coniferousforestsoils,anddeciduousforestsoils.Managementreducespotential(historic)oxidationto30percentoforiginalratesbasedonavailabledata(DelGrossoetal.,2000a;Mosieretal.,1991;Robertsonetal.,2000;Smithetal.,2000).KuchlerpotentialvegetationmapscanbeusedtodeterminethenaturalvegetationacrosstheUnitedStatesiftheentitydoesnothaveinformationforlandparcelsinoperation.

ThisnewlydevelopedmethodologymakesuseofrecentU.S.‐basedresearchthatisnotaddressedbyIPCCortheU.S.Inventory.Themethodincorporatesentityspecificannualdatasuchascurrentmanagementofthelandparcel,cultivationforcropproduction,grazingactivity,recentlyharvestedforests,orfertilizedgrasslandsorforests.

Thismethodrequireslanduseandtypeofvegetationforthepast80years.KuchlerpotentialvegetationmapscanbeusedtodeterminethenaturalvegetationacrosstheUnitedStates(grassland,coniferousforest,ordeciduousforest)iftheentitydoesnothavethisinformationforlandparcelsintheoperation.Theentitywillneedtoidentifyifthecurrentmanagementofthelandparcelincludescultivationforcropproduction,grazingingrasslands,recentlyharvestedforests,orfertilizedgrasslandsorforests.Assumingtheparceloflandisnotundercultivation,fertilized,grazedgrasslands,orrecentlyharvestedforest,theentitywillneedtoprovidethetimesincethelandhasbeenmanagedwithoneofthesepractices.

Noancillarydataarerequiredforthismethod.

3.5.5.5 ModelOutput

ThemodelprovidesavaluefordiminishedCH4oxidationcapacity.ThechangeinCH4oxidationcapacitywillbenegative,andsothereisnopotentialforincreasedCH4oxidationwiththismethod.Unlikeothermethodsinthissection,theemissionsintensityisnotrelevantforthismethod.

Lackofprecisioninknowledgeofpriorlanduse. UncertaintiesassociatedwithestimatingCH4oxidationratespriortoconversion(PCH4in

Equation3‐17).Inareviewofavailabledata,DelGrossoetal.(2000a)notedannualCH4

oxidationratesof<1.8kgCH4ha‐1year‐1forgrasslandandagriculturalsoils,1.4to4.1kgCH4ha‐1year‐1forconiferousandtropicalforestsoils,and5.3to12kgCH4ha‐1year‐1fordeciduousforestsoils.

Equation3‐17:Methane(CH4)Oxidation

CH4SoilOxidation=(PCH4×AF)×SF×A×CH4GWP

Where:

CH4SoilOxidation=CH4oxidationinsoils(metrictonsCO2‐eqyear‐1)

PCH4 =PotentialCH4oxidationbasedonhistoricnaturalvegetation;grasslands=3.2;coniferousforests=2.8,deciduousforests=11.8(kgCH4ha‐1year‐1)

AF =CH4oxidationattenuationfactor;croplandincludingset‐aside(CRP)grassland,grazingland,andfertilizedorrecentlyharvestedforests=0.30;naturalvegetation,0‐100yearsafterabandonmentofagriculturalproductionortimberharvest=0.3+(0.007×yearssinceabandonment);>100yearspost‐managementorneverusedforagriculturalmanagementortimberharvest=1.0

SF =Scalingfactor,1/1000(metrictonskg‐1)

A =Area(ha)

CH4GWP =GlobalwarmingpotentialofCH4(metrictonsCO2‐eq(metrictonsCH4)‐1)

Uncertaintyassociatedwiththeattenuationfactor.Inareviewoftemperateregioncomparisonsofpairedsitesinnaturalvegetationvs.agriculturalmanagement,Smithetal.(2000)foundthatagriculturalconversiontocroplandorpasturereducedoxidationby71percentonaverage.

UncertaintiesinmodelparametersarecombinedusingaMonteCarlosimulationapproach.Uncertaintyisassumedtobeminorforthemanagementactivitydataprovidedbytheentity,althoughthismaynotbethecaseifthereislimitedknowledgeaboutland‐usechange.Table3‐12providestheprobabilitydistributionfunctionsassociatedwithestimatinguncertaintyinmethaneoxidation.

Table3‐12AvailableUncertaintyDataforMethaneOxidation

ParameterEstimatedValue

EffectiveLowerLimit

EffectiveUpperLimit

CH4oxidationratespriortoconversion(PCH4)grasslands(kgCH4ha‐1year‐1)

3.2 0 6.9 NormalDelGrossoetal.(2000a)

CH4oxidationratespriortoconversion(PCH4)coniferousforests(kgCH4ha‐1year‐1)

2.8 0.1 5.5 Normal DelGrossoetal.(2000a)

CH4oxidationratespriortoconversion(PCH4)deciduousForests(kgCH4ha‐1year‐1)

11.8 1.9 21.6 Normal DelGrossoetal.(2000a)

CH4oxidationattenuationfactor:croplandincludingset‐aside(CRP)grassland,grazingland,andfertilizedorrecentlyharvestedforests

0.30 0.07 1 Log‐NormalSmithetal.(2000)

CH4oxidationattenuationfactor:naturalvegetation,0‐100yearsafterabandonmentofagriculturalproductionortimberharvest

0.3+(0.007×yearssince

abandonment)

0.07+(0.007×yearssince

abandonment)

1 Log‐NormalSmithetal.(2000)

CH4oxidationattenuationfactor:>100yearspost‐managementorneverusedforagriculturalmanagementortimberharvest

1 0.07 1 Log‐Normal Smithetal.(2000)

3.5.6 MethaneandNitrousOxidefromFloodedRiceCultivation

ThereareanumberofpossibilitiesforestimatingGHGemissionsfromfloodedricesystems.ProcessbasedmodelsarebeingdevelopedtoquantifyGHGemissions,suchastheDNDC(e.g.,Zhangetal.,2011)andDAYCENTmodels(Chengetal.,2013).While,thesemodelshavebeenevaluatedforvariousregionsandcountriesinAsia,theyhavenotbeensufficientlyevaluatedforU.S.ricesystems,whicharesignificantlydifferentfromthosefoundinAsia(establishmentpractices,residuemanagement,watermanagement,andvarieties).Therefore,theselectedmethodisbasedontheIPCCTier1methodology.WhiletheIPCCmethodologyhasalsobeenlargelydevelopedfromAsianricestudies,itismoretransparentanduncertaintiescanbederivedintheemissionsestimates.Itisanticipatedthattheprocess‐basedmodelsmaybefurthertestedandcalibratedinthenearfutureforU.S.conditionsandpossiblyusedinafutureversionofthesemethods.

SeveralmanagementpracticeshavethepotentialtoinfluenceCH4andN2Oemissionsfromfloodedricesystems.However,therearecurrentlynotenoughdataavailabletoquantitativelyaccountfor(orestablishscalingfactorsfor)theeffectsofallofthesemanagementpractices.Thereissufficientinformationtoaccountfortheinfluenceofwatermanagement,residuemanagement,andorganicamendmentsonCH4emissionsfromfloodedrice(Lascoetal.,2006;Yanetal.,2005).

Methane:Themethodologyassumesabaselineemissionfactoror“typical”dailyrateatwhichCH4isproducedperunitoflandarea.Thisbaselinefactorrepresentsfieldsthatarecontinuouslyfloodedduringthecultivationperiod,notfloodedatallduringthe180dayspriortocultivation,andreceivenoorganicamendments.Differencesbetweenthebaselinescenarioandotherscenariosareaccountedforbytheuseofscalingfactorsthatareusedtoadjustthebaselineemissionfactorfor

MethodforEstimatingMethaneandN2OEmissionsfromRiceCultivation

IPCCequationsdevelopedbyLascoetal.(2006)forCH4anddeKleinetal.(2006)forN2O.‐ ThebaselineemissionfactorortypicaldailyrateatwhichCH4isproducedper

unitoflandarearepresentsfieldsthatarecontinuouslyfloodedduringthecultivationperiod,notfloodedatallduringthe180dayspriortocultivation,andreceivenoorganicamendments.Differencesbetweenthebaselinecontinuouslyfloodedfieldswithoutorganicamendmentsareaccountedforbyscalingfactors(e.g.,waterregimeadjustments(pre‐andduringthecultivationperiod),ororganicamendments).CH4scalingfactorstoaccountforwaterregimesandorganicamendmentscomefromLascoetal.(2006).

‐ N2OemissionfactorsrelyonLascoetal.(2006),andthescalingfactortoaccountfordrainageeffectscomesfromAkiyamaetal.(2005;USDA,2011).

ThismethodusestheIPCC(2006)equationswiththeadditionofascalingfactorforestimatingN2Oemissionsfromdrainage(Akiyamaetal.,2005;U.S.EPA,2011).ThemethodformethaneemissionsusesentityspecificseasonalparceldataasinputintotheIPCCequation.

Thismethodwaschosentominimizeuncertainty.Processmodelswereconsidered,butnotchosenforthismethodduetoaneedforfurtherresearchonU.S.ricecultivationconditionsandpractices.

theeffectsofwatermanagement(occurringbothbeforeandduringthecultivationperiod)andtheamountoforganicamendments.TherateatwhichCH4isemitteddependsonwaterflooding/drainageregimesandonratesandtypesoforganicamendmentsappliedtothesoil.Assuch,scalingfactorsforabroadrangeofscenariosareprovidedwiththismethodology.Thefactorsaredifferentiatedbyhydrologicalcontext(e.g.,irrigated,rainfed,upland—allricefieldsintheUnitedStatesareirrigated),cultivationperiodfloodingregime(e.g.,continuous,multipleaeration),timesincelastflooding(priortocultivation;e.g.,over180days,under30days)andtypeoforganicamendment(e.g.,compost,farmyardmanure).

ThefollowingequationhasbeenadoptedfromthemethodologydevelopedbytheIPCCtoestimateCH4emissionsfromalandparcel(Lascoetal.,2006):

Thedailyemissionfactorisestimatedbasedontheconditions(i,j,k,etc.)thatinfluenceCH4emissionsforfloodedriceproduction,includingtheecosystemtype,waterregime,andorganicamendmentrate.Asmoredatabecomeavailable,additionalconditionsthatinfluenceCH4emissionsmaybeadded.The“i"intheequationsbelowrepresentsthespecificscenarioor“otherconditions”thatcansignificantlyinfluenceCH4emissionsonaparcel.Inthefuture,additionalscenarioswithfactorsthataffectCH4emissionsmaybeincludedastherelationshipbetweentheseconditionsbecomesclear.Thefollowingequationisusedtoestimatethedailyemissionfactorforalandparcel:

Equation3‐18:Flooded RiceMethaneEmissions

CH4Rice=CH4GWP×Σijk(EFijkxtijkxAijkx10‐3)

Where:

CH4Rice =Annualmethaneemissionsfromricecultivation(metrictonsCO2‐eqyear‐1)

EFijk =Adailyemissionfactorfori,j,andkconditions(kgCH4ha‐1day‐1)

tijk =Cultivationperiodofricefori,j,andkconditions(days)

Aijk =Annualharvestedareaofricefori,j,andkconditions(hayear‐1)

CH4GWP =GlobalwarmingpotentialforCH4(metrictonsCO2‐eq(metrictonsCH4)‐1)

i,j,andk =Representdifferentecosystems,waterregimes,typeandamountoforganicamendments,soiltype,ricecultivar,sulfatecontainingamendments,andotherconditionsunderwhichCH4emissionsfromricemayvary.

Thescalingfactorfororganicamendmentstoalandparcelisestimatedusingthefollowingequation:

ThescalingfactorsforEquation3‐19andEquation3‐20arefromLascoetal.(2006)andshownbelow.

Table3‐13:RiceWaterRegimeEmissionScalingFactors(DuringCultivationPeriod)

WaterRegimeDuringtheCultivationPeriod(assumesirrigated) SFwContinuouslyflooded 1Intermittentlyflooded–singleaeration 0.6Intermittentlyflooded–multipleaeration 0.52Source:Lascoetal.(2006),Table5.12.

Table3‐14:RiceWaterRegimeEmissionScalingFactors(BeforeCultivationPeriod)

WaterRegimeBeforetheCultivationPeriod SFpNonfloodedpre‐season<180days 1Nonfloodedpre‐season>180days 0.68Floodedpre‐season>30days 1.9Source:Lascoetal.(2006),Table5.13.

Equation3‐19:Flooded RiceMethaneEmissionFactor

EFi=EFcxSFwxSFpxSFoxSFs,r

Where:

EFi=adjusteddailyemissionfactorforaparticularharvestedarea(kgCH4ha‐1day‐1)

EFc=baselineemissionfactorforcontinuouslyfloodedfieldswithoutorganicamendments(kgCH4ha‐1day‐1)

SFw=scalingfactortoaccountforthedifferencesinwaterregimeduringthecultivationperiod(fromLascoetal.2006,Table5.12)(unitless)

SFp=scalingfactortoaccountforthedifferencesinwaterregimeinthepre‐seasonbeforethecultivationperiod(fromLascoetal.2006,Equation5.3andTable5.14)(unitless)

SFo=scalingfactorshouldvaryforbothtypeandamountoforganicamendmentapplied(Equation3‐20)(unitless)

SFs,r=scalingfactorforsoiltype,ricecultivar,etc.,ifavailable

Equation3‐20:OrganicAmendmentsScalingFactor

SFo=(1+(ROAixCFOAi))0.59

Where:

SFo =scalingfactorforbothtypeandamountoforganicamendment

ROAi=rateofapplicationoforganicamendment(s)(metrictonsha‐1)

CFOAi =conversionfactorfororganicamendments(fromLascoetal.2006,Table5.14)(unitless)

Table3‐15:RiceOrganicAmendmentEmissionScalingFactors;adaptedfromLascoetal.(2006)

OrganicAmendments CFOAStrawincorporatedshortly(<30days)beforecultivation 1Strawincorporatedlong(>30days)beforecultivation 0.29Compost 0.05Farmyardmanure 0.14Greenmanure 0.50Source:Lascoetal.(2006),Table5.14.

SoilN2O:TheIPCCmethodology(deKleinetal.,2006)hasbeenadaptedtoestimatedirectN2Oemissionsfromricefields.Theemissionfactorforricesoilsaccountsfornitrogenadditionsfrommineralfertilizers,organicamendments,andcropresidues.Notethataneffectofnitrogenmineralizedfrommineralsoilasaresultoflossofsoilcarbonisnotincludedinthisequation.Floodedricecultivationleadstominimallossesofsoilcarbonduetoperiodicflooding,whichisthedefaultassumptionwiththeIPCCmethod(Lascoetal.,2006),andthereforeitisnotnecessarytoincludetheeffectofenhancednitrogenmineralizationfromlossofsoilC.

ThefollowingequationisusedtoestimatethesoilN2Oemissionsfromaparcelofland:

TheemissionfactorandSFDfactorsarebasedonresearchconductedbyAkiyamaetal.(2005).TheIPCC(2006)doesnotaccountfordifferencesinwatermanagement,andusesanemissionfactorof0.3,butAkiyamaetal.(2005)providefurtherdisaggregationoftheemissionfactorsbasedonwatermanagement.Therefore,theselectedemissionfactorvalueis0.0022basedonAkiyamaetal.(2005),andthescalingfactorsare0forcontinuouslyfloodedriceand0.59foraeratedsystems(i.e.,drainageeventsduringthegrowingseason).

IndirectN2OEmissions:ForindirectN2Oemissionsfromfloodedrice,thesamemethodisusedasdescribedinSection3.5.4.2,byapplyingEquation3‐13,TotalIndirectSoilN2OEmissionsfromMineralSoils;Equation3‐14,IndirectSoilN2OEmissionsfromMineralSoils—Volatilization;andEquation3‐15,IndirectSoilN2OEmissionsfromMineralSoils—LeachingandRunoff.Inthelatter

Equation3‐21:DirectSoilN2OEmissionsfromfloodedRice

N2ORice=Nt×EF×(1+SFD)×N2OMW×N2OGWP

Where:

N2ORice=DirectemissionsofN2Ofromsoilsinfloodedriceproductionsystems(metrictonsCO2‐eqyear‐1)

Nt =Totalnitrogeninputsfromallagronomicsources:mineralfertilizer,organicamendments,residues,andadditionalmineralizationfromland‐usechangeortillagechange(metrictonsNyear‐1)

EF =EmissionfactororproportionofNttransformedtoN2O(kgN2O‐N(kgN)‐1)

SFD =Scalingfactortoaccountfordrainageeffects;0forcontinuouslyflooded(dimensionless)

N2OMW=RatioofmolecularweightsofN2OtoN2O‐N=44/28(metrictonsN2O(metrictonsN2O‐N)‐1)

N2OGWP=GlobalwarmingpotentialforN2O(metrictonsCO2‐eq(metrictonsN2O)‐1)

twoequations,usetheIPCCdefaultfractionsforFRSN,FRON,andFRleach,whichareprovidedintheequationboxes.

Theactivityandrelateddatarequirementsforthismethodinclude:

Harvestedarea(ha); Cultivationperiodindays; Watermanagementpracticesthroughouttheyear(e.g.,aerationornot); Organicmatteramendment(includingresidue)rate; OrganicfertilizerN; Fertilizernitrogenmanagement(rate); Typeoffertilizer(s)applied(qualitative); CropresidueN;and Cropyield,metrictonsdrymattercropyieldyear‐1.

Noancillarydataareneededforthismethod.

3.5.6.5 ModelOutput

ModeloutputisthecombinedemissionsofCH4andN2OinCO2equivalents,expressedonanareabasis.TheintensityofCH4emissionsandnitrousoxide(i.e.,emissionsperunitoflandareacultivated)isrelatedtothequantityofcropsgrownandcanbeestimatedwiththefollowingequation:

Thismethodhasseverallimitationsthatwillpotentiallycreatebiasorimprecisionintheresults.Currently,scalingfactorsaccountonlyforwaterandorganicmattermanagementanddonotaccountforothermitigationoptions.Asindicatedearlierthereareothermanagementopportunitiesthatmayreduceemissions,butfurtherresearchisrequiredintheseareas.Baselineemissionsarehighlyvariable,butthismethodologyprovidesonlyonefactorvaluerepresentingthebaselineemissions.Inaddition,themethodologyassumesaperiodofdrainage;however,drainevents(eventhoseofsimilarduration)canvarymarkedlybasedonsoilandclimaticconditions,fromdryandcrackingonthesurfacetosaturatedattheendofadrainageevent.Theinfluenceofdrainageonthesoilsaturationisnotaddressedwiththecurrentmethod.Inaddition,thereiscurrentlyinsufficientinformationtodevelopamethodfortheuseofsulfurproductsasamendments;futureguidancemaybeupdatedwithamethodforthispractice.

Equation3‐22:FloodedRiceCombinedMethaneandNitrousOxideEmissionsIntensity

EI=(CH4Rice+N2ORice)/Y

Where:

EI =Emissionsintensity(metrictonsCO2‐eqpermetrictonsdrymattercropyield)

CH4Rice=Annualmethaneemissionsfromricecultivation(metrictonsCO2‐eqyear‐1)

N2ORice=DirectemissionsofN2Ofromsoilsinfloodedriceproductionsystems(metrictonsCO2‐eq‐year‐1)

Y =Totalyieldofcrop(metrictonsdrymattercropyieldyear‐1)

CH4emissionsaretheresultofanumberofinteractingbiologicalprocesses,whichbynaturevaryspatiallyandtemporally.Thegreatestamountofuncertaintyisthebaselineemissionfactor.Whenusingthismethodology,theemissionfactorisanaverageemissionfactorforcontinuouslyfloodedricesystemsthathavenotbeenfloodedthe180dayspriortocultivationandhavenotreceivedorganicamendments.InthecaseofCH4emissionsfromricecultivation,theuncertaintyrangesofTier1values(emissionandscalingfactors)areadopteddirectlyfromLascoetal.(2006).Rangesaredefinedasthestandarddeviationaboutthemean,indicatingtheuncertaintyassociatedwithagivendefaultvalueforthissourcecategory.

UncertaintiesinmodelparametersarecombinedusingaMonteCarlosimulationapproach.Uncertaintyisassumedtobeminorforthemanagementactivitydataprovidedbytheentity.Table3‐16providestheprobabilitydistributionfunctionsassociatedwithestimatinguncertaintyinmethaneandN2Oemissionsfromricecultivation.

Table3‐16:AvailableUncertaintyDataforMethane,DirectandIndirectN2OEmissions

MethanefromFloodedRiceCultivation

ParameterAbbreviation/

SymbolEstimatedValue

EffectiveLowerLimit

EffectiveUpperLimit

DistributionDataSource

Baselineemissionfactorforcontinuouslyfloodedfieldswithoutorganicamendments

EFc 1.3 0.8 2.2 UniformIPCC(2006)

Waterregimeduringthecultivationperiod–Scalingfactor

SFwforcontinuouslyflooded

1 0.79 1.26 UniformIPCC(2006)

SFwforsingleaeration

0.6 0.46 0.8 UniformIPCC(2006)

SFwformultipleaerations

0.52 0.41 0.66 UniformIPCC(2006)

Waterregimebeforethecultivationperiod–Scalingfactor

SFpfornon‐floodedpre‐season<180

days1 0.88 1.14 Uniform IPCC

(2006)

SFpfornon‐floodedpre‐season>180

days0.68 0.58 0.8 Uniform IPCC

(2006)

SFpforfloodedpre‐season>30days

1.9 1.65 2.18 Uniform IPCC(2006)

Organicamendmentconversionfactor

CFOAiforstrawincorporationlessthan30daysbefore

cultivation

1 0.97 1.04 Uniform IPCC(2006)

MethanefromFloodedRiceCultivation (continued)

SymbolEstimatedValue

EffectiveLowerLimit

EffectiveUpperLimit Distribution

DataSource

CFOAiforstraw

incorporationmorethan30daysbeforecultivation

0.29 0.2 0.4 Uniform IPCC(2006)

CFOAiforcompost

0.05 0.01 0.08 UniformIPCC(2006)

CFOAiforfarmyardmanure 0.14 0.07 0.2 Uniform

IPCC(2006)

CFOAiforgreenmanure

0.5 0.3 0.6 Uniform IPCC(2006)

N2OfromFloodedRice

SymbolMean

RelativeUncertaintyLow(%)

RelativeUncertaintyHigh(%)

DistributionDataSource

EmissionfactororproportionofNttransformedtoN2O

EF 0.0022 0.24% 0.24% Normal Akiyamaetal.(2005)

Scalingfactortoaccountfordrainageeffects

SFDforaeratedsystems 0.59 0.35% 0.35% Normal

Akiyamaetal.(2005)

3.5.7 CO2fromLiming

AdditionoflimetosoilsistypicallythoughttogenerateCO2emissionstotheatmosphere(deKleinetal.,2006).However,prevailingconditionsinU.S.agriculturallandsleadtoCO2uptakebecausethemajorityoflimeisdissolvedinthepresenceofcarbonicacid(H2CO3).Therefore,theadditionoflimeleadstoacarbonsinkinthemajorityofU.S.croplandandgrazinglandsystems.Whetherlimingcontributestoasinkorsourcedependsonthepathwaysofdissolutionandratesofbicarbonateleaching.Theemissionsfactorprovidedinthisguidancehasbeenestimatedfroma

MethodforEstimatingCO2 EmissionsfromLiming

ThismethodusestheIPCCequation(deKleinetal.,2006)withU.S.specificemissionsfactors.

EntityspecificannualparceldataasinputintotheIPCCequation(e.g.,theamountoflime,crushedlimestone,ordolomiteappliedtosoils).

ThismethodwasselectedasitwastheonlyreadilyavailablemodelforestimatingCO2

emissionsfromliming.

reviewofexistingmodelsandmassbalanceanalysesconductedfortheapplicationoflimeintheUnitedStatesandisaTier2methodasdefinedbytheIPCC.

Sincecrushedlimestone(CaCO3)contains12percentC,anapplicationof1,000kgCaCO3places120kgConthesoilsurface.Itisassumedthattwo‐thirdsofthis(80kg)isacidifiedtoHCO3‐andleachedtotheoceanwhereitwillbesequesteredfordecadestocenturies(OhandRaymond,2006).Becausethistransferrepresentsamovementfromonelong‐termpool(geologicformations)toanother(ocean),thiscarbontransferdoesnotrepresentanetuptakeofCO2fromtheatmosphere.However,withthistransfer,thereis80kgCofatmosphericCO2uptakeintosoils.TheuptakeofCO2fromtheatmosphere,aftersubtractingtheone‐thirdofcarboninthelimethatisacidifieddirectlytoCO2(40kgC),yieldsatotalnetCO2uptakeof40kgCper1,000kgCaCO3applied.Thisresultsinacarboncoefficientoremissionfactorof40/1000=‐0.04kgCperkgCaCO3.Thisequatestoacarbonsink(40kgCsequestered/120kgC×100).DolomitecontainsonlyslightlymorecarbonthandoesCaCO3(13percentvs.12percent)sothefactorsareessentiallythesame.

Theemissionfactoriscountry‐specificbasedonarevisionoftheestimatesproposedinWestandMcBride(2005),whicharecurrentlyusedintheU.S.NationalGHGInventory(U.S.EPA,2011).TheunderlyingdifferencewiththeearlieremissionfactorfromWestandMcBride(2005)isthattherevisedvalueassumesthattheamountofbicarbonatecarriedintorivershasalongturnovertimeandisessentiallynotreturnedtotheatmosphereoverdecadaltocenturytimescales.

ThemodeltoestimateCO2emissionsfromliminghasbeenadaptedfrommethodsdevelopedbytheIPCC(deKleinetal.,2006),withrefinementintheemissionfactorsbasedonconditionsinU.S.agriculturallands.Thefollowingequationisusedtoestimateemissionsfromcarbonatelimeadditionstoalandparcel:

Themethodrequiresdataontheamountoflime(crushedlimestoneordolomite)appliedtosoils.

Noancillarydataareneededinordertoapplythemethod.

Equation3‐23:ChangeinSoilCarbon StocksfromLimeApplication

ΔCLime=M×EF×CO2MW

Where:

ΔCLime =Annualchangeinsoilcarbonstocksfromlimeapplication(metrictonsCO2‐eq)

M =Annualapplicationoflimeascrushedlimestoneordolomite

(metrictonsofcrushedlimestoneordolomiteyear‐1)

EF =MetrictonCO2emissionspermetrictonoflime‐0.04

(metrictoncarbon(metrictonlime)‐1)

CO2MW =RatioofmolecularweightofCO2tocarbon(44/12)(metrictonsCO2(metrictonsC)‐1)

3.5.7.5 ModelOutput

Modeloutputisgeneratedonbothanabsolutequantityofemissionsandemissionsintensity.Thelatterisbasedontheamountofemissionsperunitofyieldforcropsincroplandsystemsorgrazingsystems.Theemissionsintensityisestimatedwiththefollowingequation:

Yieldsarebasedonthetotalamountofproductfromthelandmanagedwithlimeapplication.

LimitationsincludevariationinsoilcarbonemissionsduetosoilpHandrateofnitrogenfertilizerapplication,whichinfluencethechemicalpathwayoflimedissolution(Hamiltonetal.,2007;WestandMcBride,2005).Morespecifically,theEFwillnotaccuratelycapturetheresultoflimedissolutioninthepresenceofstrongernitricacid(HNO3),whichisproducedwhennitrifyingbacteriaconvertammonium(NH4+)basedfertilizerandothersourcesofNH4+tonitrate(NO3‐).

Uncertaintiesinthelimeemissionsmethodsincludeimprecisionatthefarmscale,becausethemethodofestimationisbasedonstream‐gaugedatathatarecollectedatthewatershedscale.UncertaintiesinmodelparametersarecombinedusingaMonteCarlosimulationapproach.Uncertaintyisassumedtobeminorforthemanagementactivitydataprovidedbytheentity.Table3‐17providestheprobabilitydistributionfunctionsassociatedwithCO2emissionspermetrictonoflimeapplied.

Table3‐17:AvailableUncertaintyDataforCO2fromLiming

Parameter MeanRelative

UncertaintyLow(%)

Emissionsfactor(metrictonCO2emissionspermetrictonoflime)

‐0.04 46% 46% NormalAdaptedfromWestand

McBride(2005)

Equation3‐24:EmissionsIntensityfromLimeApplication

EI=ΔCLime/Y

Where:

EI =Emissionsintensity(metrictonsCO2permetrictondrymattercropyield)

ΔCLime =Annualchangeinsoilcarbonstocksfromlimeapplication(metrictonsCO2)

3.5.8 Non‐CO2EmissionsfromBiomassBurning

Non‐CO2GHGemissionsfrombiomassburningincludeCH4andN2O.COandNOxarealsoemittedandareprecursorsthatarelaterconvertedintoGHGsfollowingadditionalreactions(i.e.,releaseofthesegasesleadstoGHGformation).CO2isalsoemittedbutnotaddressedforcropresiduesorgrasslandburningbecausethecarbonisreabsorbedfromtheatmosphereinnewgrowthofcropsorgrasseswithinanannualcycle.

Therehasbeenlimiteddevelopmentandtestingofprocess‐basedapproachesforestimatingnon‐CO2GHGemissionfrombiomassburning.Moreover,country‐specificdataarelimitedontheamountofnon‐CO2GHGemissions.Therefore,thisguidancehasadoptedtheIPCCTier1methodasdescribedbyAaldeetal.(2006).

Themodeltoestimatenon‐CO2GHGemissionsandprecursorshasbeenadaptedfrommethodsdevelopedbyIPCC(Aaldeetal.,2006).Thefollowingequationisusedtoestimateemissionsduetoburningbiomassonaparcelofland:

Combustionefficiency,asdefinedinIPCC(2006)combinestheproportionofbiomassthatisactuallyburnedinafirewiththeamountofcarbonreleasedasaproportionofthetotalcarbonintheburnedbiomass.Themassofthefuelcombustedincludesliveanddeadbiomass(i.e.,dead

MethodforEstimatingNon‐CO2 EmissionsfromBiomassBurning

ThemethodusestheIPCCequationandemissionfactorsdevelopedbyAaldeetal.(2006).

Entityspecificannualparceldata(e.g.,areaburnedforcroplandsandgrazingland;croptypeandharvestyielddata;residue‐yieldratios(Westetal.,2010);typeofforage,grazingarea,andamountofbiomassbeforethefireingrazinglandsthatareburned;andcombustionefficiency)areinputstotheIPCCequation.

Thismethodwasselectedasitwastheonlyreadilyavailablemodelforestimatingnon‐CO2emissionsfrombiomassburning.

Equation3‐25:GHGEmissionsfromBiomassBurning

GHGBiomassBurning=A×M×C×EF×10‐3×GHGGWP

Where:

GHGBiomassBurning=AnnualemissionsofGHGorprecursorduetobiomassburning(metrictonsofCO2‐eqyear‐1)

A =Areaburned(ha)

M =Massoffuelavailableforcombustion(metrictonsdrymatterha‐1year‐1)

C =Combustionefficiency,dimensionless

EF =Emissionfactor(gGHG(kgofburnedbiomass)‐1)

GHGGWP =GlobalwarmingpotentialforeachGHG(metrictonsCO2‐eq(metrictonsGHG)‐1)

biomassincludesplantresiduesingrazingandcroplandsystems)andisapproximatedforalandparcelwiththefollowingequation:

PeakabovegroundbiomassisestimatedwithEquation3‐3forcropsandgrassvegetation.Forcroplandsthatareburnedfollowingharvest,theresiduemassisestimatedbysubtractingtheharvestindex(HI)fromoneandconvertingtoapercentage,whichistheresidualbiomassleftinthefield.DefaultharvestindicesaregiveninTable3‐5.TheestimatedmassoffuelforgrazingsystemsbasedonEquation3‐3doesnotincludethedeadbiomass.Ifthereissignificantresiduallitteringrazingsystems,thenmultiplythemassoffuelbytwoasaconservativeestimateofthetotalliveanddeadbiomassonthelandparcel.Alternatively,entitiesmayenteranestimatefortheproportionofresiduallittermassrelativetothelivebiomass,insteadofusingtwo,whichdoublesthemassoffuel.AsummaryofemissionfactorsbylandusecategoryisprovidedinTable3‐18.

Thefollowingactivityandrelateddataareneededtoapplythemethod:

Areaburnedforcroplandsandgrazingland; Croptypeandharvestyielddataforcropsgrown

infieldswithresidueburningmanagement; Residue:yieldratios(optional); Typeofforage,grazingarea,andamountof

biomassbeforethefireingrazinglandsthatareburned;and

Combustionefficiency(optional).

Alistofdefaultcombustionefficienciesisprovidedforresiduesandforages(Table3‐19andTable3‐20),buttheentitycanprovidevaluespecifictotheiroperation.Defaultdrymattercontentsandresidue‐yieldratiosareprovidedinTable3‐5,butcanalsobeenteredbytheentityiftheinformationisavailable.

Table3‐18:EmissionFactorsforBiomassBurning

Land‐UseCategoryCO CH4 N2O NOx

(gkg‐1)

Grasslandburning 65 2.3 0.21 3.9

Croplandresidue 92 2.7 0.07 2.5

Forestbiomass(withconversiontocroplandorgrazinglands)

107 4.7 0.26 3.0

Source:Aaldeetal.(2006).

Table3‐19:DefaultCombustionEfficienciesforSelectedCrops

Crop CombustionEfficiency(C)

Corn 0.88x0.93=0.82Cotton 0.88x0.93=0.82Lentils 0.88x0.93=0.82Rice 0.88x0.93=0.82Soybeans 0.88x0.93=0.82Sugarcane 0.68x0.81=0.55Wheat 0.88x0.93=0.82Source:EPA(2013),Table6‐25.

Equation3‐26:MassofFuel

M=(Hpeak/C)×(D/100)

Where:

M =Massoffuelavailableforcombustion(metrictonsdrymatterha‐1year‐1)

Hpeak =Annualpeakabovegroundherbaceousbiomasscarbonstock(metrictonsCha‐1year‐1)

C =Carbonfractionofabovegroundbiomass(dimensionless)

D =Percentageofbiomasspresentatthestageofburningrelativetopeak(%)

Insomeyears,theentitymaynotharvestthecropduetodrought,pestoutbreaks,orotherreasonsforcropfailure.Inthosecases,theentityshouldprovidetheaverageyieldthatithasharvestedinthepast,andanapproximatepercentageofaveragecropgrowththatoccurredpriortoburning.Theyieldisestimatedbasedonmultiplyingtheaveragecropyieldbythepercentageofcropgrowthobtainedpriortoburning.

3.5.8.3 ModelOutput

Modeloutputisgeneratedonbothanabsolutequantityofemissionsandemissionsintensity.Thelatterisbasedontheamountofemissionsperunitofyieldforcropsincroplandsystemsoranimalproductsingrazingsystems.Theemissionsintensityisestimatedwiththefollowingequation:

Yieldsarebasedonthetotalamountofproductfromthelandmanagedwithburning.

Table3‐20:DefaultCombustionEfficienciesforSelectVegetationTypes

VegetationTypeCombustionEfficiency(C)

BorealForest(all) 0.34Wildfire 0.40

Crownfire 0.43Surfacefire 0.15

Postloggingslashburn 0.33Landclearingfire 0.59

TemperateForest(all) 0.45Postloggingslashburn 0.62

Felledandburned(land‐clearingfire) 0.51Shrublands(all) 0.72

Shrubland(general) 0.95Calluna health 0.71

Fynbos 0.61Savannawoodlands(earlydryseasonburns)(all)

Savannawoodland(early) 0.22Savannaparkland(early) 0.73

Savannawoodlands(mid/latedryseasonburns)(all)

Savannawoodland(mid/late) 0.72Savannaparkland(mid/late) 0.82

Tropicalsavanna 0.73Othersavannawoodlands 0.68

Savannagrasslands(earlydryseasonburns)(all)

Tropical/sub‐tropicalgrassland 0.74SavannaGrasslands/Pastures(mid/latedryseasonburns)(all) 0.77

Tropical/sub‐tropicalgrassland 0.92Tropicalpasture 0.35

Savanna 0.86Source:Aaldeetal.(2006),Table2.4(C×M)andTable2.6(C)

Equation3‐27:BiomassBurningEmissionsIntensity

EI=GHGBiomassBurning/Y

Where:

EI =Emissionsintensity(metrictonsCO2permetrictondrymattercropyield,metrictonsCO2perkgcarcassyield,metrictonsCO2perkgfluidmilkyield)

GHGBiomassBurning=AnnualCO2equivalentemissionsfromburning(metrictonsCO2‐eqyear‐1)

Uncertaintyintheemissionestimatesisattributedtoimprecisionincarbonfractions,drymattercontents,harvestindices,combustionefficiencies,andtheemissionfactors.UncertaintiesinmodelparametersarecombinedusingaMonteCarlosimulationapproach.Uncertaintyisassumedtobeminorforthecropyields,peakforage,andrelativeamountofcroporforagegrowthcomparedtothepeakproduction.However,thesevaluesarelikelytohavesomelevelofuncertainty,andmethodswillneedtoberefinedinthefuturetobetteraddresstheseuncertainties,particularlythemassoffuelingrazinglands.Table3‐21providestheprobabilitydistributionfunctionsforestimatinguncertaintyinnon‐CO2emissionsfrombiomassburning.

Table3‐21:AvailableUncertaintyDataforNon‐CO2EmissionsfromBiomassBurning

UncertaintyLow(%)

CH4EFforgrassland(gCH4kg‐1)

2.3 8% 8% Normal IPCC(2006)

CH4EFforcropresidue(gCH4kg‐1)

2.7 50% 50% Normal IPCC(2006)

N2OEFforgrassland(gN20kg‐1)

0.21 93% 93% Normal IPCC(2006)

N2OEFforcropresidue(gN20kg‐1)

0.07 50% 50% Normal IPCC(2006)

Combustionefficiencyforshrublands

0.72 68% 68% Normal IPCC(2006)

Combustionefficiencyforgrasslandswithearlyseasonburns

0.74 50% 50% Normal IPCC(2006)

Combustionefficiencyforgrasslandswithmidtolateseasonburns

0.77 66% 66% Normal IPCC(2006)

Combustionefficiencyforsmallgrains

0.9 50% 50% NormalExpert

Assessmentbyauthors

Combustionefficiencyforlargegrainandothercropresidues

0.8 50% 50% NormalExpert

Assessmentbyauthors

CombustionefficiencyBorealforest(all) 0.34 102% 102% Normal IPCC(2006)

Wildfire 0.40 340% 340% Normal IPCC(2006)Crownfire 0.43 104% 104% Normal IPCC(2006)Surfacefire 0.15 96% 96% Normal IPCC(2006)

Postloggingslashburn 0.33 130% 130% Normal IPCC(2006)CombustionefficiencyTemperateforest(all) 0.45 51% 51% Normal IPCC(2006)

Postloggingslashburn 0.62 264% 264% Normal IPCC(2006)CombustionefficiencyShrublands(all)

0.72 147% 147% Normal IPCC(2006)

Callunahealth 0.71 121% 121% Normal IPCC(2006)Fynbos 0.61 195% 195% Normal IPCC(2006)

CombustionefficiencySavannawoodlands(earlydryseasonburns)(all)

0.40 93% 93% Normal IPCC(2006)

UncertaintyLow(%)

CombustionefficiencySavannawoodlands(mid/latedryseasonburns)(all)

0.74 99% 99% Normal IPCC(2006)

Savannawoodland(mid/late)

0.72 270% 270% Normal IPCC(2006)

Tropicalsavanna 0.73 598% 598% Normal IPCC(2006)Othersavannawoodlands 0.68 931% 931% Normal IPCC(2006)CombustionefficiencySavannagrasslands(earlydryseasonburns)(all)

0.74 183% 183% Normal IPCC(2006)

Tropical/sub‐tropicalgrassland

0.74270% 270% Normal IPCC(2006)

Tropical/sub‐tropicalgrassland

0.92151% 151% Normal IPCC(2006)

Tropicalpasture 0.35 427% 427% Normal IPCC(2006)Savanna 0.86 85% 85% Normal IPCC(2006)

3.5.9 CO2fromUreaFertilizerApplications

UreafertilizerapplicationtosoilscontributesCO2emissionstotheatmosphere.ThesourceoftheCO2thatisincorporatedintotheureaduringthefertilizerproductionprocessisfromfossilfuelsourcesintheU.S.fertilizerplants.TheCO2capturedduringtheproductionprocessisconsideredanemissionsremovalinthemanufacturer’sreportingsoitsreleasefollowingureafertilizationonsoilsisincludedinthefarm‐scaleentityreporting.IfmanufacturersdonotestimateCO2captureduringureaproductionandincludetherecapturedCO2asanemission,thereisnoneedforafarm‐scaleentitytoreportrelease.

TheTier1methodhasbeenadoptedfromtheIPCC(deKleinetal.,2006).Noothermethodshavebeendevelopedortestedsufficientlyforanoperationalsystem.

ThemodeltoestimateCO2emissionsfromureaapplicationhasbeenadoptedfromthemethodologydevelopedbytheIPCCandusestheIPCCdefaultemissionfactor(deKleinetal.,2006).ThefollowingequationisusedtoestimatetheCO2emissionfromalandparcelwhereurea‐basedfertilizershavebeenapplied:

MethodforEstimatingCO2 EmissionsfromUreaFertilizerApplication

ThismethodusesIPCCequationandemissionfactorsdevelopedbydeKleinetal.(2006). ThismethodusesentityspecificannualparceldataasinputintotheIPCCequation(e.g.,

theamountofureafertilizerappliedtosoils). ThismethodassumesthatthesourceofCO2usedtomanufactureureaisfossilfuelCO2

capturedduringNH3manufacture.

Thismethodrequiresdataontheamountofureafertilizerappliedtosoils.

3.5.9.5 ModelOutput

Modeloutputisgeneratedonbothanabsolutequantityofemissionsandemissionsintensity.Thelatterisbasedontheamountofemissionsperunitofyieldforcropsincroplandsystemsoranimalproductsingrazingsystems.Theemissionsintensityisestimatedwiththefollowingequation:

Yieldsarebasedonthetotalamountofproductfromthelandmanagedwithureaapplication.

Urea(CO(NH2)2)isconvertedintoammoniumandCO2inthepresenceofwaterandtheenzymeurease.TheCO2willdissolveinwatertoformcarbonate,bicarbonate,andcarbonicacidasafunctionofsoilpHandtemperature.Someofthebicarbonatemaybetransferredtogroundwater,waterways,andeventuallytheocean,andthereforereducetheCO2emissionstotheatmosphere(deKleinetal.,2006;Hamiltonetal.,2007)).However,thereisinsufficientinformationavailabletoincludethispossibilityintheureamethod,soitisassumedthatanyincreaseinbicarbonatewillleadtoproductionofCO2.

Equation3‐28:CO2 EmissionsfromUreaFertilization

CUrea=M×EF×CO2MW

Where:

CUrea =Annualreleaseofcarbonfromureaaddedtosoil(metrictonsCO2‐eqyear‐1)

M =Annualamountofureafertilization(metrictonsureayear‐1)

EF =Emissionfactororproportionofcarboninurea,0.20 (metrictonC(metrictonurea)‐1)

CO2MW=RatioofmolecularweightofCO2tocarbon(44/12) (metrictonsCO2(metrictonsC)‐1)

Equation3‐29:EmissionsIntensityfromUreaFertilization

EIUrea=CUrea/Y

Where:

EIUrea=Emissionsintensity(metrictonsCO2permetrictondrymattercropyield,metrictonsCO2perkgcarcassyield,metrictonsCO2perkgfluidmilkyield)

CUrea =Annualchangeinsoilcarbonstocksduetoureaapplication(metrictonsCO2year‐1)

Uncertaintyisassumedtobeminorforthemanagementactivitydataprovidedbytheentity,althoughthismaynotbethecaseifthereislimitedknowledgeaboutlandusehistoryforindividualparcels.Uncertaintymayalsoexistintheemissionfactor,assumingthatsomeofthebicarbonateisnotconvertedtoCO2.However,themethodassumesallCO2isemittedbecauseuncertaintyestimatesarenotavailableforthisemissionfactor.Therefore,nouncertaintyisestimatedforthissourceofGHGemissionsbasedonthisconservativeassumptionthatallCO2isemitted.

3.6 SummaryofResearchGapsforCropandGrazingLandManagement

ThissectiondiscussesresearchgapsassociatedwithcroplandandgrazinglandmanagementimpactsonsoilcarbonstockchangesandGHGemissions.Thelistisnotnecessarilyexhaustive,buthighlightssomekeygapsthatwillneedfurtherresearchbeforethereissufficientevidenceforadditionalcriteriatobeincludedinthemethodology.Ingeneral,themajorityofpriorexperimentaleffortshavefocusedoncomponentsofGHGs,butfewstudieshavebeenconductedontotalGHGbudgetstoincludeCO2,N2O,andCH4incombination,whichisneededtoquantifyinteractingeffectsonthenetemissionsofthesegases(Liebigetal.,2010).Inaddition,limitedresearchhasbeenconductedtoaddresstheinfluenceofcatastrophicweathereventsonGHGemissions,suchasmajorfloods,tornadoes,andhurricanes.

CarbonStocks:10Thefollowingprocessesandpracticesrequirefurtherstudytoimprovethefundamentalunderstandingorfilldatagapsinthecarboninventorymethods.Inparticular,deficienciesinunderstandingcontinuetounderminethedevelopmentofrobustestimatesofnetGHGemissionsinrangelandsandpastures.Suchdeficienciesstemfromalackofmeasurementsacrossthemajorgrasslandecoregions,aswellaslimitationsassociatedwithbasicunderstandingofmechanisticprocessesrelatedtoGHGfluxes.Therearealsomajorgapswithrespecttoagroforestry,woodyplantencroachment,andperennialwoodycropsystems.

Moredataonallometricrelationshipsforagroforestry,woodyplantencroachment,andperennialwoodycropsystems,suchasorchards.

Improvedabilitytoquantifytheinfluenceofagroforestry,woodyplantencroachment,andperennialwoodycropsonsoilorganiccarbonstocks,includingoptimaldensityoftrees,thetypeoftrees,andthelandscapepositionofsilvopasturesystems.

Improvedmechanisticunderstandingandabilitytoquantifythefateofcarbonwithtransportandsedimentationfollowingerosionevents.

Fieldestimatesoftheamountofcarbonaddedtosoilsthroughdynamicreplacementonerodiblelands.

Improvedmechanisticunderstandingofcarbondynamicsinthesubsoilhorizons. Furtherstudyontheeffectofirrigationonplantproductionanddecompositiontoquantify

theneteffectonsoilorganiccarbonstocks. Furtherresearchonthevariationintypesandresidencetimesofbiocharamendments,in

additiontobiocharimpactonotherGHGemissions,primingofsoilorganicmatterdecomposition,andtheoverallphysicalbreakdownanddisintegrationofbiocharovertime(Jafféetal.,2013).

Dataonlong‐termresponsesofsoilorganiccarbontovariationinstockingrate,grazingmethod(i.e.,continuous,rotational,short‐durationrotational,andultra‐highstockingdensity),andvegetationcomposition(i.e.,forbandgrassmixtures,cool‐andwarm‐seasongrassmixtures,grassandlegumemixtures,grassandwoodymixtures,andplantarchitecturetypes),andwhethertheseresponsesaremediatedbydifferentsoilstypes,climaticconditions,botanicalcomposition,grazingmethodused,fertilizerregime,etc.

10Exceptagroforestrycarbonstockchanges,whicharecoveredlaterinthissection.

FurtherstudytoaddressmitigationofGHGsinaridrangelands,particularlyinshrublands,includinginteractionsbetweenmanagementandenvironmentalconditions(Ingrametal.,2008).Additionaldatacollectionandmodelimprovementarealsoneededinaridrangelands,asuncertaintyisextremelylargeforthesoilcarbonsequestrationestimatesassociatedwithreducedstockingratesandseedingoflegumes(Brownetal.,2010;Brown,2010).OurbasicknowledgeofcarbonsequestrationandGHGmitigationinaridandsemiaridenvironmentsislimited,andtheeffectofmanagementisrelativelyunderstudied.

Needforlife‐cycleassessmentofgrazingsystemswithparticularattentiontobalanceofsoilorganiccarbon,N2Oemissionsfromsoil,andCH4emissionsfromruminantsandsoil,dependingonstockingrate,stockingmethod,foragetypeassociatedwithqualityofintake,andenvironmentalconditionsofgrazingsystem.

DatafromadaptivemanagementapproachestoinformunderstandingofsoilorganiccarbonsequestrationandGHGemissionsunderdifferentgrazingmanagementstrategies.Thisapproachcouldhelpstrengthenconservation‐orientedprogramstoobtaingreaterimpactforreducingGHGemissionsandsequesteringsoilorganicC.

Additionalfieldexperimentsanddataonsoilcarbonemissionsresultingfromthecombinedapplicationoflimeandnitrogenfertilizers.

SoilNitrousOxideEmissions:Thefollowingpracticeshave,insomestudies,significantlyaffectedN2Oemissions,butrequireadditionalresearchinside‐by‐sidecomparisonstudiesacrossdifferentsoiltypesandclimate,especiallyforextensivelygrownrowcropsthatreceivehighlevelsofnitrogenfertilizers(cornandwheatinparticular):

EffectsofsplitordelayednitrogenapplicationsonloweringN2OfluxesandonincreasingNUEtoprovideequivalentyieldsatlowertotalnitrogeninput.

Capacityofspatiallyprecisefertilizerapplicationtechnology(variablerateapplicators)tolowerN2Ofluxes(bothdirectandindirect)andincreaseNUE.

Effectsofbandednitrogenfertilizerapplications,showninsomestudiestoincreaseNUEandinotherstoincreaseN2Oemissions.

ThegeneralizabilityofhigherN2OEFsandnitratelossatnitrogenfertilizerratesgreaterthancropneeds(i.e.,atratesgreaterthanthoserecommendedbyMaximumReturntoNitrogenapproaches).

ThegeneralizabilityofdifferentfertilizerformulationsonN2Oemissions,inparticularforureavs.anhydrousammoniavs.injectedsolutions.

Thegeneralizabilityofcoatedfertilizerssuchaspolymercoatedurea,ureaseinhibitors,biocharadditions,andnitrificationinhibitorsforloweringN2Oemissionsandnitrateloss.

MoreresearchontheresponsesofsoilN2Oemissionstovariationsinstockingrates,grazingmethods(continuous,rotational,short‐durationrotational,andultra‐highstockingdensity),andvegetationcomposition(forbandgrassmixtures,cool‐andwarm‐seasongrassmixtures,grassandlegumemixtures,grassandwoodymixtures,andplantarchitecturetypes),bothindividuallyandincombinations.

ThepotentialformobilewaterandsheltersourcesinpasturestoreduceN2Oemissionsbyallowingforamoreevendistributionofmanure.

InfluenceofcropresidueharvestingonN2Oemissions,aswellassoilorganiccarbonstocks,giventheinterestinusingcropresiduesasafeedstockforbioenergyproduction.

InfluenceofcovercropsonN2Oemissions,includingeffectsofplanttype(e.g.,legumevs.nonlegume)andresiduemanagement(e.g.,harvestedvs.incorporated).

InfluenceofmanureandcompostonN2Oemissionsinsofaraseffectsmaydifferfromsyntheticnitrogeninputswithrespecttorate,timing,placement,andformoforganicnitrogenadded(e.g.,liquidvs.drymanurevs.compostwithdifferentC:Nratios).

ImprovedquantificationofspatialandtemporalvariationofN2Oemissionsindifferentcroppingsystemsandlandscapestoprovideamoreaccurateassessmentofseasonalandannualemissionsacrosswholefields.

Improvedestimatesofindirectemissions,andinparticularthepercentageofnitrogenthatislostfromafieldthroughvolatilizationorleaching/runoff,andlaterconvertedtoN2Oindownstreamanddownwindecosystems.AdditionalstudyonpracticesthatcanreducenitratelossesaswellaspracticesthatcanreduceNH3andNOxlosses.

ResearchisalsoneededtoimprovemodelingandempiricalquantificationofsoilN2OemissionsinordertoprovideestimatesofN2Ofluxesthatintegrateacrossmultiplemanagementpracticessimultaneously:

FurtherdevelopmentandvalidationofquantitativesimulationmodelscapableofaccuratelypredictingN2Ofluxesinresponsetodifferingmanagementpractices,withparticularrespecttorate,timing,placement,andformulationofaddedfertilizers,bothsyntheticandorganic;tillagetypeandintensity;andresiduemanagement.

MoredataregardingseasonalandannualN2Oemissions,includingemissionsduringthenon‐growingseasonandinparticularwinterandfreeze‐thawperiods.

BetterknowledgeoffluxesacrossallLandResourceRegions(LRRs)concentratedespeciallyinthoseareasandcroppingandgrazedsystemsexpectedtocontributemosttolocalandregionalN2Ofluxes,withside‐by‐sidecomparisonsofdifferentmanagementpractices.

DevelopmentofstandardizedmethodologiesandcreationofnewtechnologiesforrapidassessmentofN2Ofluxesinthefield.

AnimprovedunderstandingofthesourcesofN2Oincroppedsoils(e.g.,nitrificationvs.denitrification)andconsequencesforfeedbacksamongadaptivemanagement,soilphysicalandbiologicalattributes,andSOCdynamics.

DevelopmentofasetofgeographicallystratifiedtestsitesatwhichfactorsknowntoaffectagronomicN2Oemissionscouldbetestedinthecontextofdifferentmanagementsystems.ThiswouldprovidearobustempiricaldatasetforestablishingTier2and3models.

FloodedRiceProductionEmissions:TheprimaryresearchgapisthelimitedamountofresearchconductedintheUnitedStatesonGHGfromricesystems.Therefore,mostofthecurrentconclusionsaboutmanagementinfluencesonriceCH4emissionsarebasedonAsianstudieswherericeistransplantedasopposedtodirectseeded.ThismaybeproblematicbecausewaterismanageddifferentlyinAsiantransplantedfloodedricesystemsduringtheestablishmentperiodthaninU.S.systems.Untilrecently,nostudiesevaluatedseasonalorannualN2OemissionsfromricesystemsintheUnitedStates(Adviento‐Borbeetal.,2007;Pittelkowetal.,2013).IntheUnitedStates,muchoftheresearchonGHGemissionscomesfromLouisiana,Texas,andCalifornia.Lindau’slabconductedonstationresearchinLouisianatoevaluateCH4emissions(e.g.,Lindauetal.,1995;Lindauetal.,1998).Sass’sgroupalsoevaluatedCH4emissionsonexperimentalstationsinTexas(e.g.,Huangetal.,1997;Sassetal.,1994).InCalifornia,variousresearchergroups(e.g.,Bossioetal.,1999;Fitzgeraldetal.,2000)havebeenconductingresearchbothonstationandoffstationandhaverecentlyalsoincludedN2Omeasurements(Adviento‐Borbeetal.,2007;Pittelkowetal.,2013).

ThefollowingpracticeshaveinsomestudiessignificantlyaffectedCH4orN2Oemissionsbutrequirefurtherside‐by‐sidecomparisonswithexperimentaldesignsacrossdifferentsoiltypesandclimateswithintheUnitedStates.

Watermanagementpractices(inparticularmidseasondrainsorintermittentirrigation)areoftensuggestedasviableoptionstomitigateCH4emissions.Whiledatasupportthisconclusion,thesemanagementpracticeshavenotbeenwidelytestedintheUnitedStates.In

studieswherethesoilhasbeendrainedduringtheseason,investigatorshavereporteddelayedcropmaturation(aproblemintemperateclimateswithrelativelyshortgrowingseasons),reducedyieldsandgrainquality,andincreasedweedanddiseasepressure.Therefore,althoughmidseasondrainageismentionedasamitigationoption,moreresearchisrequiredbeforeitisrecommendedforuseinU.S.ricesystems.

ReturningricestrawtosoiloftenresultsinincreasedCH4emissions,buttheremovalofstrawrequiresenergyandtime.Furthercompoundingtheproblemisthattherearerelativelyfewusesforricestraw.Theremovalofricestrawalsoremovesnutrientswhichwouldneedtobereplaced.Ofparticularconcernispotassium,asricestrawcontainsanaverageof1.4percentofpotassium.Therefore,itispossibletoremovemorethan100kg/haofpotassiumthroughremovalofricestraw,whichwillneedtobereplacedinordertomaintainasustainablecroppingsystem.

InCalifornia,farmerstypicallyincorporatericestrawandfloodtofacilitatestrawdecompositionduringthewinter.ThispracticeincreasesCH4emissionsfromricefieldsduringthewinterandthefollowinggrowingseason.However,ithasalsosignificantlyimprovedhabitatforoverwinteringwaterfowlinthePacificFlyway.Fitzgeraldetal.(2000)reportedthatuptohalfoftheannualCH4emissionsoccurredduringthewinterfallowperiodwhenstrawwasincorporatedandflooded.Recentstudiessuggestthat50percentmaybeahighestimateandthatfurtherresearchisneeded(Adviento‐Borbeetal.,2007;Pittelkowetal.,2013).

WhilemanystudieshaveshownvarietaldifferencesinhowmuchCH4isemitted,thesestudiesareallrelativelyoldandmanyofthevarietiesarenolongerwidelyused.Furtherresearchoncurrentvarietiesneedstobeconducted.

LimiteddataonnitrogenplacementsuggeststhatdeepplacementoffertilizerreducesCH4emissions,butmoreresearchisneededtoconfirmthefindings.

Side‐by‐sidecomparisonswithexperimentaldesignsareneededofwet‐anddry‐seededricetoevaluatetheirinfluenceonCH4andN2Oemissions.ThesearethetwomostcommonriceestablishmentpracticesintheUnitedStates.

SomestudiesfromChinasuggestthatmorecarbonissequesteredinricesystemsthaninupland(aerobic)systems,butthishasnotbeenevaluatedintheUnitedStates.

Agroforestry:Asufficientdatabasefordevelopingthemethodstoreadilymeasureand/ormodelthevariousGHGimpactsofagroforestryiscurrentlylacking.FullGHGmonitoringandaccountinginagroforestrywillrequireamixofmethodologiesfromamongtheGHGaccountingframeworksbecauseofthediversityinusesassociatedwithagroforestrysystems.Thefollowingresearchgapsarehighlighted.

Assessmentofapproachesforestimatingwoodybiomassinagroforestryplantings,whichincludescomparisonofexistingequationsandlookuptableswithagroforestry‐generatedvolumeandbiomassequationstodeterminebestapproachforestimatingcarboninthewoodybiomassofagroforestryplantings.

Developmentofeffectivestrategiesformeasuring/monitoringcarbonsequestrationandGHGemissionsinsoilandwoodycomponents.

EffectofdifferentspeciesmixturesandcombinationsofmanagementactivitiesonsoilcarbonsequestrationandminimizingtotalGHGemissions.

ImpactofmanagementoptionsandenvironmentinteractionsoncarbonsequestrationandtotalGHGemissionswithinagroforestrysystems.

Developmentoftoolsrelevanttotheinventory/measurement/estimationofthese“treesoutsideofforests.”Inaddition,testingthevalidityofcurrentcarbonaccountingtools(e.g.,DAYCENT,HOLOS)inprovidingaccurateestimatesofcarbonsequesteredinthewoodybiomassofagroforestryplantings.

Understandingsoilcarbondynamicsinagroforestrysystems,alongwiththeimpactofsoilerosion,transportanddepositiononcarbonstocks.

Developinginventorymethodologies(suchastheuseofLightDetectionandRanging)toestablishacost‐effectivenationalagroforestryinventorycompatibleforinclusionwithcurrentinventoriescontributingtoregional/nationalGHGassessments.

Developingstandardizedexperimentalprocedures,measurement,andmonitoringprotocols,suchasthosebeingdevelopedthroughtheGreenhouseGasReductionthroughAgriculturalCarbonEnhancementnetwork(GRACEnet)11toagroforestrypracticeswiththestandardizedmeasurementandmonitoringforagriculturalN2Oemissions.

MethaneOxidationinSoils:SoilCH4oxidationisknowntodecreaseby~70percentuponconversionoflongstandingnaturalvegetationtocropandpastureland(seeSection3.5.5).CH4oxidationratesforsoilsundernaturalvegetationarenotwellknownforallclimatesandsoils,soadditionalmeasurementswouldbeuseful.AswithN2O,thefurtherdevelopmentandvalidationofquantitativesimulationmodelscapableofaccuratelypredictingCH4fluxeswouldalsobehelpfulforbettergeneralizingeffectsandforfutureinclusionoffactorsthatmaybediscoveredtorestoreoxidationincroppedsoils.ThereisalsolimitedresearchontheeffectofgrazinglandmanagementonCH4oxidationalthoughvariationinstockingrates,grazingmethods,andassociatedpracticesmayhaveaninfluenceonthisprocess.

InorganicSoilCarbon:TheeffectofmanagementonsoilinorganiccarbondynamicsandexchangeofCO2withtheatmosphereisalsoinneedoffurtherresearch.ThefollowinglistisabriefsummaryofsomeofthekeygapsidentifiedforquantificationofGHGemissions:

Wheninorganiccarbonisaddedtosoilasagriculturallimeorasabreakdownproductofurea,partoftheinorganiccarbonbecomesbicarbonate.ImprovedunderstandingofthefateofthisbicarbonateindifferentsoilsandlandscapeswouldhelptobettercharacterizethepresenceandstrengthoftheresultingbicarbonateCO2sink.

ImprovedquantificationofemissionsoruptakeofatmosphericCO2withadditionofcarbonatelimestosoilswillrequiremethodstodeterminethedominanceofweatheringduetocarbonicacid(H2CO3)vs.thestrongernitricacid(HNO3)incroplandandgrazinglandsoils.

Improvedmechanisticunderstandingandquantificationofinorganiccarbondynamicsareneededinirrigatedsystems,aswellasinnonirrigatedsystems—particularlyinaridandsemiaridregions.

11GRACEnetisaresearchprograminitiatedbyUSDAAgriculturalResearchServiceto“identifyandfurtherdevelopagriculturalpracticesthatwillenhancecarbonsequestrationinsoils,promotesustainability,andprovideasoundscientificbasisforcarboncreditsandtradingprograms”(USDAARS,2013).

Appendix3‐A:SoilN2OModelingFrameworkSpecifications

SoilN2Oemissionsareestimatedusingacombinationofprocess‐basedmodeling,empiricalscalarsbasedonexperimentaldata,andscalingfactorsforpracticesinfluencingtheN2Oemissionsasrepresentedinthebaseemissionrates(Section3.5.4.1,Equations3‐8and3‐9,andTextbox3‐1).Thisappendixprovidesmoreinformationabouttheprocess‐basedmodels,inadditiontothederivationofempiricalscalarsandthepractice‐basedscalingfactors.

DAYCENTandDNDCmodelswereusedtoestimateN2Oemissionsforthetypicalfertilizerrateanda0‐levelnitrogenfertilizationrateassociatedwithmajorcropsineachUSDALRR.CropssimulatedarelistedinTable3‐A.1;baseemissionratesforothercrops(e.g.,sugarcane,millet,rye)wereestimatedusingtheTier1emissionfactor(onepercentofnitrogeninputs).Toestimateemissionfactorsfromthemodeloutput,theN2Oemissionsatthe0‐leveladditionwassubtractedfromtheN2Oemissionforthetypicalfertilizationrate.Thedifferencewasthendividedbythesyntheticagronomicnitrogeninputtoestimatetheemissionfactoratthetypicalrateoffertilization.ScalarswereusedtoscaletheN2Oemissionsforfertilizationratesthatweregreaterthanthetypicalrate.Thescalarswerederivedfromempiricaldatabasedonthechangeinemissionfactorsacrossarangeoffertilizationrates.SeeTextbox3‐1formoreinformationabouthowtheresultingemissionfactorswereusedtoestimatebaseemissionratesforthedirectsoilN2Omethod.

Meta‐analyseswereusedtoderivepractice‐basedscalingfactorsfromexperimentaldata.ThescalingfactorswereusedtoadjustthebaseemissionratesforspecificpracticesthatinfluencesoilN2Oemissions.Thescalingfactorsincludedtheeffectofnitrificationinhibitors(Sinh),slow‐releasefertilizers(SSR),pasture/range/paddockmanure(SPRP),andtillage(Still).TheresultingscalingfactorsareusedinEquation3‐9toscalethebaseemissionratesforlandparcelsmanagedwiththesepractices.

Figure3‐A.1providesanoverviewofthedecisionsandstepsinvolvedinestimatingN2Oemissionsfrommineralsoils.

Figure3‐A.1:DecisionTreeforEstimatingN2OEmissionsfromMineralSoils

3‐A.1DescriptionofProcess‐BasedModels

DAYCENT12isageneralterrestrialbiogeochemicalmodelthatsimulatescarbonandnitrogentransformationsinvolvedinprimaryproductivity,decompositionandnutrientdynamics(DelGrossoetal.,2000b;Partonetal.,2001).Themodelalsosimulatesheatandwaterfluxesverticallythroughthesoilprofile(one‐dimensional).Lateralflowofwaterisnotsimulatedexceptthatoverlandrunoffoccurswhenrainfalleventsofsufficientmagnitudeoccurgiventhepermeabilityofthesurfacesoillayer.Keysubmodelsincludeplantgrowthwithdynamiccarbonallocationamongplantcomponents,soilorganicmatterdecompositionandnutrientmineralization,andN2Oemissionsfromnitrificationanddenitrification.Plantgrowthiscontrolledbynutrientavailability,soilwaterandtemperature,andvegetationtypespecificparameterscontrollingmaximumplantgrowthrates,maximum/minimumC:Nratiosofbiomasscomponents,andphenology.Decompositionofsenescedplantmaterialandsoilorganicmatteriscontrolledbythequalityandquantityoflitterinputs,soiltexture,water,andtemperature.N2OemissionsarecontrolledbysoilNH4andNO3,watercontent,temperature,gasdiffusivity,andlabilecarbonavailability.Landmanagement/disturbanceeventssuchascultivation,waterandnutrientadditions,fire,andgrazing,canbereadilyimplementedinthemodel.ThemodelhasbeenappliedtosimulatesoilGHGfluxesatscalesrangingfromplotstoregionstotheglobe(DelGrossoetal.,2010;DelGrossoetal.,2005;DelGrossoetal.,2009).TheabilityofDAYCENTtosimulatecropyields,SOM,N2Oemissions,andNO3leachinghasbeentestedagainstavarietyoffieldexperimentsincroplandandgrasslandintheUnitedStates(DelGrossoetal.,2005;DelGrossoetal.,2008a;DelGrossoetal.,2008b).

DNDC13isaprocess‐basedbiogeochemicalmodelthatisusedtopredictplantgrowthandproduction,carbonandnitrogenbalance,andgenerationandemissionofsoil‐bornetracegasesby

12TheversionofDAYCENTcodedandparameterizedfortheU.S.NationalGHGinventory(U.S.EPA,2013)wasusedtoderiveexpectedbaseemissionrates.13DNDC9.5compiledonFeb.25,2013,wasusedtoderiveexpectedbaseemissionrates.

meansofsimulatingcarbonandnitrogendynamicsinnaturalandagriculturalecosystems(Lietal.,2000;Miehleetal.,2006;Stangetal.,2000)andforestedwetlands(Zhangetal.,2002).Themodelintegratesdecomposition,nitrification‐denitrification,photosynthesisandhydrothermalbalancewiththeecosystem.Thesecomponentsaremainlydrivenbyenvironmentalfactors,includingclimate,soil,vegetation,andmanagementpractices.ThemodelhasbeentestedandusedforestimatingGHGemissionsfromforestedecosystemsinawiderangeofclimaticregions,includingboreal,temperate,subtropical,andtropical(Kesiketal.,2006;Kieseetal.,2005;Kurbatovaetal.,2008;Lietal.,2004;Stangetal.,2000;Zhangetal.,2002),andsimilarlyforgrasslandsandcultivatedwetlands(Giltrapetal.,2010;Rafiqueetal.,2011).

Modelinputs,forbothmodels,includetheweatherdata,14soilcharacteristics,andmanagementdataforthesesimulations.Atotalof1,200samplesweredrawnforcroplandsitesimulationsandanother1,200samplesforgrasslandsitesimulations.Thesamplenumberwasoriginallydeterminedfromaplantoselectthreesoiltypesfrom20countiesdominatedbyagricultureineachof20LRRs(3x20x20=1,200).Theemissionratesthatwereproducedbybothmodelswillbeavailableonlineinsupplementarymaterialfiles.Anexampleoftheratesforcorn,winterwheat,andgrassaregiveninFigure3‐A.2.

Figure3‐A.2:ExampleofMedianBaseEmissionRatesforCorn,WinterWheat,andGrassProductioninLandResourceRegionswithCoarse,Medium,andFineTexturedSoils

Table3‐A.1providesthe2.5,50,and97.5percentilebaseemissionratesforeachcrop,LRR,andsoiltexturecombination.EmissionratesarekgN2O‐Nperhawhencropsarefertilizedattypicalnitrogenrates.

14ThemodelsusedDAYMETweatherforthecentroidofgrassland/croplandineachcounty.

Table3‐A.1BaseEmissionRate(kgN2O‐Nha‐1)PercentilesbyLandResourceRegion(LRR),Crop,andSoilTextureatTypicalNitrogenFertilizerRates

LRR Crop SoilGroupEmissionRate(25thPercentile)

EmissionRate(50thPercentile)

EmissionRate(97.5thPercentile)

A Grass Coarse 0.02 0.56 5.28A Grass Medium 0.41 1.20 3.86A Grass Fine 0.49 1.34 5.30A Tomato Coarse 0.04 1.08 4.83A Tomato Medium 0.28 1.69 8.31A Tomato Fine 0.49 2.09 15.73A Wheat,Spring Coarse 0.03 0.61 3.53A Wheat,Spring Medium 0.16 1.00 2.87A Wheat,Spring Fine 0.40 1.32 3.50A Wheat,Winter Coarse 0.05 0.55 4.00A Wheat,Winter Medium 0.19 0.91 2.99A Wheat,Winter Fine 0.35 1.21 2.77B Grass Coarse 0.01 0.40 5.25B Grass Medium 0.02 0.45 5.41B Grass Fine 0.05 0.74 8.20B Pea Coarse 0.00 0.36 2.43B Pea Medium 0.00 0.61 3.80B Pea Fine 0.02 0.53 3.02B Wheat,Spring Coarse 0.00 0.49 2.71B Wheat,Spring Medium 0.01 0.80 4.43B Wheat,Spring Fine 0.04 0.87 3.56B Wheat,Winter Coarse 0.00 0.40 2.05B Wheat,Winter Medium 0.01 0.54 3.58B Wheat,Winter Fine 0.04 0.75 3.72C Alfalfa Coarse 0.01 0.58 0.99C Alfalfa Medium 0.01 0.66 1.60C Alfalfa Fine 0.00 0.86 2.25C Corn Coarse 0.21 0.78 3.00C Corn Medium 0.27 0.93 8.23C Corn Fine 0.60 1.60 12.96C Grass Coarse 0.05 0.32 1.17C Grass Medium 0.08 0.36 1.37C Grass Fine 0.07 0.42 1.16C Rice Coarse 0.04 0.63 1.34C Rice Medium 0.03 0.70 2.19C Rice Fine 0.02 0.95 7.50C Safflower Coarse 0.17 0.89 2.86C Safflower Medium 0.38 1.15 7.46C Safflower Fine 0.56 2.09 12.92C Sunflower Coarse 0.07 0.58 2.13C Sunflower Medium 0.15 0.73 6.45C Sunflower Fine 0.29 1.37 9.16C Tomato Coarse 0.48 1.15 2.90C Tomato Medium 0.57 1.21 8.01C Tomato Fine 0.79 2.25 18.94C Wheat,Winter Coarse 0.05 0.86 1.81C Wheat,Winter Medium 0.06 0.96 3.30C Wheat,Winter Fine 0.15 1.47 5.08

D Alfalfa Coarse 0.01 0.55 1.47D Alfalfa Medium 0.01 0.49 2.91D Alfalfa Fine 0.01 0.67 4.79D Corn Coarse 0.20 0.85 2.03D Corn Medium 0.26 0.87 3.28D Corn Fine 0.30 1.32 5.99D Cotton Coarse 0.01 1.04 2.53D Cotton Medium 0.02 0.97 3.37D Cotton Fine 0.09 1.63 5.68D Grass Coarse 0.02 0.39 3.14D Grass Medium 0.02 0.46 6.27D Grass Fine 0.05 0.55 6.91D Wheat,Winter Coarse 0.00 0.35 1.27D Wheat,Winter Medium 0.00 0.36 2.21D Wheat,Winter Fine 0.04 0.56 5.10E Grass Coarse 0.01 0.46 7.35E Grass Medium 0.02 0.63 8.00E Grass Fine 0.12 0.66 5.52E Wheat,Spring Coarse 0.02 0.59 2.46E Wheat,Spring Medium 0.05 0.70 4.67E Wheat,Spring Fine 0.07 0.87 2.92E Wheat,Winter Coarse 0.02 0.39 1.97E Wheat,Winter Medium 0.06 0.53 4.80E Wheat,Winter Fine 0.10 0.63 2.89F Corn Coarse 0.28 0.76 1.57F Corn Medium 0.36 0.92 2.92F Corn Fine 0.45 1.29 4.92F Grass Coarse 0.12 0.57 2.80F Grass Medium 0.15 0.66 2.69F Grass Fine 0.16 0.80 3.52F Soybean Coarse 0.20 0.95 3.26F Soybean Medium 0.26 1.05 3.23F Soybean Fine 0.29 1.48 4.40F Wheat,Spring Coarse 0.10 0.69 1.85F Wheat,Spring Medium 0.11 0.93 2.92F Wheat,Spring Fine 0.12 1.19 4.90F Wheat,Winter Coarse 0.14 0.85 3.17F Wheat,Winter Medium 0.19 1.03 6.43F Wheat,Winter Fine 0.18 1.41 11.05G Corn Coarse 0.11 0.69 1.88G Corn Medium 0.16 0.90 3.41G Corn Fine 0.23 1.62 6.59G Grass Coarse 0.09 0.55 1.85G Grass Medium 0.09 0.54 1.92G Grass Fine 0.18 0.91 3.67G Wheat,Winter Coarse 0.08 0.49 1.64G Wheat,Winter Medium 0.09 0.64 2.05G Wheat,Winter Fine 0.10 0.91 4.43H Corn Coarse 0.31 0.92 5.62H Corn Medium 0.62 1.49 11.03H Corn Fine 0.81 2.67 20.40

H Cotton Coarse 0.14 0.70 2.28H Cotton Medium 0.18 1.17 4.38H Cotton Fine 0.41 1.55 8.88H Grass Coarse 0.30 0.88 2.53H Grass Medium 0.29 0.95 3.53H Grass Fine 0.57 1.64 4.34H Wheat,Winter Coarse 0.15 0.65 2.29H Wheat,Winter Medium 0.21 0.99 3.81H Wheat,Winter Fine 0.32 1.30 9.16I Cotton Coarse 0.25 0.63 4.38I Cotton Medium 0.23 0.63 8.15I Cotton Fine 0.34 1.27 8.70I Grass Coarse 0.36 1.02 4.24I Grass Medium 0.42 1.09 5.49I Grass Fine 0.56 1.90 5.27I Sorghum Coarse 0.34 0.78 5.69I Sorghum Medium 0.31 0.79 8.75I Sorghum Fine 0.43 1.60 9.35I Wheat,Spring Coarse 0.38 0.78 6.87I Wheat,Spring Medium 0.41 0.82 12.28I Wheat,Spring Fine 0.60 1.60 15.24I Wheat,Winter Coarse 0.19 0.43 4.66I Wheat,Winter Medium 0.20 0.58 6.57I Wheat,Winter Fine 0.22 1.06 7.75J Corn Coarse 0.48 1.10 4.33J Corn Medium 0.61 1.54 7.48J Corn Fine 0.71 2.63 17.71J Grass Coarse 0.48 1.41 3.95J Grass Medium 0.61 1.86 5.13J Grass Fine 0.69 2.41 5.77J Sorghum Coarse 0.35 0.90 3.81J Sorghum Medium 0.47 1.31 6.67J Sorghum Fine 0.52 1.96 14.66J Wheat,Spring Coarse 0.37 0.89 3.65J Wheat,Spring Medium 0.48 1.30 5.93J Wheat,Spring Fine 0.72 2.31 13.76J Wheat,Winter Coarse 0.24 0.80 3.30J Wheat,Winter Medium 0.33 1.02 5.63J Wheat,Winter Fine 0.32 1.13 11.65K Alfalfa Coarse 0.16 0.90 2.35K Alfalfa Medium 0.28 1.39 2.95K Alfalfa Fine 0.16 1.25 2.96K Corn Coarse 0.40 1.14 2.41K Corn Medium 0.72 1.75 4.57K Corn Fine 0.45 1.81 5.27K Grass Coarse 0.35 1.07 3.77K Grass Medium 0.56 1.45 4.17K Grass Fine 0.35 1.54 5.64K Soybean Coarse 0.26 0.94 2.07K Soybean Medium 0.57 1.37 2.80K Soybean Fine 0.37 1.43 3.35

K Wheat,Spring Coarse 0.35 1.04 2.33K Wheat,Spring Medium 0.77 1.65 4.58K Wheat,Spring Fine 0.46 1.79 5.19L Corn Coarse 0.41 1.42 3.31L Corn Medium 0.63 1.97 5.92L Corn Fine 1.36 3.09 15.09L Grass Coarse 0.47 1.39 6.01L Grass Medium 0.56 1.82 7.02L Grass Fine 0.63 2.08 6.61L Soybean Coarse 0.31 1.29 2.45L Soybean Medium 0.45 1.66 3.10L Soybean Fine 0.95 2.31 6.22L Wheat,Winter Coarse 0.44 1.65 3.14L Wheat,Winter Medium 0.54 1.97 3.34L Wheat,Winter Fine 1.06 2.75 8.73M Corn Coarse 0.55 1.51 4.33M Corn Medium 0.87 2.28 11.87M Corn Fine 0.99 2.76 15.46M Grass Coarse 0.49 1.31 4.06M Grass Medium 0.68 1.91 4.97M Grass Fine 0.65 1.94 5.19M Soybean Coarse 0.41 1.29 2.66M Soybean Medium 0.71 1.86 5.03M Soybean Fine 0.78 2.08 7.52M Wheat,Winter Coarse 0.55 1.62 2.91M Wheat,Winter Medium 0.85 2.16 5.17M Wheat,Winter Fine 0.84 2.45 7.72N Corn Coarse 0.60 1.48 12.11N Corn Medium 0.76 2.11 19.17N Corn Fine 1.14 2.80 32.82N Grass Coarse 0.42 1.64 3.94N Grass Medium 0.57 2.08 5.03N Grass Fine 0.91 2.61 5.95N Soybean Coarse 0.58 1.31 4.04N Soybean Medium 0.73 1.80 5.24N Soybean Fine 1.00 2.07 11.18O Corn Coarse 0.60 1.55 4.52O Corn Medium 0.67 2.14 9.63O Corn Fine 1.07 3.08 24.03O Cotton Coarse 0.51 1.19 4.95O Cotton Medium 0.61 1.84 14.76O Cotton Fine 0.99 3.24 25.42O Grass Coarse 0.39 1.70 3.92O Grass Medium 0.44 2.24 7.03O Grass Fine 0.76 2.81 7.97O Rice Coarse 0.52 1.11 5.15O Rice Medium 0.73 1.29 9.18O Rice Fine 1.00 2.45 11.14O Soybean Coarse 0.53 1.22 3.73O Soybean Medium 0.55 1.66 6.67O Soybean Fine 0.86 2.18 14.83

P Corn Coarse 0.43 0.93 4.56P Corn Medium 0.60 1.85 12.27P Corn Fine 0.76 2.23 27.80P Cotton Coarse 0.37 0.81 4.04P Cotton Medium 0.63 1.68 10.68P Cotton Fine 0.73 2.18 20.32P Grass Coarse 0.29 1.26 4.30P Grass Medium 0.41 1.95 5.44P Grass Fine 0.50 2.79 7.47P Soybean Coarse 0.36 0.80 2.98P Soybean Medium 0.56 1.65 5.62P Soybean Fine 0.67 1.72 12.55R Alfalfa Coarse 0.09 1.35 3.01R Alfalfa Medium 0.26 1.63 3.10R Alfalfa Fine 0.25 1.85 3.61R Corn Coarse 0.25 1.35 2.84R Corn Medium 0.51 1.81 4.92R Corn Fine 0.53 2.25 4.97R Grass Coarse 0.30 1.77 7.53R Grass Medium 0.49 1.96 7.25R Grass Fine 0.56 2.82 9.59R Soybean Coarse 0.20 1.24 2.69R Soybean Medium 0.45 1.62 3.06R Soybean Fine 0.41 1.95 3.80S Alfalfa Coarse 0.16 1.03 2.23S Alfalfa Medium 0.36 1.54 2.99S Alfalfa Fine 0.44 1.53 3.44S Corn Coarse 0.44 1.14 2.84S Corn Medium 0.86 1.81 6.89S Corn Fine 0.97 2.20 12.36S Grass Coarse 0.60 1.37 3.02S Grass Medium 0.77 1.85 4.99S Grass Fine 0.93 2.35 6.43S Soybean Coarse 0.39 1.04 1.66S Soybean Medium 0.77 1.59 3.48S Soybean Fine 0.89 1.78 4.72T Corn Coarse 0.45 0.92 5.78T Corn Medium 0.48 1.15 11.08T Corn Fine 0.63 2.76 24.52T Grass Coarse 0.33 1.05 4.89T Grass Medium 0.41 1.23 8.49T Grass Fine 0.50 2.32 9.65T Soybean Coarse 0.40 0.81 4.06T Soybean Medium 0.48 0.98 8.03T Soybean Fine 0.50 1.79 17.49T Wheat,Winter Coarse 0.33 0.81 4.89T Wheat,Winter Medium 0.36 1.10 8.05T Wheat,Winter Fine 0.46 2.72 17.87U Corn Coarse 0.36 0.64 2.64U Corn Medium 0.34 0.66 4.67U Corn Fine 0.47 1.18 14.76

U Grass Coarse 0.33 0.99 4.74U Grass Medium 0.35 0.79 4.09U Grass Fine 0.39 1.72 5.90U Potato Coarse 0.57 0.82 2.53U Potato Medium 0.63 1.05 13.93U Potato Fine 0.79 1.53 13.88U Wheat,Spring Coarse 0.23 0.55 2.08U Wheat,Spring Medium 0.30 0.54 5.11U Wheat,Spring Fine 0.32 0.84 10.58

3‐A.2EmpiricalScalarsforBaseEmissionRates

AsdescribedinTextbox3‐1,thebaseemissionratemodeledbyDAYCENTandDNDCisusedtocalculateanemissionfactorforthetypicalfertilizercasethatisthenscaledtoreflecttheincreaseinemissionfactorwithincreasingnitrogeninputs(SEFinTextbox3‐1).TocalculateSEFameta‐analysiswasperformedusingdatafromallfieldstudiesintheliteraturewhereatleastthreedifferentlevelsofnitrogeninput,includingazeronitrogenrate,wereappliedtothesamecropatthesamesiteduringthesamegrowingseason.EmissionfactorswerecalculatedasthedifferencebetweentheN2Ofluxesat0NandatxNdividedbytheN2Ofluxat0N.Thenullhypothesiswasthatemissionfactorswillbeconstantacrossdifferentnitrogenrates.

Atotalof44datasetsthatmeetthebasecriteriawereidentified.Fromeachdataset,slopesforeachfertilizeradditionintervalwerecalculatedandcomparedtotheslopeofthefirstinterval(0Ntothefirstnitrogenadditionlevel).Thevalueoftheslopeisameasureofhowmuchtheemissionfactorchangesperadditionalunitofnitrogenfertilizerinput(kgNha‐1)foragivenstudysiteyear.Thus,theslopemeasuresthedegreeofnonlinearityoftheemissionfactor.Theslopeiszeroiftheemissionfactorisconstant,asassumedbytheIPCCTier1method.Apositiveslopeindicatesthatthetotalemissionfunctionisconvexwithrespecttototalnitrogeninput,i.e.,thattheunitoffluxincrease(theemissionfactor)isgreaterwitheachsuccessiveunitofnitrogeninput.Uncertaintywasquantifiedwithaconfidenceintervalobtainedbyperformingabootstrapanalysis(n=100,000)ontheoriginalslopes.

Thereweresufficientdatatoanalyzefivedifferentsub‐categories:corn,grassland,othercrops,clay‐texturedsoils,andother‐texturedsoils.Themeanslopewassignificantlygreaterthanzeroforallanalyzedcategoriesbutonlythegrasslandcategorywassignificantlydifferentfromtheothers.ThusintheERbequationinTextbox3‐1therearetwovaluesforSEF,oneforgrasslandsandanotherforallothercrops.

Thestudiesusedinthemeta‐analysisareprovidedbelow.

Breitenbeck,G.A.,andJ.M.Bremner.1986.Effectsofrateanddepthoffertilizerapplicationonemissionofnitrousoxidefromsoilfertilizedwithanhydrousammonia.Biologyandfertilityofsoils,2(4):201‐204.

Cardenas,L.M.,R.Thorman,N.Ashlee,M.Butler,etal.2010.QuantifyingannualN2Oemissionfluxesfromgrazedgrasslandunderarangeofinorganicfertilisernitrogeninputs.Agriculture,EcosystemsandEnvironment,136:218‐226.

Chang,C.,C.M.Cho,andD.H.Janzen.1998.Nitrousoxideemissionfromlong‐termmanuredsoils.SoilScienceSocietyAmericaJournal,62:677‐682.

Ding,W.,Y.Cai,X.Cai,K.Yagi,etal.2007.Nitrousoxideemissionsfromanintensivelycultivatedmaize‐wheatrotationinsoilintheNorthChinaPlain.ScienceandtheTotalEnvironment,373.

Halvorson,A.D.,S.J.DelGrosso,andC.A.Reule.2008.Nitrogen,Tillage,andCropRotationEffectsonNitrousOxideEmissionsfromIrrigatedCroppingSystems.JournalofEnvironmentalQuality,37(4):1337‐1344.

Hoben,J.P.,R.J.Gehl,N.Millar,P.R.Grace,etal.2011.Nonlinearnitrousoxide(N2O)responsetonitrogenfertilizerinon‐farmcorncropsoftheUSMidwest.GlobalChangeBiology,17(2):1140‐1152.

Kaiser,E.A.,K.Kohrs,M.Kücke,E.Schnug,etal.1998.Nitrousoxidereleasefromarablesoil:importanceofN‐fertilization,cropsandtemporalvariation.SoilBiologyandBiochemistry,30:1553‐1563.

Kammann,C.,L.Grünhage,C.Müller,S.Jacobi,andH.‐J.Jäger.1998.SeasonalvariabilityandmitigationoptionsforN2Oemissionsfromdifferentlymanagedgrasslands.EnvironmentalPollution,102(S1):179‐186.

Kim,D.‐G.,G.Hernandez‐Ramirez,andD.Giltrap.2013.Linearandnonlineardependencyofdirectnitrousoxideemissionsonfertilizernitrogeninput:ameta‐analysis.Agriculture,EcosystemsandEnvironment,168:53‐65.

Letica,S.A.,C.A.M.deKlein,C.J.Hoogendoorn,R.W.Tillman,etal.2010.Short‐termmeasurementofN2Oemissionsfromsheep‐grazedpasturereceivingincreasingratesoffertilisernitrogeninOtago,NewZealand.AnimalProductionScience,50:17‐24.

Lin,S.,J.Iqbal,R.Hu,J.Wu,etal.2011.Nitrousoxideemissionsfromrapefieldasaffectedbynitrogenfertilizermanagement:acasestudyincentralChina.AtmosphericEnvironment,45:1775‐1779.

Ma,B.L.,T.Y.Wu,N.Tremblay,W.Deen,etal.2010.Nitrousoxidefluxesfromcornfields:on‐farmassessmentoftheamountandtimingofnitrogenfertilizer.GlobalChangeBiology,16(1):156‐170.

McSwiney,C.P.,andG.P.Robertson.2005.NonlinearresponseofN2Ofluxtoincrementalfertilizeradditioninacontinuousmaize(ZeamaysL.)croppingsystem.GlobalChangeBiology,11(10):1712‐1719.

Mosier,A.R.,A.D.Halvorson,C.A.Reule,andX.J.Liu.2006.NetglobalwarmingpotentialandgreenhousegasintensityinirrigatedcroppingsystemsinnortheasternColorado.JournalofEnvironmentalQuality,35(4):1584‐1598.

Signor,D.,C.E.P.Cerri,andR.Conant.2013.N2OemissionsduetonitrogenfertilizerapplicationsintworegionsofsugarcanecultivationinBrazil.EnvironmentalResearchLetters,8(1):015013.

Song,C.,andJ.Zhang.2009.Effectsofsoilmoisture,temperature,andnitrogenfertilizationonsoilrespirationandnitrousoxideemissionduringmaizegrowthperiodinnortheastChina.ActaAgriculturaeScandinavia,59:97‐106.

vanGroenigen,J.W.,G.J.Kasper,G.L.Velthof,A.vandenPol‐vanDasselar,etal.2004.Nitrousoxideemissionsfromsilagemaizefieldsunderdifferentmineralnitrogenfertilizerandslurryapplications.PlantandSoil,263.

Velthof,G.L.,O.Oenema,R.Postma,andM.L.VanBeusichem.1997.Effectsoftypeandamountofappliednitrogenfertilizeronnitrousoxidefluxesfromintensivelymanagedgrassland.NutrientCyclinginAgroecosystems,46:257‐267.

Zebarth,B.J.,P.Rochette,andD.L.Burton.2008.N2Oemissionsfromspringbarleyproductionasinfluencedbyfertilizernitrogenrate.CanadianJournalofSoilScience,88:197‐205.

Zhang,J.,andX.Han.2008.N2Oemissionfromthesemi‐aridecosystemundermineralfertilizer(ureaandsuperphosphate)andincreasedprecipitationinnorthernChina.AtmosphericEnvironment,42:291‐302.

3‐A.3Practice‐BasedScalingFactors

Datawereanalyzedtoderivescalingfactorsforthefollowingpractices:nitrogenfertilizerplacement,nitrificationinhibitors,no‐tillmanagement,andslow‐releasefertilizers.PracticeswereincludediftherewassufficientevidencefromfieldexperimentstosuggestthatthepracticeinfluencedN2Oemissions,orforwhichapreviousmeta‐analysishadbeenconductedandshownthatthepracticehadaneffectonN2Oemissions(i.e.,no‐tillmanagement;vanKesseletal.,2012).AllpracticeswerefoundtohaveasignificanteffectonN2Oemissionwiththeexceptionofnitrogenplacement.ThescalingfactorsareprovidedinTable3‐9.

Documentationfortheno‐tillscalingfactorcanbefoundinvanKesseletal.Scalingfactorsfornitrificationinhibitorswerederivedusingalinearmixed‐effectmodelingapproach(PinheiroandBates,2000),similartothemethodusedbyOgleetal.(2005)toderivefactorsthatwereusedinthe2006IPCCGuidelines(IPCC,2006).Variancesassociatedwithindividualexperimentalresultswerenottakenintoconsiderationinthemeta‐analysesbecausemanystudiesdidnotprovidethisinformation.AgoalforfutureanalysessupportingtheUSDAmethodswillbetoincludevariances,undertheassumptionthatstudieswillreportthisinformationinfuturepublications.Covariateswereincludedintheanalysistodetermineifthepracticehadadifferenteffectdependingonthelanduse,climate,soiltype,watermanagement,tillagepractice,orcroptype.Covariateswereretainedinthemodelifthevariablewassignificantatanalphalevelof0.05.Forotherscalingfactors,therewereinsufficientdatatousethelinearmixed‐effectmodelingapproach,andsoaveragedifferencesbetweenthecontrolandtreatmentswereestimatedfromthestudiestoestimateascalingfactor.Theresultingestimateswereevaluatedforstatisticalsignificantfromavalueof0(ornoeffect)usinganalphalevelof0.05.A95percentconfidenceintervalwasderivedforeachscalingfactorandprovidedinTable3‐6asanupperandlowerboundontheestimatedfactor.

Thestudiesusedineachmeta‐analysisareprovidedbelow.

NitrogenFertilizerPlacement:Burton,D.L.,X.Li,andC.A.Grant.2008.Influenceoffertilizernitrogensourceandmanagement

practiceonN2OemissionsfromtwoBlackChernozemicsoils.CanadianJournalofSoilScience,88:219‐227.

Drury,C.F.,W.D.Reynolds,C.S.Tan,T.W.Welacky,etal.2006.EmissionsofNitrousOxideandCarbonDioxide.SoilScienceSocietyofAmericaJournal,70(2):570‐581.

Engel,R.,D.L.Liang,R.Wallander,andA.Bembenek.2010.InfluenceofUreaFertilizerPlacementonNitrousOxideProductionfromaSiltLoamSoil.JournalofEnvironmentalQuality,39(1):115‐125.

Halvorson,A.D.,andS.J.DelGrosso.2013.Nitrogenplacementandsourceeffectsonnitrousoxideemissionsandyieldsofirrigatedcorn.JournalofEnvironmentalQuality,42(Inpress).

Hou,A.X.,andH.Tsuruta.2003.NitrousoxideandnitricoxidefluxesfromanuplandfieldinJapan:effectofureatype,placement,andcropresidues.NutrientCyclinginAgroecosystems,65:191‐200.

Hultgreen,G.,andP.Leduc.2003.Theeffectofnitrogenfertilizerplacement,formulation,timing,andrateongreenhousegasemissionsandagronomicperformance:AgricultureAgri‐FoodCanada,PrairieAgriculturalMachineryInstitute.

Liu,X.J.,A.R.Mosier,A.D.Halvorson,andF.S.Zhang.2005.Tillageandnitrogenapplicationeffectsonnitrousandnitricoxideemissionsfromirrigatedcornfields.PlantandSoil,276:235‐249.

Maharjan,B.,andR.T.Venterea.Inreview.Nitritedynamicsexplainfertilizermanagementeffectsonnitrousoxideemissionsinmaize.SubmittedtoSoilBiologyandBiochemistry.

Zebarth,B.J.,P.Rochette,D.L.Burton,andM.Price.2008.EffectoffertilizernitrogenmanagementonN2Oemissionsincommercialcornfields.CanadianJournalofSoilScience,88:189‐195.

NitrificationInhibitors:Akiyama,H.,H.Tsuruta,andT.Watanabe.2000.N2OandNoEmissionsfromSoilsafterthe

ApplicationofDifferentChemicalFertilizers.Chemosphere‐GlobalChangeScience,2(3):313‐320.

Ball,B.C.,K.C.Cameron,H.J.Di,andS.Moore.2012.EffectsofTramplingofaWetDairyPastureSoilonSoilPorosityandonMitigationofNitrousOxideEmissionsbyaNitrificationInhibitor,Dicyandiamide.SoilUseandManagement,28(2):194‐201.

Bremner,J.M.,G.A.Breitenbeck,andA.M.Blackmer.1981.EffectofNitrapyrinonEmissionofNitrousOxidefromSoilFertilizedwithAnhydrousAmmonia.GeophysicalResearchLetters,8(4):353‐356.

Bronson,K.F.,A.R.Mosier,andS.R.Bishnoi.1992.NitrousOxideEmissionsinIrrigatedCornasAffectedbyNitrificationInhibitors.SoilScienceSocietyofAmericaJournal,56(1):161‐165.

Cui,M.,X.C.Sun,C.X.Hu,H.J.Di,etal.2011.EffectiveMitigationofNitrateLeachingandNitrousOxideEmissionsinIntensiveVegetableProductionSystemsUsingaNitrificationInhibitor,Dicyandiamide.JournalofSoilsandSediments,11(5):722‐730.

deKlein,C.A.M.,K.C.Cameron,H.J.Di,G.Rys,etal.2011.RepeatedAnnualUseoftheNitrificationInhibitorDicyandiamide(Dcd)DoesNotAlterItsEffectivenessinReducingN2OEmissionsfromCowUrine.AnimalFeedScienceandTechnology,166‐167:480‐491.

Delgado,J.A.,andA.R.Mosier.1996.MitigationAlternativestoDecreaseNitrousOxidesEmissionsandUrea‐NitrogenLossandTheirEffectonMethaneFlux.JournalofEnvironmentalQuality,25(5):1105‐1111.

Dittert,K.,R.Bol,R.King,D.Chadwick,etal.2001.UseofanovelnitrificationinhibitortoreducenitrousoxideemissionfromN15‐labelleddairyslurryinjectedintosoil.RapidCommunicationsinMassSpectrometry,15:1291‐1296.

Dobbie,K.E.,andK.A.Smith.2003.ImpactofDifferentFormsofNFertilizeronN2OEmissionsfromIntensiveGrassland.NutrientCyclinginAgroecosystems,67(1):37‐46.

Ghosh,S.,D.Majumdar,andM.C.Jain.2003.MethaneandNitrousOxideEmissionsfromanIrrigatedRiceofNorthIndia.Chemosphere,51(3):181‐195.

Hadi,A.,O.Jumadi,K.Inubushi,andK.Yagi.2008.MitigationOptionsforN2OEmissionfromaCornFieldinKalimantan,Indonesia.Soilscienceandplantnutrition,54(4):644‐649.

Halvorson,A.D.,S.J.DelGrosso,andF.Alluvione.2010.TillageandInorganicNitrogenSourceEffectsonNitrousOxideEmissionsfromIrrigatedCroppingSystems.SoilScienceSocietyofAmericaJournal,74(2):436‐445.

Halvorson,A.D.,S.J.DelGrosso,andC.P.Jantalia.2011.NitrogenSourceEffectsonSoilNitrousOxideEmissionsfromStrip‐TillCorn.JournalofEnvironmentalQuality,40(6):1775‐1786.

Halvorson,A.D.,andS.J.D.Grosso.2012.NitrogenSourceandPlacementEffectsonSoilNitrousOxideEmissionsfromNo‐TillCorn.JournalofEnvironmentalQuality,41(5):1349‐1360.

Halvorson,A.D.,C.S.Snyder,A.D.Blaylock,andS.J.DelGrosso.Inreview.EnhancedEfficiencyNitrogenFertilizers:PotentialRoleinNitrousOxideEmissionMitigation.AgronomyJournal.

Jumadi,O.,Y.Hala,A.Muis,A.Ali,M.Palennari,K.Yagi,andK.Inubushi.2008.InfluencesofChemicalFertilizersandaNitrificationInhibitoronGreenhouseGasFluxesinaCorn(ZeaMaysL.)FieldinIndonesia.Microbesandenvironments,23(1):29‐34.

Kelly,K.B.,F.A.Phillips,andR.Baigent.2008.ImpactofdicyandiamideapplicationonnitrousoxideemissionsfromurinepatchesinnorthernVictoria,Australia.AustralianJournalofExperimentalAgriculture,48:156‐159.

Kumar,U.,M.C.Jain,H.Pathak,S.Kumar,etal.2000.NitrousOxideEmissionfromDifferentFertilizersandItsMitigationbyNitrificationInhibitorsinIrrigatedRice.Biologyandfertilityofsoils,32(6):474‐478.

Linzmeier,W.,R.Gutser,andU.Schmidhalter.2001.NitrousOxideEmissionfromSoilandfromaNitrogen‐15‐LabelledFertilizerwiththeNewNitrificationInhibitor3,4‐DimethylpyrazolePhosphate(Dmpp).Biologyandfertilityofsoils,34(2):103‐108.

Macadam,X.M.B.,A.Prado,P.Merino,J.M.Estavillo,etal.2003.Dicyandiamideand3,4‐DimethylPyrazolePhosphateDecreaseN2OEmissionsfromGrasslandbutDicyandiamideProducesDeleteriousEffectsinClover.Journalofplantphysiology,160(12):1517‐1523.

Magalhaes,A.M.T.,P.M.Chalk,andW.M.Strong.1984.EffectofNitrapyrinonNitrousOxideEmissionfromFallowSoilsFertilizedwithAnhydrousAmmonia.NutrientCyclinginAgroecosystems,5(4):411‐421.

Majumdar,D.,S.Kumar,H.Pathak,M.C.Jain,etal.2000.ReducingNitrousOxideEmissionfromanIrrigatedRiceFieldofNorthIndiawithNitrificationInhibitors.Agriculture,ecosystems&environment,81(3):163‐169.

Majumdar,D.,H.Pathak,S.Kumar,andM.C.Jain.2002.NitrousOxideEmissionfromaSandyLoamInceptisolunderIrrigatedWheatinIndiaasInfluencedbyDifferentNitrificationInhibitors.Agriculture,ecosystems&environment,91(1):283‐293.

Malla,G.,A.Bhatia,H.Pathak,S.Prasad,etal.2005.MitigatingNitrousOxideandMethaneEmissionsfromSoilinRice‐WheatSystemoftheIndo‐GangeticPlainwithNitrificationandUreaseInhibitors.Chemosphere,58(2):141‐147.

McTaggart,I.P.,H.Clayton,J.Parker,L.Swan,etal.1997.NitrousOxideEmissionsfromGrasslandandSpringBarley,FollowingNFertiliserApplicationwithandwithoutNitrificationInhibitors.Biologyandfertilityofsoils,25(3):261‐268.

Menendez,S.,P.Merino,M.Pinto,C.González‐Murua,etal.2006.3,4‐DimethylpyrazolPhosphateEffectonNitrousOxide,NitricOxide,Ammonia,andCarbonDioxideEmissionsfromGrasslands.JournalofEnvironmentalQuality,35(4):973‐981.

Merino,P.,J.M.Estavillo,L.A.Graciolli,M.Pinto,etal.2002.MitigationofN2OEmissionsfromGrasslandbyNitrificationInhibitorandActilithF2AppliedwithFertilizerandCattleSlurry.SoilUseandManagement,18(2):135‐141.

Parkin,T.B.,andJ.L.Hatfield.2010.InfluenceofNitrapyrinonN2OLossesfromSoilReceivingFall‐AppliedAnhydrousAmmonia.Agriculture,ecosystems&environment,136(1):81‐86.

Pathak,H.,A.Bhatia,S.Prasad,S.Singh,etal.2002.EmissionofNitrousOxidefromRice‐WheatSystemsofIndo‐GangeticPlainsofIndia.Environmentalmonitoringandassessment,77(2):163‐178.

Sanz‐Cobena,A.,L.Sánchez‐Martín,L.García‐Torres,andA.Vallejo.2012.GaseousemissionsofN2OandNOandNO3‐leachingfromureaappliedwithureaseandnitrificationinhibitorstoamaize(Zeamays)crop.Agriculture,ecosystems&environment,149:64‐73.

Shoji,S.,J.Delgado,A.Mosier,andY.Miura.2001.UseofControlledReleaseFertilizersandNitrificationInhibitorstoIncreaseNitrogenUseEfficiencyandtoConserveAirAndwaterQuality.CommunicationsinSoilScienceandPlantAnalysis,32(7‐8):1051‐1070.

Smith,L.C.,C.A.M.deKlein,andW.D.Catto.2008.EffectofDicyandiamideAppliedinaGranularFormonNitrousOxideEmissionsfromaGrazedDairyPastureinSouthland,NewZealand.NewZealandJournalofAgriculturalResearch,51(4):387‐396.

Vallejo,A.,L.Garcia‐Torres,J.A.Diez,A.Arce,etal.2005.ComparisonofNlosses(NO3‐,N2O,NO)fromsurfaceapplied,injectedoramended(DCD)pigslurryofanirrigatedsoilinaMediterraneanclimate.PlantandSoil,272:313‐325.

Vallejo,A.,U.M.Skiba,L.Garcia‐Torres,A.Arce,etal.2006.Nitrogenoxidesemissionfromsoilsbearingapotatocropasinfluencedbyfertilizationwithtreatedpigslurriesandcomposts.SoilBiologyandBiochemistry,38:2782‐2793.

Venterea,R.T.,B.Maharjan,andM.S.Dolan.2011.Fertilizersourceandtillageeffectsonyield‐scaledN2Oemissionsinacorncroppingsystem.JournalofEnvironmentalQuality.

Weiske,A.,G.Benckiser,andJ.C.G.Ottow.2001.EffectoftheNewNitrificationInhibitorDmppinComparisontoDcdonNitrousOxide(N2O)EmissionsandMethane(CH4)OxidationDuring3YearsofRepeatedApplicationsinFieldExperiments.NutrientCyclinginAgroecosystems,60(1):57‐64.

Zaman,M.,M.L.Nguyen,J.D.Blennerhassett,andB.F.Quin.2008.ReducingNH3,N2Oand‐NLossesfromaPastureSoilwithUreaseorNitrificationInhibitorsandElementalS‐AmendedNitrogenousFertilizers.Biologyandfertilityofsoils,44(5):693‐705.

Slow‐releaseFertilizers:Akiyama,H.,H.Tsuruta,andT.Watanabe.2000.N2OandNOEmissionsfromSoilsafterthe

ApplicationofDifferentChemicalFertilizers.Chemosphere‐GlobalChangeScience,2(3):313‐320.

Akiyama,H.,andH.Tsuruta.2002.EffectofchemicalfertilizerformonN2O,NOandNO2fluxesfromAndisolfield.NutrientCyclinginAgroecosystems,63:219‐230.

Burton,D.L.,X.Li,andC.A.Grant.2008.InfluenceoffertilizernitrogensourceandmanagementpracticeonN2OemissionsfromtwoBlackChernozemicsoils.CanadianJournalofSoilScience,88:219‐227.

Cheng,W.,Y.Nakajima,S.Sudo,H.Akiyama,etal.2002.N2OandNOemissionsfromafieldofChinesecabbageasinfluencedbybandapplicationofureaorcontrolled‐releaseureafertilizers.NutrientCyclinginAgroecosystems,63:231‐238.

Chu,H.Y.,Y.Hosen,andK.Yagi.2007.NO,N2O,CH4andfluxesinwinterbarleyfieldofJapaneseAndisolasaffectedbyNfertilizermanagement.SoilBiology&Biochemistry,39:330‐339.

Halvorson,A.D.,S.J.DelGrosso,andF.Alluvione.2010a.NitrogenSourceEffectsonNitrousOxideEmissionsfromIrrigatedNo‐TillCorn.JournalofEnvironmentalQuality,39(5):1554‐1562.

Halvorson,A.D.,S.J.DelGrosso,andF.Alluvione.2010b.TillageandInorganicNitrogenSourceEffectsonNitrousOxideEmissionsfromIrrigatedCroppingSystems.SoilScienceSocietyofAmericaJournal,74(2):436‐445.

Hyatt,C.R.,R.T.Venterea,C.J.Rosen,M.McNearney,etal.2010.Polymer‐CoatedUreaMaintainsPotatoYieldsandReducesNitrousOxideEmissionsinaMinnesotaLoamySand.SoilScienceSocietyofAmericaJournal,74(2):419‐428.

Ji,Y.,G.Liu,J.Ma,H.Xu,etal.2012.Effectofcontrolled‐releasefertilizeronnitrousoxideemissionfromawinterwheatfield.NutrientCyclinginAgroecosystems,94:111‐122.

Jiang,J.Y.,Z.H.Hu,W.J.Sun,andY.Huang.2010.NitrousoxideemissionsfromChinesecroplandfertilizedwitharangeofslow‐releasenitrogencompounds.AgricultureEcosystems&Environment,135:216‐225.

Jumadi,O.,Y.Hala,A.Muis,A.Ali,M.Palennari,K.Yagi,andK.Inubushi.2008.InfluencesofChemicalFertilizersandaNitrificationInhibitoronGreenhouseGasFluxesinaCorn(ZeaMaysL.)FieldinIndonesia.Microbesandenvironments,23(1):29‐34.

Maharjan,B.,R.T.Venterea,andC.Rosen.2013.FertilizerandIrrigationManagementEffectsonNitrousOxideEmissionsandNitrateLeaching.AgronomyJournal.

Nash,P.R.2010.Alternativetillageandnitrogenmanagementoptionstoincreasecropproductionandreducenitrousoxideemissionsfromclaypansoils:M.S.Thesis,UniversityofMissouri.

Shoji,S.,J.Delgado,A.Mosier,andY.Miura.2001.UseofControlledReleaseFertilizersandNitrificationInhibitorstoIncreaseNitrogenUseEfficiencyandtoConserveAirAndwaterQuality.CommunicationsinSoilScienceandPlantAnalysis,32(7‐8):1051‐1070.

Venterea,R.T.,B.Maharjan,andM.S.Dolan.2011.Fertilizersourceandtillageeffectsonyield‐scaledN2Oemissionsinacorncroppingsystem.JournalofEnvironmentalQuality.

Yan,X.Y.,Y.Hosen,andK.Yagi.2001.NitrousoxideandnitricoxideemissionsfrommaizefieldplotsasaffectedbyNfertilizertypeandapplicationmethod.Biologyandfertilityofsoils,34:297‐303.

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Appendix3‐B:GuidanceforCropsNotIncludedintheDAYCENTModel

TheDAYCENTmodelisrecommendedforuseinestimatingSoilCarbonStockChanges(Section3.5.3),andwasused(alongwiththeDNDCmodel)togeneratebaseemissionratesforEquation3‐9(SeeAppendix3‐AforadiscussionofhowmodelswereusedtoestimateN2Oemissionsfrommineralsoils).Inaddition,nitrogenmineralizedfromsoilorganicmatter(Nmin);additionalnitrogeninputsfromachangeinsoilorganicmattermineralizationduetoaland‐usechangeortillagechange(Ndmin);nitrogenmineralizationfromorganicamendments(e.g.,manure,sewagesludge,compost);andnitrogenmineralizationfromcrop,grass,andcovercropresidues(Nresid)aregeneratedbytheDAYCENTmodel.

TheDAYCENTmodelisnotusedtogenerateestimatesforallcropsgrownintheUnitedStates.TheDAYCENTmodeliscurrentlyusedtoestimateSOCstocksforthefollowingcropsandsectors:agroforestry,almond,alfalfa,windbreak,woodlot,sorghum,springwheat,winterwheat,woodlot—softwoods,woodlot—hardwoods,clover,cotton,drylandbeans,corn,oats,millet,grass‐cloverpasture,grass,peas,potato,sugarbeets,sunflower,soybean,sugarcane,peanut,tobacco,uplandrice,windbreakthree‐row,andwalnut.Thesecropsrepresent90percentofthecropsgrownintheUnitedStates,andmorecropsaretestedandaddedtotheDAYCENTmodel‐basedassessmentonaregularbasis.

However,ifanentityismanagingacropthatisnotincludedintheDAYCENTlistofcrops,the2006IPCCGuidelinesmaybeusedtoestimateemissionsorsinksforthesourceslistedabove.ThisapproachisconsistentwiththeU.S.EnvironmentalProtectionAgencyNationalInventoryReport(U.S.EnvironmentalProtectionAgency,2013),andacompletediscussionofthisalternativemethodologyinprovidedinAnnex3(Section3.12)oftheNationalInventoryReport.15Specifically,theNationalInventoryReportusesacombinationofTier1,2,and3approachestoestimatedirectandindirectN2Oemissionsandsoilchangesinagriculturalsoils.ThisreportfollowsthesameapproachforthecropsnotincludedintheDAYCENTmodelwhenestimatingsoilcarbonstockchangesanddirectN2Oemissions(SeeTable3‐B‐1).

Table3‐B‐1AlternativeMethodologiesforCropsNotIncludedintheDAYCENTModel

Source Tier1 Tier2

SoilcarbonstockchangesIPCC2006Guidelines(SeeChapter5,Section5.2.3.3)

DirectN2OemissionsfrommineralsoilsforthecropsNOTestimatedbytheDAYCENTmodel

IPCC2006Guidelineswithmanagementbasedscalingfactors(SeeSection3.5.4)

Nsmin, Notestimated

Nitrogeninputsfromorganicamendments(NmanandNcomp)

IPCC2006Guidelines(SeeChapter11Section11.2.1.1)

Nresid Equation3‐B‐1Residuenitrogen(Seebelow)

15SeeU.S.EnvironmentalProtectionAgency,NationalGHGInventoryAnnex3:http://www.epa.gov/climatechange/Downloads/ghgemissions/US‐GHG‐Inventory‐2013‐Annex‐3‐Additional‐Source‐or‐Sink‐Categories.pdf

Defaultvaluesfordrymattercontent,root:shootratioandharvestindexareprovidedinTable3‐5inSection3.5.1.2.DefaultvaluesfromtheIPCCguidelinesvaluesareprovidedinTable3‐B‐2forthenitrogencontentofabovegroundandbelowgroundresiduesinmajorcroptypesandindividualcrops.

Equation3‐B‐1:ResidueN

ForCrops:

Nresid=[((Ydm/HI)–Ydm)x(1–Rr)xNa]+[(Ydm/HI)xR:SxNb]

ForGrazingForage:

Nresid=[Ydmx(1–Fr–Rr)xNa]+[YdmxR:SxNb]

Where:

Nresid =Nitrogeninresiduesaboveandbelowgroundontheparcelofland (metrictonsNyear‐1ha‐1)

Ydm =Cropharvestorforageyield,correctedformoisturecontent (metrictonsbiomassha‐1) =YxDM

Y =Cropharvestortotalforageyield(metrictonsbiomassha‐1)

DM =Drymattercontentofharvestedbiomass(dimensionless)

Fr =Proportionofliveforageremovedbygrazinganimals(dimensionless)

Rr =Proportionofcrop/forageresidueremovedduetoharvest,burningorgrazing(dimensionless)

Na =Nitrogenfractionofabovegroundresiduebiomassforthecroporforage(dimensionless)

Nb =Nitrogenfractionofbelowgroundresiduebiomassforthecroporforage(dimensionless)

R:S =Root‐shootratio(unitless)

Table3‐B‐2:NitrogenContentofAbovegroundandBelowgroundResiduesofMajorandIndividualCrops

CropNitrogenContentof

AbovegroundResidues(kgN(kgdm)‐1)

NitrogenContentofBelowgroundResidues

(kgN(kgdm)‐1)

Majorcroptypes

Grains 0.006 0.009Beansandpulses 0.008 0.008Grass‐clovermixtures 0.025 0.016Nitrogen‐fixingforages 0.027 0.022Non‐nitrogen‐fixingforages 0.015 0.012Perennialgrasses 0.015 0.012Rootcrops,other 0.016 0.014Tubers 0.019 0.014IndividualcropsAlfalfa 0.027 0.019Barley 0.007 0.014Drybean 0.01 0.01Maize 0.006 0.007Millet 0.007 NANon‐legumehay 0.015 0.012Oats 0.007 0.008Peanut(w/pod) 0.016 NAPotato 0.019 0.014Rice 0.007 NARye 0.005 0.011Sorghum 0.007 0.006Soybean 0.008 0.008Springwheat 0.006 0.009Wheat 0.006 0.009Winterwheat 0.006 0.009Source:deKlein(2006).

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