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7.0147.014

Lecture 20: Biogeochemical CyclesLecture 20: Biogeochemical Cycles

April 1, 2007April 1, 2007

Global Nutrient Cycling - Biogeochemical Cycles

Useful Conversion Factors

1012 g = 1 teragram = 1 Tg

109 g = 1 gigaton = 1 gt

106 g = 1 metric ton = 1 tonne

Mean Residence Time (MRT)

= pool size / flux in or out (or their average) of pool

Fractional Turnover = 1 / MRT

= fraction that is removed and replaced per unit time

OCEAN

Dead OrganicMatter

MarineBiomass

Sinking

Dead OrganicMatter

Bioelementsin Solution

Uptake

Weathering

Volatile ElementsVolatile ElementsPrecipitation

Decomposition

Runoff Losses

TerrestrialBiomass

Death

Evaporation

H2O andvolatile

elementsonly

Pools (or Pools (or ““reservoirsreservoirs”” or or ““compartmentscompartments””)) FluxesFluxes

Solar Energy BudgetSolar Energy BudgetTotal From Sun 100%

(1.3 x 1021 kcal per year)

Reflected 30%

Absorbed (heat) 47%

Used in Evaporation 23%

Used in Winds & Currents 0.2%

Used in Photosynthesis 0.002%

Used toDrive theCycles

SolarRadiation

UV radiation

Absorbedby ozone

Absorbedby earth

Visiblelight

Reflected byatmosphere

Radiated byatmosphere

as heat

Greenhouseeffect

Energy In = Energy Out

Source of energy to most ecosystems on Earth is Solar Radiation

Heat radiatedby earth

The Geologic Cycle (slow!)The Geologic Cycle (slow!)Powered by Solar & Geothermal EnergyPowered by Solar & Geothermal Energy

GEOTHERMAL

Magma MetamorphicRocks

SedimentaryRocks

Soils

SurfaceRocks

Igneous(Lava)

Igneous

Melting

WeatheringErosion

UpliftingUncovering

SOLAR

Critical forCritical forDriving theDriving the

CycleCycle

eg.Ca,K,Fe,P

2

““All the rivers run into the sea;All the rivers run into the sea;yet the sea is not full.yet the sea is not full.””

- Ecclesiastes 1:7- Ecclesiastes 1:7

The Global Water Cycle

Ice33,000,000

Soil Waters122,000

Groundwater15,300,000

Atmosphere13,000

Oceans1,350,000,000

111,000 71,000

River flow40,000

40,000

385,000 425,000

Net transport toland

Reference: Schlesinger, 1997

Pools (km3)

Fluxes (km3/yr)

=

The Global Phosphorus Cycle

Dust Transport

1.0

Soils200,000

Mineable rock10,000

P- Mines

Land plants3000

60Internalcycling

12

Sediments4x109

Riverflow21

1000Internalcycling

Oceans90,000

2

Fertilizers &Detergents

Pools (1012 g P)

Fluxes (1012 g P/yr)

Geologic TimeScales

(108 years)Reference: Schlesinger, 1997

crops

Negligibleatmosphericreservoir

Global Nitrogen- Transformations

ReducedReduced

OxidizedOxidized

AnoxicAnoxicOxicOxic

Organic - N NH4+

NO2-

NO3-

N2(gas)

N2O(gas)

NO(gas)

Ammonification Nitrogen Fixation

Denitrification

(anaerobic respiration)

Nitrification

(chemosynthesis)

Assim

ilatory

Nitrate R

eduction

ReleasesEnergy

Requires EnergyRequires Energy

3

The Global Nitrogen CycleN-fixation inlightening 10

Landplants3,500

1200

Soil organic – N12,000

IndustrialN-fixation

80

Internalcycling

Groundwater

Riverflow36

8000 Internalcycling Oceans

570,000

Permanent Burial10

Atmosphere4x109

BiologicalFixation 140

Denitrification<200

Denitrification110

Fluxes (1012 g N/yr)

Pools (1012 g N)

Reference: Schlesinger, 1997

Fertilizedcrops

20

Cultivatedlegumes

40

Natural sources Human sources

Am

ount

of n

itrog

en (g

igat

ons/

year

)

160

140

120

100

80

60

40

20

0

Nitrogen Fluxes

Lightning

Biologicalfixation

Fossil fuels

Nitrogenfertilizer

Nitrogen-fixing crops

140 Gt is equivalent to10 million trucks of dry

N fertilizer

Hubbard Brook Experimental Forest, New Hampshire

Choose two similar watersheds.Document nutrient levels in soil organic matter, plants, and streams.

Figure 51.10a upper Figure 51.10a lower

Clearcut Control

Devegetate one watershed and leave the other intact.Monitor the amount of dissolved substances in streams.

4

Devegetated

Net

dis

solv

ed s

ubst

ance

(kg/

ha)

1965–66 1966–67 1967–68 1968–69 1969–70

Control

1000

800

600

400

200

0

Year

Figure 51.10b

Nutrient runoff results

NO3- K+ , Ca+

Burning fossilfuel

The Global Carbon Cycle The Global Carbon Cycle –– Simplified! Simplified!(uncertainties abound)(uncertainties abound)

Burial0.1

Ocean40,000

92 90

Atmospheric Pool750

Measuredannual

increase+3.5

Soils1500

Net destructionof vegetation

1.5

10250

GPP

R50

Rivers1

Landplants560

Pools (1015 g C)

Fluxes (1015 g C/yr)

R

+2 in vegetation ? +2 in ocean ?

6

+3.5 increase in +3.5 increase in atmatm

+7.5 anthropogenic sources+7.5 anthropogenic sources

Year

CO

2 con

cent

ratio

n (p

pm)

360

350

340

330

320

310

1960 1970 1980 1990

Atmospheric COAtmospheric CO2 2 Concentrations are increasingConcentrations are increasing

Question:Question: Summer and winter areout of phase in the N. and S.Hemisphere. Why doesn’t thiscancel out the seasonal effect?

-450 -350-400 -300 -250 -100-150-200 0-50 50Thousands of years

400

150

200

350

300

250

Atm

osph

eric

CO

2 (pp

mV)

Ice Core Data Showing Changes inIce Core Data Showing Changes inAtmospheric COAtmospheric CO22 Concentrations Concentrations

5

CO2

nutrients: N, P, etc

Biogeochemical Cycles are Coupled to Each OtherBiogeochemical Cycles are Coupled to Each Other

Biosphere II ExperimentBiosphere II Experiment

Phot

osyn

thes

is O

Phot

osyn

thes

is O

22

Cement infrastructureCement infrastructureRich soilRich soil

Ca(OH)2 + CO CO2 2 →CaCO3 + H20

Why didn’t CO2increase?

Res

pira

tion

Res

pira

tion

CO

CO 22

Phot

osyn

thes

is C

O

Phot

osyn

thes

is C

O 22 Respiration

Respiration

O O22

gasp!

Δ O2

Δ CO2Understandable

Understandable

only inonly in

hindsighthindsight

6

““Biosphere IBiosphere I”” Experiment Experiment

Popu

latio

n si

ze (b

illion

s)Po

pula

tion

size

(billi

ons)

COCO22

NN22O (310 x COO (310 x CO22))

CHCH4 4 (21x CO(21x CO22))

Population sizePopulation size

380380

340340

300300

260260

18001800

14001400

10001000

60060000 10001000500500 15001500 20002000

CO

CO

22 an

d N

and

N22O

O

ppm

Vpp

mV

Time, Calendar years (A.D.)Time, Calendar years (A.D.)

CH

CH

44 pp

bVpp

bV

66

44

22

00

Falkowski and Tchernov 2004

Take Home MessagesTake Home Messages Cycles of essential elements for life on Earth (Cycles of essential elements for life on Earth (eg.C,N,Peg.C,N,P……))

sustain life, and are mediated by lifesustain life, and are mediated by life

The cycles are tightly coupled. You canThe cycles are tightly coupled. You can’’t change one withoutt change one withoutchanging the otherschanging the others

Human activities have accelerated the global fluxes of elementsHuman activities have accelerated the global fluxes of elementsto unprecedented rates in the history of the Earth, resulting into unprecedented rates in the history of the Earth, resulting inchanging concentrations in land, atmosphere, and water.changing concentrations in land, atmosphere, and water.

The long term effects of these changes on the Earth System areThe long term effects of these changes on the Earth System arenot easily understood or predicted.not easily understood or predicted.

Many environmental problems have their origin in acceleratedMany environmental problems have their origin in acceleratedbiogeochemical fluxes of life-essential elements, which, whenbiogeochemical fluxes of life-essential elements, which, whenout of balance, become out of balance, become ‘‘pollutantspollutants’’

““Some call it pollution, we call it lifeSome call it pollution, we call it life””

•• http://http://www.youtube.com/watch?vwww.youtube.com/watch?v=7sGK=7sGKvDNdJNAvDNdJNA