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NITROGEN POLLUTION AND ITS IMPACTS

OUTLINE:

1. Background – forms and cycling 2. Pools and Sources 3. Cycling, transport dynamics and loadings from watersheds

4. Landuse and N exports

5. Management Options to reduce N 6. Effects and impacts of excess N on ecosystems Notes based on –

• Environmental Protection Agency. 2002. Nitrogen: Multiple and Regional Impacts. EPA-430-R-01-006.

• Follett, R. 2008. Chapter 2: Transformation and ransport processes of nitrogen in Agricultural Systems. In Nitrogen in the Environment: Sources, Problems, and Management.

• Passeport et al. 2013; Environmental Management

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Forms of N: Inorganic: Reduced

1. NH4+ (ammonium – g, aq, s)

2. N2 (nitrogen – g) 3. N2O (nitrous oxide – g) 4. NO (nitric oxide – g) 5. NH3 (g, aq)

Oxidized

1. NO2- (nitrite – aq)

2. NO2 (nitrogen dioxide - g) 3. NO3

- (nitrate – aq) Organic N – urea, amines, proteins and nucleic acids Tied to carbon molecules

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Reactive vs. Non-reactive species Non-reactive – Nitrogen gas N2 – atmosphere - largest N store on the planet (78%) Reactive (Nr) – • Ammonia • Ammonium • Nitrogen oxides • Nitric acid • Nitrous oxide • Organic compounds

N2 ↔Nr Concern – Anthropogenic activities increasing Nr accumulation in environmental reservoirs – and increasing impacts on these reservoirs.

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Nitrogen Pools

airlandsea

Table 1. Estimates of the active pools in the global nitrogen cycle.million tonnes N

AirN2 3 900 000 000N2O 1 400

LandPlants 15 000Animals 200

of which people 10Soil organic matter 150 000

of which microbial biomass 6 000

SeaPlants 300Animals 200In solution or suspension 1 200 000

of which NO3--N 570 000

of which NH4+-N 7 000

Dissolved N2 22 000 000

Table 1. Estimates of the active pools in the global nitrogen cycle.million tonnes N

AirN2 3 900 000 000N2O 1 400

LandPlants 15 000Animals 200

of which people 10Soil organic matter 150 000

of which microbial biomass 6 000

SeaPlants 300Animals 200In solution or suspension 1 200 000

of which NO3--N 570 000

of which NH4+-N 7 000

Dissolved N2 22 000 000

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plantsanimalssoil organic matter

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Sources Natural Sources:

1. lightning N2 + O2 → 2NO

2. biological nitrogen fixation

N2 → NH3 → amino acids → proteins Symbiotic bacteria in rhizobium/roots of plants Types of plants that host these bacteria? Other types of bacteria?

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Plants – alfalfa, clovers, peas, beans, etc. Prior to anthropogenic influences - there was a balance between BNF and denitrification which kept Nr low Anthropogenic sources

1. Burning of fossil fuels

2. Widespread cultivation of N fixing crops

3. Use of fertilizers in agriculture – synthetic and animal manures

4. Urban, suburban, rural sources – wastewater, lawn fertilizers, septic tanks, CSOs.

Human activity has had a profound effect on the N cycle. 1. Energy production - under high temperature combustion processes, unreactive nitrogen is converted to reactive nitrogen by two processes. a. Thermal NOx 1000oK N2 + O2 → 2NO

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b. Fuel NOx - the oxidation of organic N in fuels Natural gas - very low 0% Coal - up to 3%

2. Fertilizer-Most anthropogenic fertilizers are either NH3 or urea produced from NH3. This material is produced by the Haber-Bosch process. 4N2 + 12H2 = 8NH3

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Types of N fertilizers – • Ammonium nitrate • Ammonium sulfate • Calcium nitrate • Sodium nitrate • Urea [CO(NH2)2] • Urea-ammonium nitrate

3. Production of legumes and other crops allows for the conversion of N2 to reactive N by increasing biological nitrogen fixation. Legumes include: Soybeans

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Ground nuts (peanuts) Pulses (lentils) Forage (alfalfa, clover)

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1 Tg = 1012 grams Increase in anthropogenic N from 1860 to 2000 (Tg N /year) Process 1860 2000 Cultivation of legumes – BNF

15 33

Combustion 1 25 Fertilizer 0 100 Total Increase = 16 to 158 Tg N / year

0

2

4

6

1850 1870 1890 1910 1930 1950 1970 1990 20100

50

100

150

200

Population Haber Bosch C-BNF Fossil Fuel Total Nr

Global Population and Reactive Nitrogen TrendsGlobal Population and Reactive Nitrogen Trends

From Galloway et al. 2002. In review.

Hum

an P

opul

atio

n (b

illio

ns)

TgN

yr -1

Natural N Fixation

0

2

4

6

1850 1870 1890 1910 1930 1950 1970 1990 20100

50

100

150

200

Population Haber Bosch C-BNF Fossil Fuel Total Nr

Global Population and Reactive Nitrogen TrendsGlobal Population and Reactive Nitrogen Trends

From Galloway et al. 2002. In review.

Hum

an P

opul

atio

n (b

illio

ns)

TgN

yr -1

Natural N Fixation

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Major Sources1.Power plant emissions2.Vehicle emissions3. Septic tank leakage4. Manure/livestock runoff5. Agricultural emissions6.Fertilizer runoff(e.g. lawns and fields)7. Wastewater effluent8. Food and feed imports

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What happens to inputs of Nr? • Denitrification (loss as N2 gas) • Biomass storage (crops, forests, animals) • Soil storage • Groundwater storage • Exported to streams/estuaries • Exported in food and feed

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Cycling, Transport Dynamics and N loadings from

watersheds

• Nitrate – high mobility • Ammonium – low mobility, fixation with clay particles in

subsoils • Organic N – moderate mobility • Particulate versus dissolved species (?)

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Key N Cycle Processes: Mineralization & Immobilization: • Mineralization: Conversion of organic N into inorganic N

forms (NH4+, NO3

-) – via processes like decomposition -- organic N forms decompose and form ammonium first.

• 1.5 to 3.5 % of the organic N may mineralize annually

• Immobilization: Conversion of inorganic N forms to

organic N forms (via processes like plant & biological uptake) o NH4+ preferably immobilized compared to NO3-

The carbon and nitrogen pool ratios in soils and implications for mineralization and immobilization

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P

C

NC

ORGANICPOOLS

NH4

NO3P

INORGANIC POOLS

LITTER

HUMUS

mineralization

immobilization

N P

C – carbonN – nitrogenP - phosphorus NH4- ammonium

NO3 - nitrate

Organic & Inorganic C, N, P, pools in soil

decomposition

P

C

NC

ORGANICPOOLS

NH4

NO3P

INORGANIC POOLS

LITTER

HUMUS

mineralization

immobilization

N P

C – carbonN – nitrogenP - phosphorus NH4- ammonium

NO3 - nitrate

Organic & Inorganic C, N, P, pools in soil

decomposition

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Fixation/Sorption by Clay Minerals • Positively charged ammonium ions are adsorbed on the

negatively charged clay & humus sites – remember the CEC!

• Because of the adsorption of ammonium to clay particles

ammonium is the less mobile form of N --- nitrate (negatively charged) is more mobile that ammonium.

• Thus soils with higher clay content will retain or tie-up

more ammonium (compared to sandy soils)

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• Clay type (e.g., vermiculite) will also influence this fixation

Ammonia Volatilization

• Conversion of ammonium from the soil to ammonia gas – NH3

• Typical sites for occurrence – locations where animal

manures are spread on land.

• Moist and high pH conditions increase the rate of volatilization.

******Nitrification • Conversion of Ammonium Nitrate • Performed by bacteria classed as autotrophs – because they

obtain their energy from the oxidation of ammonium ions. • Two types of bacteria involved – Nitrosomonas and

Nitrobacter – two stage process of conversion • Ammonium Nitrite Nitrate

• Nitrification reaction results in the release of hydrogen ions

– which means nitrification increases soil acidity!

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• Factors that regulate Nitrification:

o Availability of ammonium o Aeration – nitrifying bacteria are aerobic and need O2 o Moisture – nitrification is retarded at high soil

moisture conditions (poor aeration) o Carbon – don’t need carbon as an energy source o Temperature – optimum 20C to 30C. o Exchangeable base forming cations: Nitrification

proceeds at a higher rate in soils that have an abundance of exchangeable base forming cations like Ca2+ and Mg2+

o Clay types: (e.g., smectite & allophane) Clays that stabilize organic matter and hold ammonium ions tightly can result in reduced rates of nitrification

o Pesticides & nitrification inhibitors Denitrification

• Process of conversion of nitrate NO3- to nitrogen gas N2 • Typically occurs in saturated soils – like wetlands, riparian

soils, flooded rice fields,……. • Bacteria involved – are heterotrophs – extract their energy

from carbon (as opposed to autotrophs) • Conditions for Denitrification:

o Nitrate availability o Energy source – Carbon

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o Anaerobic conditions – soil air less than 10% O2 or less that 0.2 mg/L

o Temperature between 2 and 50C – optimum is between 25 and 35C.

o Acidity – very low acidity (pH < 5.0) can inhibit denitrification

• Denitrification can occur in large saturated expanses of soil

or even in small pockets of saturation called micro-sites • Denitrification rates could vary from 5 to 15 kgN/ha/yr for

agricultural fields to as high as 30 to 60 kgN/ha/yr for wetlands, saturated soils receiving high N loads or riparian soils.

• During a year - denitrification losses will be high when

there is sufficient moisture in the soil and the soil is warm.

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Hydrologic delivery processes:

1. Precipitation 2. Surface runoff 3. Sediment and soil associated 4. Vertical drainage, leaching 5. Groundwater flow

N flux = (N concentration * water flux)

Surface & sediment

drainage

Groundwater flow

Surface & sediment

drainage

Groundwater flow

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Primary pathways of transport for various N forms

Either in dissolved or particulate forms Dissolved < 0.45 or 0.7 micron Particulate > 0.45 or 0.7 micron Surface runoff – DON, diss-NH4

+, NO3-

Vertical drainage - DON, diss-NH4+, NO3

- Near subsurface flow - DON, diss-NH4

+, NO3-

Groundwater flow - DON, diss-NH4+, NO3

- Sediment - PON, particulate-NH4

+ DON – dissolved organic N PON – particulate organic N Concentration of nitrogen forms in runoff will depend on sources and the intersection of those sources by hydrologic flow paths

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Storm event patterns of N across various seasons – how they relate to sources and hydrologic flow paths Trajectories are dictated by – • Where the N source is located in soil profile (near surface

versus deep) • Which hydrologic flow path intersects this source

NO3

NH4

DON

Typical solute trajectories during a summer rainfall event

NO3

NH4

DON

Typical solute trajectories during a summer rainfall event

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PN export with storms – • PN exports increases with discharge and typically peaks on

the rising limb of the hydrograph. • Large storms can contribute large PN exports

NO3

NH4

DON

Possible solute trajectories during a snowmelt event in a forestedwatershed

NO3

NH4

DON

Possible solute trajectories during a snowmelt event in a forestedwatershed

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Inamdar et. al., 2015. – • Irene in 59 hours exported 1/3rd the annual N runoff export

from a 12 ha watershed. • 87% of that N export from TS Irene was in particulate

form!

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N exports with landuse

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Forested watersheds: • Primary inputs – atmospheric deposition (nitrate,

ammonium) • Atmospheric deposition for Newark region ~ 3-4 kgN/ha/yr • Primary N species exported – nitrate and DON; PN could

be high during large storms or watersheds with significant erosion and sediment export

• Inorganic N species constitute 50 to 80% • In some undisturbed tropical forested watersheds – DON is

the predominant form of N (could be as high as 60-80%) • Nitrate-N concentrations less than 1.0 mg/L • Exports occur with surface as well as with groundwater

flow o DON – surface waters o Nitrate – groundwaters

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N concentrations (ueq/L)

0

10

20

30

40

50

60

70

1/1/1995 1/1/1996 12/31/1996 12/31/1997 12/31/1998

conc

entra

tion

(ueq

/L)

nitrate N DON ammonium

0

10

20

30

40

50

60

70

nh4 no3 don

N species

% N

of t

otal

Adirondack forested watershed N exports

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Agricultural watersheds: • Primary inputs – fertilizers, manure, atmospheric

deposition • Poultry manure N:P = 3:1 << that required for plant uptake

~ 8:1. Manure application rate for corn ~ 214 kgN/ha/yr • N species – nitrate, ammonium, organic N • Nitrate concentrations in runoff and groundwater may be as

high as 10 mg/L • Particulate concentrations in runoff compared to dissolved

forms are high

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0.0

2.0

4.0

6.0

8.0

10.0

12.0

pre-BMP post-BMP

Ann

ual N

load

(kg/

ha)

particulatedissolved-organicnitrateammonium

52

27

165

52

17

29

2

QN1

Agricultural Watershed in Coastal Plain of Virginia

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Urban watersheds: • Primary sources – wastewater, leaking septic tanks, leaking

sewer lines, CSOs, lawn fertilizers, golf courses, atmospheric deposition

• Golf course N application rate = 173 kgN/ha/yr! • Key N species – nitrate, ammonium N

• N concentrations may vary considerably with season and

hydrologic conditions (baseflow versus stormflow)

• Runoff and N – short-circuited, bypasses natural sinks in the landscape

• Increasing concerns in regions like the Chesapeake Bay watershed.

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Management strategies for N Source Control • Emissions Control

o Reduce emissions from power utilities o transportation

• Fertilizer applications

o Reduce applications rates o Timing and location of fertilizer application o Retardants (e.g., nitrification inhibitors)

• Manure management & application

o Storage o Timing of applications o Location and amount of application

• Suburban & rural areas –

o Septic tanks o Homeowner lawn fertilizers – timing and amounts o Golf courses

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Transport & delivery controls Intercept and reduce nutrients during transport • Riparian zones, wetlands, buffer strips Ripar – beside the stream

o Denitrification of N o Vegetative uptake of N o Deposition of sediment & sediment-N o Conversion of inorganic N to N2 & organic N

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N loadings (kg/ha/yr) retained in the riparian buffer Reference Location Input Output Retention Total N Peterjohn & Correll 1984

Rhode River

83 9 74

Nitrate-N Peterjohn & Correll 1984

Rhode river

45 6.4 38.6

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Effectiveness of riparian zones as N sinks is site specific • Flow paths

o Deep versus shallow • Carbon availability/vegetation

o Why? • Vegetation growth status

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Other types of management practices for N control (notes and graphics from Passeport et al., 2013, Environmental Management)

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Urban landscape management practices • Reduce management intensity • Increase biodiversity • Enhance native species

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Effects of Excess N:

Human health and non-ecosystem effects: • Particulate matter (PM) – especially < 2.5 μm • Ozone formation (NOx triggers ozone)

o 80 ppb – for 8hrs – can cause problems • Acid deposition (deterioration of exposed structures) • Nitrate concentrations in waters (blue baby syndrome)

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Ecosystem Impacts of excess N

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N-poor terrestrial landscapes and impacts of N enrichment e.g., Ombrotrophic Bogs and carnivorous plants

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Surface water Impacts

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Eutrophication – Coastal & inland • Eutrophication – excess growth of algae (N, P) • N especially a concern for coastal water bodies – N limited. • N deposition plays a greater role

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Dead Zones Anoxic or Hypoxic waters – an increasing problem! • Hypoxic – when dissolved O2 < 2 to 3 mg/L • Anoxic – when devoid of O2 Dead zones occur in: • Gulf of Mexico • Chesapeake Bay

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• Long Island Sound • Florida Bay • Baltic Sea • Adriatic Sea Gulf of Mexico – Hypoxic zone – 20,000 sq km! – attributed to N loadings from the Mississippi River watershed.