Institute of Food and Agricultural Sciences (IFAS) Biogeochemistry of Wetlands SiScience an dAd App litilications NITROGEN

Wetland Biogeochemistry Laboratory Soil and Water Science Department University of Florida

Instructor : Patrick Inglett [email protected]

6/22/20086/22/2008 P.W.WBL Inglett1 1

Nitrogen

‰ Introduction ‰ N Forms, Distribution, Importance ‰ Basic processes of N Cycles ‰ Examples of current research ‰ Examples of applications ‰ Key points learned

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1 Nitrogen Learning Objectives

‰ Identify the forms of N in wetlands ‰ Understand the importance of N in wetlands/global processes ‰ Define the major N processes/transformations ‰ Understand the importance of microbial activity in N transformations ‰ Understand the potential regulators of N processes ‰ See the application of N cycle principles to understanding natural and man-made ecosystems

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Nitrogen Cycling

Plant biomass N

N2 NH3 N2 N2O (g) Litterfall Nitrogen Fixation Volatilization

Mineralization. Water - Nitrification + + NO3 NH4 Organic N NH4 Column

AEROBIC - Plant Peat NO3 + + [NH4 ]s uptake accretion [NH4 ]s Denitrification ANAEROBIC Microbial + Organic N Biomass N Adsorbed NH4

N2, N2O (g)

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2 Forms of Nitrogen

‰ Organic Nitrogen ‰ Inorganic Nitrogen + • Proteins • Ammonium (NH4 ) - • Amino Sugars • Nitrate N (NO3 ) - • Nucleic Acids • Nitrite N (NO2 )

• Urea • Nitrous ox ide (N2O)

• Dinitrogen (N2)

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N Transformations

Solid Gaseous Phase: Phase: Particulate N N2 + Bound: NH4 N2O - - NO3 NO2

Aqueous Phase: + NH4 DON - DIN NO3 - Particulate N NO2

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3 Reservoirs of Nitrogen

‰ Lithosphere 163,600 x 1018 g ‰ Atmosphere 3,860 x 1018 g ‰ Hydrosphere 23 x 1018 g ‰ Biosphere 0.28 x 1018 g

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Forms of Nitrogen Total Soil N

Organic Inorganic

+ Protein NH4 + Amino sugars Clay-fixed NH4 Heterocyclic N

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4 Nitrogen Transformations

1. Ammonification/Mineralization 2. Immobilization 3. Nitrification 4. Denitrification 5. Dissimilatory nitrate reduction to ammonia (DNRA) 6. Nitrogen fixation 7. Ammonia volatilization

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Transformation of N Species Organic N 2 1

+ NH4 5 8e- 7 3 NO - 3 3 3 4e- - NH2OH 4 2e - NH3 - 2e NO2 2 e- 4 2e- N2O 6 3e- 4 N2 1e-

0 +1 +2+3 +4 -3 -2 -1 +5 Oxidation Number

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5 Nitrogen Cycle

Plant/Algae

+ - Org-N NH4 NO3

N-FixersMicrobialNit rBiomassifi ers DitifiDen it rifiers

N2 N2/N2O

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Productivity

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6 Greenhouse Gas Emissions

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Nitrogen Budget

ƒ Biological N2 fixation ƒ Dry and wet deposition ƒ Non-point sources ƒ Wastewaters

ƒ Plant biomass ƒ Microbial biomass ƒ Soil organic N ƒ Porewater (DIN, DON) ƒ Exchangeable N

ƒ Clay fixed NH4-N

ƒ Volatilization ƒ Outflow ƒ Gaseous losses ƒ Plant harvest 6/22/2008 P.W. Inglett 14

7 Ammonium Ion Wet Deposition 1985-2003 1985

2003

http://nadp.sws.uiuc.edu/isopleths/ 6/22/2008 P.W. Inglett 15

Nitrate Ion Wet Deposition 1985-2003 1985

2003

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8 Inorganic N Wet Deposition 1985 1985-2003

2003

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Biological Nitrogen Fixation

Plant biomass N

N2 NH3 N2 N2O (g) Litterfall Nitrogen Fixation Volatilization

Mineralization. Water - Nitrification + + NO3 NH4 Organic N NH4 Column

AEROBIC - Plant Peat NO3 + + [NH4 ]s uptake accretion [NH4 ]s Denitrification ANAEROBIC Microbial + Organic N Biomass N Adsorbed NH4

N2, N2O (g)

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9 Nitrogen Fixation

12 -1 Type of fixation N2 fixed (10 g yr ) Non-biological Industrial about 50 Combustion about 20 Lightning about 10 Total about 80 Biological Agricultural land about 90 FtdForest and non-agriltlldicultural land abt50bout 50 Sea about 35 Total about 175

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Biological N2 Fixation

+ - N N + 8H + 8e + 16ATP → 2NH3 + H2 + 16ADP + 16Pi

Nitrogenase

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10 Biological N2 Fixation

‰ Prokaryotes

Aerobic Anaerobic Associated w/ Plants Azotobacter Clostridium Rhizobium Beijerinckia Desulfovibrio Frankia Klebsiella Cyanobacteria Azospirillum Cyanobacteria

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Biological N2 Fixation

‰ Heterocysts (cyanobacteria) ‰ Leghaemoglobin (rhizobium) ‰ Exopolysaccharides ‰ Temporal isolation

http://www.bact.wisc.edu/Microtextbook/ images/book_4/chapter_2/2-53.jpg

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11 Temporal Isolation y

Heterocystous Nitrogenase activit Non-heterocystous

12:00 24:00 12:00 Time (h)

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Biological N2 Fixation Regulators?

‰ Light ‰ Concentration of Oxygen ‰ Growth limitation ‰ Macro nutrients (C, P) ‰ Co-factors (Fe, Mo)

Inc. Growth Inc. N

Demand Species Inc. N2 shift Fixation

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12 Nitrogen Mineralization

Plant biomass N

N2 NH3 N2 N2O (g) Litterfall Nitrogen Fixation Volatilization

Mineralization. Water - Nitrification + + NO3 NH4 Organic N NH4 Column

AEROBIC - Plant Peat NO3 + + [NH4 ]s uptake accretion [NH4 ]s Denitrification ANAEROBIC Microbial + Organic N Biomass N Adsorbed NH4

N2, N2O (g)

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Organic Nitrogen

‰ Proteins ƒ Amino acids (30-50% of total organic N) ‰ Nucleic Acids ƒ 2-5 % of total organic N ‰ Amino Sugars ƒ 3-13% of total organic N ‰ Urea

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13 Protein Decomposition

Proteins Peptides AiAmino ac ids

Arg Pro ProteasesPro Peptidases Arg Ala Ala Asp

Phe Phe Asp Lys Pro Lys

Pro

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H R1 O H R2 O H R3 O N -C -C -OH N -C -C -OH N -C -C -OH H H H H H H

-2H2O

H R1 O R2 O R3 O N -C -C - N -C -C - N -C -C - OH H H H HHH

Peptide Bonds

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14 H R1 O H R2 O H R3 O N -C -C -OH N -C -C -OH N -C -C -OH H H H H H H

+ 2H2O

H R1 O R2 O R3 O N -C -C - N -C -C - N -C -C - OH H H H H H H

Peptide Bonds

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Detrital Matter Enzyme Leaching Complex Polymers Hydrolysis Monomers Cellulose, Hemicellulose, Sugars, Amino Acids Proteins, Lipids, Waxes, Lignin Fatty Acids

Uptake

Bacterial Cell

Electron Acceptors: Amino acids - 4+ O2, NO3 , Mn , 3+ 2- Fe , SO4 , CO2

Products: + NH4 CO2, H2O, Energy + 2+ NH4 , Mn , ATP 2+ 2- Fe ,S ,CH4

+ NH4 6/22/2008 P.W. Inglett 30

15 Urea Decomposition

NH2 Urease C = O + H2O CO2 + 2NH3

NH2

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Urea Decomposition

Urease activity (14C Tracer)

[Thorén (2007)] 6/22/2008 P.W. Inglett 32

16 Ammonification [Mineralization] Organic N Ammonium N [Brea kdown of O rgani c M att er]

Immobilization

Inorganic N Organic N

(NH4-N and NO3 -N) [Buildup of Organic Matter]

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Ammonification/NAmmonification/N--MineralizationMineralization

Organic N Ammonium R

- HOOCC NH 2 NH4 + H

NN--ImmobilizationImmobilization

Inorganic N Organic N R NH 4 - + HOOCC NH 2 - NO3 H

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17 Microbial Biomass C & N

[Everglades[Everglades--WCAWCA--2A]2A]

y = 10.7 x + 2.2 25 R2 = 0.78

20

15 ) -1 10

5 (g C kg robial biomass C

c 0

Mi 00.51.0 1.5 2.0

Microbial biomass N (g N kg-1)

White and Reddy, 2000

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Decomposition ‰Microbial Biomass ƒ C/N ratio = 10 ƒ Effic iency o f car bon ass im ilati on Aerobic = 20-60% Anaerobic = 10% ‰Plant Detritus [100 units] ƒ 40% carbon (dry weight basis) = 40 units C ƒ Amount of C assimilated during decomposition Aerobic = 16 units [40% efficiency] Anaerobic = 4 units [10% efficiency]

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18 Carbon/Nitrogen Ratio

Nitrogen Demand? ƒ Micro bial Biomass C/N ra tio = 10 Aerobic = 1.6 units Anaerobic = 0.4 units Critical Plant Detritus Nitrogen? Aerobic = 1.6 % Anaerobic = 0.4% Critical Carbon/Nitrogen Ratio? Aerobic = [40% C]/[1.6% N] = 25 Anaerobic = [40% C]/[0.4% N] = 100

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Carbon/Nitrogen Ratio

Mineralization [M] vs. Immobilization [I]

‰ Aerobic ƒ I > M = C/N ratio > 25 ƒ M > I = C/N ratio < 25 ‰ Anaerobic ƒ I > M = C/N ra tio > 100 ƒ M > I = C/N ratio < 100

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19 N Mineralization/Immobilization [Aerobic]

ase C/N = 15 e

C/N = 35

organic N Rel C/N = 100 n I

Decomposition (days)

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N Mineralization/Immobilization [Aerobic] 40 Immobilization Ratio

n 30

20 Mineralization

rbon/Nitroge 10 a

C C/N ratio of soil humus/ microbial biomass 0 0 150 Decomposition Period [days]

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20 Regulators of Ammonification

Substrate quality ‰ C/N ratio of the substrate ‰ Secondary compounds ‰ N forms (soluble vs. structural compounds)

Microbial Activity ‰ Extracellular enzyme activity ‰ Supply of electron acceptors (Redox) ‰ Temperature ‰ pH

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Mineralizable Nitrogen [Everglades[Everglades--WCAWCA--2A]2A]

400 y = 0.317 x + 52.4 R2 = 0.76

) 300 -1 d -1 200

(mg N kg 100 ineralization Rate M

0 0 100 200 300 400

+ -1 Extractable NH4 (mg N kg )

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21 Fate of Ammonium

Plant biomass N

N2 NH3 N2 N2O (g) Litterfall Nitrogen Fixation Volatilization

Mineralization. Water - Nitrification + + NO3 NH4 Organic N NH4 Column

AEROBIC - Plant Peat NO3 + + [NH4 ]s uptake accretion [NH4 ]s Denitrification ANAEROBIC Microbial + Organic N Biomass N Adsorbed NH4

N2, N2O (g)

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Nitrogen Uptake by Rice (15N)

TtlNTotal N upt tkake

Added 15N uptake

Nitrogen uptake N-uptak e fr om Soil organic N

Growing season

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22 Ammonium AdsorptionAdsorption-- Aerobic

Ca2+ NH + 4 + 2+ - K Mg - 2+ - - Ca + - - NH4 2+ - Ca K+ - - - + - - K K+ - - Ca2+ NH + - 4 Mg2+ - - - NH + - - Mg2+ 4 - Ca2+ - - - K+ Mg2+ - - Ca2+ K+ K+ Dissolved [Pore water] Adsorbed [Solids]

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Ammonium AdsorptionAdsorption-- Anaerobic

Ca2+ Fe2+ Mn2+ K+ Mg2+ - - Fe2+ - - NH + - - 4 Ca2+ - 2+ K+ - Mn - - - - K+ 2+ 2+ Mn - - + Mn Mg2+ - NH4 - - - Fe2+ + - - Mg2+ NH4 - 2+ Ca Fe2+ - - - K+ Mg2+ -- Ca2+ K+ Dissolved Fe2+ [Pore water] Adsorbed [Solids]

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23 6/22/2008 P.W. Inglett 47

Ammonium Fixation Illite -Clay- Clay Mineral

Ca2+ K+ - - + Si-O layer Mg2+ 2+ NH+4 - + + - Mg - NH- 4 K- NH- 4 + Al-O-OH layer - Ca2+ - K - - --Si-O layer

+ Interlayer Space NH4 - + - NH4 - Mg2+ NH- + - K+ NH + + - 4 - - 4 NH4 - - + - - NH4 Fixed - - Ammonium NH + - Interlayer Space - 4 - + - Mg2+ NH- + K+ + NH + NH4 - 4 - NH4 - 4 - NH + - - - 4 -- 2+ Mg + NH + K 4 Mg2+ K+ Ca2+

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24 Adsorption/Desorption Equilibrium

+ NH4 NH + + 4 NH4

Soil NH + + + 4 NH4 NH4

+ + NH4 NH4 + NH4

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Adsorption/Desorption Equilibrium

Fixed Exchangeable Solution

+ NH4 NH + + 4 NH4 + NH4

NH + + 4 NH4

+ NH4 + + NH4 NH4

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25 S = K × C+ S0 + + + NH4 ads = K ×[NH4 ] + NH4 fixed

(()+) K, Partition Coefficient ) -1

E NH4-C0 (mg N kg monium Adsorbed m A S0

(-) Porewater ammonium (mg N l-1)

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Ammonium Adsorption -Salinity

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26 Increasing salinity

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Ammonia Flux

NH3(g) (Volatilization)

Photosynthesis

O2 + CH2OCO2 + H2O Respiration NH3(g) + 2- 2H + CO3 CO2 + H2O

+ + NH3(aq) + H NH4 Water

Soil + NH4 (adsorbed)

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27 Diel pH changes in the Water Column

10 System with high photosynthesis 9 pH System with low 8 photosynthesis

7

6 12:00 24:00 12:00 Time (h)

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Ammonia Volatilization (assuming high pH)

Floodwater depth Plant density SAV/Periphyton Density

Temperature Wind speed Soil CEC

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28 Ammonia Volatilization

+ + ‰ NH4 =NH= NH3 +H+ H ‰ Temperature ‰ High floodwater pH ‰ Cation exchange ‰ High soil pH capacity of soil ‰ High algal activity ‰ Wind speed ‰ Plant density ‰ Floodwater depth

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Nitrification

Plant biomass N

N2 NH3 N2 N2O (g) Litterfall Nitrogen Fixation Volatilization

Mineralization. Water - Nitrification + + NO3 NH4 Organic N NH4 Column

AEROBIC - Plant Peat NO3 + + [NH4 ]s uptake accretion [NH4 ]s Denitrification ANAEROBIC Microbial + Organic N Biomass N Adsorbed NH4

N2, N2O (g)

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29 Nitrification

+ - - NH4 → NO2 → NO3

- Obligate aerobes (O2 e acceptor) ƒ Common nitrifiers (autotrophs) ƒ Methanotrophs ƒ HtthHeterotrophs

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Sites of Nitrification

Aerobic Water column

Aerobic Soil/Floodwater Interface

O2 ic Soil b Aero

O2 Aerobic Root Zone Anaerobic Soil

O2

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30 Nitrification

+ - ‰ NH4 → NO2 Nitrosomonas (‘Nitroso’ spp.) - - ‰ NO2 → NO3 Nitrobacter (‘Nitro’ spp.) - ‰ Obligate aerobes (O2 e acceptor) + ‰ Energy/e- source: NH4

‰ Carbon source: CO2

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Nitrification

Step I: Nitrosomonas ΔGo = -65 kcal/mole + - - + NH4 + 2H2O → NO2 + 6e + 8H

- + 1½ O2 + 6e + 6H → 3H2O + 1 - + NH4 + 1 /2O2 → NO2 + 3H2O + H

Step II: Nitrobacter ΔGo = -18 kcal/mole

- - - + 2NO2 + 2H2O → 2NO3 + 4e + 4H

- + O2 + 4e + 4H → 2H2O - - 2NO2 + O2 → 2NO3

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31 Heterotrophic Nitrification

‰ BtiBacteria an dfid fungi ‰ Poorly studied ‰ Only dominant under high C/low N conditions

‰ Many facultative (reduce oxidized N to N2) “Nitrifier denitrification”

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Nitrification

‰ Ammonia

NH3 toxic to nitrifying bacteria ‰ pH 7 - 8.5 optimum ‰ Sulfide S2- toxic to nitrifying bacteria

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32 Nitrification

Ideal Conditions Inhibited

+ + NH4 NH4 - - NO3 NO2

- NO2 - NO3 C

Time Time

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Nitrification

Hydroxylamine Oxygenases oxidored uct ase - - + 2 e 2 e [NOH] NH4 NH2OH

- Oxygen 2 e Chemical stress 2 e- - - N2O NO2 NO3 Nitrite Nitrite reductase reductase

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33 Inhibited Nitrification

‘Leaky Pipe’

Model: NO NO

+ - NH4 NO3 N2

Nitrifiers Denitrifiers N2O N2O

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Nitrification

Regulators: ‰ Ammonia concentration ‰ Oxygen availability

‰ Alkalinity and CO2 ‰ Temperature ‰ Nitrifying population ‰ pH ‰ CEC

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34 Organic N

Volatilization (to atmosphere) Ammonification

- - + + NH - [NH3 ](g) 4 NH4 - Soluble Adsorbed - - Soil Particle - -

- Uptake [NO3 ] ant/Microbial ant/Microbial Pl Live Organic N

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Anaerobic Ammonium Oxidation: Anammox

6/22/2008 P.W. Inglett 70 Kuypers et al. 2003. Nature 422:608–611.

35 Anaerobic Ammonium Oxidation (ANAMMOX)

+ - + 5NH4 + 3NO3 → 4N2 + 9H2O + 2H

+ - NH4 +NO+ NO2 → N2 +2H+ 2H2O

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ANAMMOX

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36 Denitrification

Plant biomass N

N2 NH3 N2 N2O (g) Litterfall Nitrogen Fixation Volatilization

Mineralization. Water - Nitrification + + NO3 NH4 Organic N NH4 Column

AEROBIC - Plant Peat NO3 + + [NH4 ]s uptake accretion [NH4 ]s Denitrification ANAEROBIC Microbial + Organic N Biomass N Adsorbed NH4

N2, N2O (g)

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Pasteur, L. 1859. Sur la fermentation nitreuse. Bull. Sot. Chim. Fr. 1:21.

“Nitrates, which the juice of beetroot naturally includes, decompose, and great quantities of nitrous vapor form at the surface of the vats”

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37 Dehérain and Maquenne, 1882. Sur la reduction des nitrates dans la terre arable. C. R. Acad. Sci. 95: 691-693

“From experience”: Nitrate is reduced under certain conditions in arable land releasing nitrogen gas Nitrate reduction occurs in arable soil which contains a higggh organic matter We have observed that this reduction occurs when the soil atmosphere is completely stripped of oxygen

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Nitrate Reduction

‰ Denitrification 5 e- ƒ NO3 → N2 ‰ Dissimilatory nitrate reduction to ammonia (DNRA) - - 8 e + ƒ NO3 → NH4 ‰ Assimilatory nitrate reduction to ammonia - + ƒ NO3 → NH4 → Proteins

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38 Nitrate Reduction

‰ Assimilatory ‰ Dissimilatory + ƒ NH4 uptake for ƒ Functions as an electron growth - biosynthesis acceptor + ƒ NH4 concentration- ƒ Oxygen concentration main regulator main regulator ƒ Growth linked ƒ Linked to catabolic ƒ Occurs mainly in reactions aerobic soils ƒ Occurs in anaerobic soils ƒ Insensitive to O2 ƒ Sensitive to O2

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Nitrate Reduction

‰ Denitrification ‰ DNRA - - - + ƒ NO3 /NO2 → N2 ƒ NO3 → NH4 5 e- transferred 8 e- transferred ƒ Low electron ƒ High electron pressure pressure ƒ Eh = 200 – 300 mV ƒ Eh = < 0 mV

ƒ O2 transient ƒ Lake sediments or conditions permanenttldt wetlands ƒ Facultative ƒ Obligate anaerobes anaerobes

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39 Regulators of Denitrification

‰ Low O 2 content - Low Eh ‰ Presence of Electron Acceptor- N Oxides ‰ Facultative Anaerobes - Enzymes ‰ Electron Donor - Carbon Source ‰Organic carbon compounds ‰Reduced sulfur compounds ‰ Temperature ‰ Nitrate diffusion/flux to anaerobic zones ‰ Plant root density

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Sites of Nitrification/Denitrification

Aerobic Water column

Aerobic Soil/Floodwater Interface

O2 ic Soil b Aero

O2 Aerobic Root Zone Anaerobic Soil

O2

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40 Dissimilatory Nitrate Reduction

- - NO3 NO2 [HNO] [H2N2O2]

Biological NH2OH N2O

[HNO] = Nitroxyl

[H2N2O2]]yp = Hyponitrite NH OH = Hydroxylamine + 2 NH4 N2

DNRA Denitrification

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Abiotic Nitrate Reduction (‘Chemo(‘Chemo--denitrification’)denitrification’)

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41 Abiotic Nitrate Reduction

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Lithotrophic Nitrate Reduction

Other potential e- donors: 2- S , H2

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42 N.R. Coupled to Fe(II) Oxidation: Paddy Soils

- w/ added NO3

w/ added - w/o added NO3 - NO3

w/o added - NO3

Fe(II) Fe(III) Ratering and Schnell. 2001. Env. Microbiol. 3(2), 100-109.

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N.R. Coupled to Fe(II) Oxidation

- - NO3 + Fe(II) → NO2 → Fe(III) + N2

- NO3 -control

Fe(III)-culture

- NO3 -culture

Fe(III)-control

Straub, et al. 2007. Appl. Env. Microbiol. 62: 1458-1460. 6/22/2008 P.W. Inglett 86

43 N2

3+ + O2/Fe NH4 - NO2 - NO3

CH4?

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44 Regulators of Denitrification

‰ Low O2 content - Low Eh ‰ Presence of Nitrogenous Oxides ‰ Heterotrophs: Carbon Source Organic carbon compounds, DOC ‰ Lithotrophs: Electron Source

Reduced Fe, Mn, S compounds, H2 ‰ Temperature ‰ Nitrate diffusion/flux to anaerobic zones ‰ Plant root density

Nitrate uptake, O2 loss

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Denitrification

Wetland Denitrification Rate

Houghton Lake Wetland (mg N kg-1 day-1) Inflow Zone 74 Interior Zone 11 Orange County ESAWT Inflow Zone 9 Interior Zone 1 Everglades-WCA-2a Inflow Zone 6 Interior Zone 1

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45 Coupled Nitrification-Nitrification- Denitrification

Nitrification rate depends on: ‰ Ammonification rate ‰ Ammonium flux into aerobic zone Denitrification rate depends on: ‰ Nitrification rate ‰ Nitrate flux into anaerobic zones Nitrification-denitrification occurs in: ‰ Flooded soil-water column with aerobic-anaerobic interfaces ‰ Aerobic-anaerobic interfaces in the root zone ‰ Water-table fluctuations/ alternate flooding and draining

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Coupled NitrificationNitrification--DenitrificationDenitrification

6/22/2008 P.W. Inglett 92 From: Reddy (1989)

46 Everglades –WCA-2A

Nitrate source/ Nitrate source/ High C/Low Eh Nitrate depleted/ Low C,P/Low Eh High C/Low Eh

10

8 ) -1

hr 6 -1

4 With Glucose

N kg Without Glucose trification Rate g

(m 2 Deni

0 0 2 4 6 8 10 Distance (km)

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Denitrification Walls: (Schipper et al. 2003)

47 ANAMMOX vs. Denitrification

- ‰ Suppl y of NO2 - - ‰ Low NO2 favors Denitrification, High NO2 (>10mM) inhibits ANNAMOX

‰ Organic matter availability

‰ Higher DOC favors Denitrification

‰ Temperature

‰ ANNAMOX dominant @ low Temp.

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48 Denitrification vs. DNRA

- ‰ Supply of NO3 - Low NO3 favors DNRA ‰ Organic matter availability High DOC favors DNRA

‰ Hydropattern Denitrifiers are largely facultative

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15 - (Of added NO3 )

ON

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49 6/22/2008 P.W. Inglett 99

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50 Potential Fate Low Organic - Matter NO3 (Low C:EA)

- + NO2 H + NH4 NO N2O

N2O N2 Dissimil N2 N2 a tory N2

+ NH4 High Organic Matter Permanently Anaerobic Aerobic (High C:EA) Anaerobic 6/22/2008 P.W. Inglett 101

Potential Fate

Burgin and Hamilton. 2007. Front Ecol Environ 5(2): 89–96.

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51 Nitrogen Cycling

Plant biomass N

N2 NH3 N2 N2O (g) Litterfall Nitrogen Fixation Volatilization

Mineralization. Water - Nitrification + + NO3 NH4 Organic N NH4 Column

AEROBIC - Plant Peat NO3 + + [NH4 ]s uptake accretion [NH4 ]s Denitrification ANAEROBIC Microbial + Organic N Biomass N Adsorbed NH4

N2, N2O (g)

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52