Nitrogen Cycle

Sources

• Lightning • Inorganic fertilizers • Nitrogen Fixation • Animal Residues • Crop residues • Organic fertilizers

Forms of Nitrogen

• Urea  CO(NH 2 ) 2 • Ammonia  NH 3 (gaseous) • Ammonium  NH 4 • Nitrate  NO 3 • Nitrite  NO 2 • Atmospheric Dinitrogen  N 2 • Organic N

Global Nitrogen Reservoirs

Nitrogen Reservoir Atmosphere Ocean



soluble salts Biomass Metric tons nitrogen

3.9*10 15 6.9*10 11 5.2*10 8

Actively cycled

No Yes Yes

Land



organic matter



Biota

1.1*10 11 2.5*10 10 Slow Yes

Roles of Nitrogen

• Plants and bacteria use nitrogen in the form of NH 4 + or NO 3 • It serves as an electron acceptor in anaerobic environment • Nitrogen is often the most limiting nutrient in soil and water.

Nitrogen is a key element for

• amino acids • nucleic acids (purine, pyrimidine) • cell wall components of bacteria (NAM).

Nitrogen Cycles

• Ammonification/mineralization • Immobilization • Nitrogen Fixation • Nitrification • Denitrification

N 2 N 2 O NH 4 NO NO 2 R-NH 2 NO 2 NO 3

Ammonification or Mineralization

N 2 N 2 O NH 4 NO 2 R-NH 2 NO NO 2 NO 3

Mineralization or Ammonification • Decomposers: earthworms, termites, slugs, snails, bacteria, and fungi • Uses extracellular enzymes  initiate degradation of plant polymers • Microorganisms uses: • Proteases, lysozymes, nucleases to degrade nitrogen containing molecules

• Plants die or bacterial cells lyse  organic nitrogen • Organic nitrogen nitrogen (NH 3 ) is converted to release of inorganic • When pH<7.5, converted rapidly to NH 4 • Example: Urea NH 3 + 2 CO 2

Immobilization

• The opposite of mineralization • Happens when nitrogen is limiting in the environment • Nitrogen limitation is governed by C/N ratio • C/N typical for soil microbial biomass is 20 • C/N < 20  Mineralization • C/N > 20  Immobilization

N 2

Nitrogen Fixation

N 2 O NH 4 NO 2 R-NH 2 NO NO 2 NO 3

Nitrogen Fixation

• Energy intensive process : •

N 2 + 8H+ + 8e 16ADP + 16 Pi + 16 ATP = 2NH 3 + H 2 +

• Performed only by selected bacteria and actinomycetes • Performed in nitrogen fixing crops (ex: soybeans)

Microorganisms fixing

• • • • •

Azobacter Beijerinckia Azospirillum Clostridium Cyanobacteria

• Require the enzyme nitrogenase • Inhibited by oxygen • Inhibited by ammonia (end product)

Rates of Nitrogen Fixation

N 2 fixing system

Rhizobium-legume Cyanobacteria- moss

Nitrogen Fixation (kg N/hect/year)

200-300 30-40 Rhizosphere associations Free- living 2-25 1-2

Applications to wetlands

• Occur in overlying waters • Aerobic soil • Anaerobic soil • Oxidized rhizosphere • Leaf or stem surfaces of plants

Bacterial Fixation

• Occurs mostly in salt marshes • Is absent from low pH peat of northern bogs • Cyanobacteria found in waterlogged soils

N 2 N 2 O NH 4

Nitrification

R-NH 2 NO NO 2 NO 2 NO 3

Nitrification

Two step reactions that occur together : • 1 rst step catalyzed by

Nitrosomonas

2 NH 4 + + 3 O 2  2 NO 2 +2 H 2 O+ 4 H + • 2 nd step catalyzed by

Nitrobacter

• 2 NO 2 + O 2  2 NO 3 -

• Optimal pH is between 6.6-8.0

• If pH < 6.0  rate is slowed • If pH < 4.5  reaction is inhibited In which type of wetlands do you thing Nitrification occurs?

N 2 N 2 O NH 4

Denitrification

NO 2 R-NH 2 NO NO 2 NO 3

Denitrification

• Removes a limiting nutrient from the environment • 4NO 3 + C 6 H 12 O 6  • Inhibited by O 2 2N 2 + 6 H • Not inhibited by ammonia 2 0 • Microbial reaction • Nitrate is the terminal electron acceptor

Looking at the Nitrogen cycle through the eye of NH 4

Surfac e water Oxidized layer Reduce d soil layer Low [NH 4 ] [NH 4 ] HIGH Slow Diffusion Biodegradati on C/N <20 C/N >20

Surfac e water Oxidized layer Reduce d soil layer Low [NH 4 ] nitrificatio n [NH 4 ] HIGH Slow Diffusion [NO 3 ] high

Surfac e water Oxidized layer Reduce d soil layer N 2 [NO 3 ] high Leaching [NO 3 ] Low Denitrificatio n