Nitrogen Cycle

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Nitrogen Cycle
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Sources

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

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Forms of Nitrogen

• Urea ? CO(NH2)2
• Ammonia ? NH3 (gaseous)
• Ammonium ? NH4
• Nitrate ? NO3
• Nitrite ? NO2
• Atmospheric Dinitrogen ?N2
• Organic N

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Global Nitrogen Reservoirs
Nitrogen Reservoir Metric tons nitrogen Actively cycled
Atmosphere 3.91015 No
Ocean ? soluble salts Biomass 6.91011 5.2108 Yes Yes
Land ? organic matter ? Biota 1.11011 2.51010 Slow Yes
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Roles of Nitrogen

• Plants and bacteria use nitrogen in the form of
  NH4 or NO3-
• It serves as an electron acceptor in anaerobic
  environment
• Nitrogen is often the most limiting nutrient in
  soil and water.

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Nitrogen is a key element for

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

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Nitrogen Cycles

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

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Ammonification or Mineralization
N2
N2O
NH4
NO2
R-NH2
NO
NO2
NO3
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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

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• Plants die or bacterial cells lyse ? release of
  organic nitrogen
• Organic nitrogen is converted to inorganic
  nitrogen (NH3)
• When pHlt7.5, converted rapidly to NH4
• Example
• Urea NH3 2 CO2

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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 lt 20 ?Mineralization
• C/N gt 20 ?Immobilization

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Nitrogen Fixation
N2
N2O
NH4
NO2
R-NH2
NO
NO2
NO3
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Nitrogen Fixation

• Energy intensive process
• N2 8H 8e- 16 ATP 2NH3 H2 16ADP 16
  Pi
• Performed only by selected bacteria and
  actinomycetes
• Performed in nitrogen fixing crops
• (ex soybeans)

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Microorganisms fixing

• Azobacter
• Beijerinckia
• Azospirillum
• Clostridium
• Cyanobacteria

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

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Rates of Nitrogen Fixation
N2 fixing system Nitrogen Fixation (kg N/hect/year)
Rhizobium-legume 200-300
Cyanobacteria- moss 30-40
Rhizosphere associations 2-25
Free- living 1-2
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Applications to wetlands

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

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Bacterial Fixation

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

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Nitrification
N2
N2O
NH4
NO2
R-NH2
NO
NO3
NO2
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Nitrification

• Two step reactions that occur together
• 1rst step catalyzed by Nitrosomonas
• 2 NH4 3 O2 ? 2 NO2- 2 H2O 4 H
• 2nd step catalyzed by Nitrobacter
• 2 NO2- O2 ? 2 NO3-

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• Optimal pH is between 6.6-8.0
• If pH lt 6.0 ? rate is slowed
• If pH lt 4.5 ? reaction is inhibited

In which type of wetlands do you thing
Nitrification occurs?
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Denitrification
N2
N2O
NH4
NO2
R-NH2
NO
NO2
NO3
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Denitrification

• Removes a limiting nutrient from the environment
• 4NO3- C6H12O6? 2N2 6 H20
• Inhibited by O2
• Not inhibited by ammonia
• Microbial reaction
• Nitrate is the terminal electron acceptor

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Looking at the Nitrogen cycle through the eye of
NH4
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Surface water
Low NH4
Oxidized layer
Biodegradation
Reduced soil layer
Slow Diffusion
C/N lt20
C/N gt20
NH4 HIGH
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Surface water
nitrification
Low NH4
Oxidized layer
NO3 high
Reduced soil layer
Slow Diffusion
NH4 HIGH
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N2
Surface water
Oxidized layer
NO3 high
Leaching
Reduced soil layer
NO3 Low
Denitrification
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QUESTIONS ?