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SourcesSourcesLightningInorganic fertilizersNitrogen FixationAnimal ResiduesCrop residuesOrganic fertilizers
FORMS OF NITROGENFORMS OF NITROGENUrea CO(NH2)2
Ammonia NH3 (gaseous)Ammonium NH4
Nitrate NO3
Nitrite NO2
Atmospheric Dinitrogen N2
Organic N
Global Nitrogen Reservoirs
Nitrogen Reservoir
Metric tons nitrogen
Actively cycled
Atmosphere 3.9*1015 No
Ocean soluble salts
Biomass6.9*1011
5.2*108
YesYes
Land organic matter Biota
1.1*1011
2.5*1010
SlowYes
ROLES OF NITROGENROLES OF NITROGENPlants 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.
NITROGEN IS A KEY ELEMENT NITROGEN IS A KEY ELEMENT FORFOR
amino acids nucleic acids (purine, pyrimidine) cell wall components of bacteria (NAM).
NITROGEN CYCLESAmmonification/mineralizationImmobilizationNitrogen Fixation NitrificationDenitrification
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
Mineralization or AmmonificationDecomposers: earthworms, termites,
slugs, snails, bacteria, and fungiUses extracellular enzymes initiate
degradation of plant polymersMicroorganisms uses:Proteases, lysozymes, nucleases to
degrade nitrogen containing molecules
Plants die or bacterial cells lyse release of organic nitrogen
Organic nitrogen is converted to inorganic nitrogen (NH3)
When pH<7.5, converted rapidly to NH4
Example:
Urea NH3 + 2 CO2
ImmobilizationmmobilizationThe opposite of mineralizationHappens when nitrogen is limiting in the
environmentNitrogen limitation is governed by C/N
ratioC/N typical for soil microbial biomass is 20C/N < 20 MineralizationC/N > 20 Immobilization
Nitrogen Fixation
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
Nitrogen FixationNitrogen FixationEnergy 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)
Microorganisms fixingMicroorganisms fixingAzobacterBeijerinckiaAzospirillumClostridiumCyanobacteria
Require the enzyme nitrogenase
Inhibited by oxygenInhibited by
ammonia (end product)
Rates of Nitrogen Rates of Nitrogen FixationFixation
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
Applications to Applications to wetlandswetlandsOccur in overlying watersAerobic soilAnaerobic soilOxidized rhizosphereLeaf or stem surfaces of plants
Bacterial FixationBacterial FixationOccurs mostly in salt marshesIs absent from low pH peat of northern bogsCyanobacteria found in waterlogged soils
NitrificationNitrification
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
NitrificationTwo step reactions that occur together :
1rst step catalyzed by Nitrosomonas2 NH4
+ + 3 O2 2 NO2- +2 H2O+ 4 H+
2nd step catalyzed by Nitrobacter2 NO2
- + O2 2 NO3-
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?
Denitrification
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
DenitrificationRemoves a limiting nutrient from the
environment4NO3
- + C6H12O6 2N2 + 6 H20
Inhibited by O2
Not inhibited by ammoniaMicrobial reactionNitrate is the terminal electron acceptor
Surface water
Oxidized layer
Reduced soil layer
[NH4] HIGH
Low [NH4]
Slow Diffusion
Biodegradation
C/N <20
C/N >20
Surface water
Oxidized layer
Reduced soil layer
[NH4] HIGH
Low [NH4]
Slow Diffusion
nitrification
[NO3] high
Surface water
Oxidized layer
Reduced soil layer
[NO3] high
Leaching
[NO3] Low
N2
Denitrification