nitrogen cycle02
DESCRIPTION
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SourcesSourcesLightningInorganic fertilizersNitrogen FixationAnimal ResiduesCrop residuesOrganic fertilizers
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FORMS OF NITROGENFORMS OF NITROGENUrea 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 3.9*1015 No
Ocean soluble salts
Biomass6.9*1011
5.2*108
YesYes
Land organic matter Biota
1.1*1011
2.5*1010
SlowYes
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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.
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NITROGEN IS A KEY ELEMENT NITROGEN IS A KEY ELEMENT FORFOR
amino acids nucleic acids (purine, pyrimidine) cell wall components of bacteria (NAM).
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NITROGEN CYCLESAmmonification/mineralizationImmobilizationNitrogen Fixation NitrificationDenitrification
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R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
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R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
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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
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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
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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
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Nitrogen Fixation
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
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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)
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Microorganisms fixingMicroorganisms fixingAzobacterBeijerinckiaAzospirillumClostridiumCyanobacteria
Require the enzyme nitrogenase
Inhibited by oxygenInhibited by
ammonia (end product)
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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
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Applications to Applications to wetlandswetlandsOccur in overlying watersAerobic soilAnaerobic soilOxidized rhizosphereLeaf or stem surfaces of plants
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Bacterial FixationBacterial FixationOccurs mostly in salt marshesIs absent from low pH peat of northern bogsCyanobacteria found in waterlogged soils
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NitrificationNitrification
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
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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-
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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?
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Denitrification
R-NH2
NH4 NO2
NO3NO2
NO
N2O
N2
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DenitrificationRemoves a limiting nutrient from the
environment4NO3
- + C6H12O6 2N2 + 6 H20
Inhibited by O2
Not inhibited by ammoniaMicrobial reactionNitrate is the terminal electron acceptor
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Surface water
Oxidized layer
Reduced soil layer
[NH4] HIGH
Low [NH4]
Slow Diffusion
Biodegradation
C/N <20
C/N >20
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Surface water
Oxidized layer
Reduced soil layer
[NH4] HIGH
Low [NH4]
Slow Diffusion
nitrification
[NO3] high
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Surface water
Oxidized layer
Reduced soil layer
[NO3] high
Leaching
[NO3] Low
N2
Denitrification
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