Nitrogen Cycle02

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