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Nitrogen Fixation
reduction of N2 to NH3
Biological (only certain bacteria), industrial (fossil fuels and high temperature), lightening fixation
Mineralization
Organic nitrogen to ammonium (R-N → NH4+)
Residues with C/N ratio < 25 results in net mineralization
No change in oxidation state
Volatilization
NH4+ + OH- → H2O + NH3
Gaseous loss of ammonia
losses enhanced by high pH, moisture, high temperatures, surface application of fertilizers
No change in oxidation state
Nitrification
Ammonium (NH4+) → nitrate (NO3-)
Carried out by chemoautotrophic bacteria and archaea
requires molecular oxygen, produces acidity
cationic to anionic form
assumes ammonium is available, 60% WFPS is optimal, acid intolerant
Denitrification
Nitrate reduction to gas, NO3- → N2
Affected by oxygen concentration (inhibits activity), carbon availability (requires active respiration), and nitrate concentration is needed
reduces fertilizer efficiency, increases destruction of ozone layer, contributes to global warming, reduces nitrate contamination of groundwater, regenerates atmospheric nitrogen