There are two main ways nitrogen is 'fixed':
- Fixation by Lightning
The energy from lightning causes nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground, where they can be assimilated by plants.
- Biological Fixation
About 90% of nitrogen fixation is done by bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium.
N2 + 3 H2 → 2 NH3
Ammonia can be used by plants directly. Ammonia and ammonium may be further reacted in the nitrification process.
Nitrification occurs by the following reactions:
2 NH3 + 3 O2 → 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 → 2 NO3-
Aerobic bacteria use oxygen to convert ammonia and ammonium. Nitrosomonas bacteria convert nitrogen into nitrite (NO2-) and then nitrobacter convert nitrite to nitrate (NO3-). Some bacteria exist in a symbiotic relationship with plants (legumes and some root-nodule species). Plants utilize the nitrate as a nutrient. Animals obtain nitrogen by eating plants or plant-eating animals.
When plants and animals die, bacteria convert nitrogen nutrients back into ammonium salts and ammonia. This conversion process is called ammonification. Anaerobic bacteria can convert ammonia into nitrogen gas through the process of denitrification:
NO3- + CH2O + H+ → ½ N2O + CO2 + 1½ H2O
Denitrification returns nitrogen to the atmosphere, completing the cycle.