Nitrogen fixing prokaryotes are fulfilling their own metabolic needs, but other organisms benefit since excess ammonia is released into the soil or water. Plants assimilate the NO 3 - released from these bacteria and convert it to organic forms, such as amino acids and proteins. Animals can only assimilate organic nitrogen which they obtain by eating plants and other animals. Denitrification occurs when bacteria obtain the oxygen necessary for their metabolism from NO 3 - rather than O 2 under anaerobic conditions. This process returns nitrogen to the atmosphere by converting NO 3 - to N 2 . 3 .
Ammonification is the decomposition of organic nitrogen back into ammonium.
Prokaryotes serve as vital links at several points in the cycle. Most of the nitrogen cycling involves nitrogenous compounds in the soil and water. Denitrification returns only a small amount of N 2 to the atmosphere. Most assimilated nitrogen comes from nitrate, which is efficiently recycled from organic forms by ammonification and nitrification. The majority of nitrogen in most ecosystems is recycled locally by decomposition and reassimilation, although nitrogen exchange between the soil and atmosphere are of long-term importance.