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Nitrogen changes between different chemical forms and oxidation states through fixation, ammonification, nitrification, denitrification, and anammox.

The nitrogen cycle begins with nitrogen gas, which most organisms cannot use directly.

Nitrogen fixation by free-living bacteria, like Azotobacter, converts nitrogen gas into ammonia, which enters the soil.

Symbiotic bacteria, like Rhizobium, fix nitrogen into ammonia in the root nodules of legumes.

During ammonification, decomposers break down amino acids and nucleotides from dead organisms and waste, releasing nitrogen as ammonium ions.

In aerobic zones, nitrification occurs in two steps. First, ammonia-oxidizers like Nitrosomonas oxidize ammonia to nitrite. Then, Nitrobacter converts nitrite to nitrate.

In low-oxygen zones, denitrification by Pseudomonas species converts nitrate into nitrogen gas.

Whereas in anoxic zones, the anammox process by the family Brocadiaceae combines ammonia and nitrite to form nitrogen gas.

This gas then returns to the atmosphere.