Figure 1. The nitrogen cycle. N 2 fixation yields ammonia ( ); bound nitrogen is the nitrogen in amino acids or in nucleic acid bases. Nitrification leads to nitrate via nitrite whereas N 2 again can be produced by denitrification; depending on the conditions, this process is associated with the liberation of varying amounts of N 2O (laughing gas). Nitrite and ammonium can be converted to N 2 via the anammox process. The nitrate ammonification is also shown. (Diagram: Anne Kemmling, Goettingen, Germany.)
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