Schematic illustration of the key processes of the nitrogen cycle. Note that the fate of nitrite determines whether nitrogen remains fixed (as nitrite, nitrate or ammonium) or is lost to the atmosphere (as NO, N2O or N2). The stippled line for comammox indicates that nitrite is an intermediate but oxidized to nitrate by the same organism. Abbreviations: Anammox, anaerobic ammonium oxidation; DNRA, dissimilatory nitrite reduction to ammonia; assim., assimilatory; dissim., dissimilatory.