Fig 1. Schematic of the nitrogen cycle, featuring the microbial processes of nitrification (NH₄⁺ to NO₂^– and then to NO₃^–) and denitrification (NO₃^– to NO₂^–, nitric oxide, N₂O then to nitrogen gas).

For every mole of NH₄⁺ nitrified to NO₃^–, two moles of oxygen are consumed (Stoichiometric relationships collectively found in [5,6,10–13]). Note that the proportion of N₂O released from nitrification and denitrification is highly variable as indicated by the dashed arrows [14]. Nitrogen assimilation, dissimilatory NO₃^– reduction to NH₄⁺ (DNRA) and anaerobic NH₄⁺ oxidation (anammox) are excluded from the figure but may be important components of the nitrogen cycle [15].