6.2. Nitrification

Once nitrogen is fixed it can either be assimilated and transformed to organic compounds, or further processed by heterotrophic nitrifying bacteria. Nitrification includes two subsequent oxidative reaction of ammonium (NH4+) or ammonia (NH3) to nitrate (NO3). Nitrifying bacteria are the most frequent in locations where more ammonia is present. In the first step, NH3 is oxidized to nitrite (NO2), this process is called nitrosifying (R6.2-R6.4). In the second step this nitrite is oxidized to nitrate (NO3), which is then available for assimilation by plants.

NH3 + O2 → NO2+ 3H+ + 2e


NH3 + O2 + 2H+ + 2e → NH2OH +  H2O


NH2OH +  H2O → NO2 + 5H+ + 4e


Nitrifying (oxidizing nitrites to nitrates)

NO2+ H2O→NO3 + 2H+ + 2e