nitrificationdenitrification

Nitrification and denitrification are two connected processes in the nitrogen cycle, often discussed together as nitrification-denitrification. Nitrification is the aerobic oxidation of ammonia or ammonium to nitrite and then to nitrate. In the first step, ammonia-oxidizing organisms, including bacteria and some archaea, convert NH3/NH4+ to nitrite. In the second step, nitrite-oxidizing bacteria convert NO2- to NO3-. Nitrification is typically carried out by autotrophic microorganisms that obtain energy from these oxidation steps and fix carbon dioxide for growth. It requires sufficient oxygen and generally neutral to slightly basic pH.

Denitrification is the anaerobic reduction of nitrate to nitrogen gas, proceeding through intermediate nitrite, nitric oxide, and nitrous oxide. Denitrifying microorganisms use nitrate as a terminal electron acceptor when oxygen is limited, often using organic carbon or other reduced compounds as an electron donor. The end product is N2, which is released to the atmosphere, and in some steps N2O can be emitted as a greenhouse gas. Denitrification is favored by low oxygen conditions, available nitrate, and adequate carbon sources.

In natural and engineered systems, nitrification and denitrification can be coupled. Nitrification supplies nitrate that denitrifiers can reduce in suboxic or anoxic microenvironments, such as in soils, sediments, and wastewater treatment. In wastewater treatment, a common approach is to separate aerobic nitrification from anoxic denitrification to remove nitrogen efficiently. Some systems aim for simultaneous or shortcut processes to save energy and resources, and new approaches (such as anammox) further modify the conventional nitrification-denitrification framework.