Nitrification

Nitrification is a biological process in which reduced nitrogen compounds, typically ammonia or ammonium, are oxidized first to nitrite and then to nitrate. In aerobic environments, this two-step process is usually carried out by distinct groups of microorganisms: ammonia-oxidizing organisms convert ammonia to nitrite, and nitrite-oxidizing organisms convert nitrite to nitrate. In some organisms, including complete ammonia oxidizers, a single organism can perform both steps.

Ammonia-oxidizing bacteria (AOB) such as Nitrosomonas and ammonia-oxidizing archaea (AOA) oxidize ammonia to nitrite, while nitrite-oxidizing bacteria (NOB) such as Nitrobacter and Nitrospira oxidize nitrite to nitrate. Nitrification is a chemolithoautotrophic process requiring oxygen and typically occurring in soils, freshwater, marine systems, and engineered environments such as wastewater treatment plants. Archaea can contribute substantialy in some environments, and complete ammonia oxidizers (comammox) have been identified, capable of performing the full oxidation in a single organism.

Ecological and practical significance: Nitrification influences soil fertility, nitrogen leaching, and nitrous oxide emissions. In agriculture and wastewater treatment, management of nitrification involves maintaining adequate oxygen, adjusting pH, and sometimes applying inhibitors to slow nitrification and reduce nitrate losses. Environmental conditions such as low oxygen or extreme pH can limit the rate of nitrification.