denitrification

Denitrification is a microbial process that reduces nitrate to nitrogen gases, primarily under anaerobic or low-oxygen conditions. In this dissimilatory pathway, nitrate serves as a terminal electron acceptor, with organic carbon or other electron donors providing energy for the microbes. Denitrification is a major part of the global nitrogen cycle, removing fixed nitrogen from soils, sediments, and waters and returning it to the atmosphere as nitrogen gas or nitrous oxide.

The process typically proceeds through a sequence of reductions: nitrate (NO3-) to nitrite (NO2-), to nitric oxide (NO), to nitrous oxide (N2O), and finally to dinitrogen (N2). Each step is catalyzed by specific enzymes: nitrate reductases (Nar or Nap) reduce nitrate to nitrite; nitrite reductases (Nir) reduce nitrite to NO; nitric oxide reductases (Nor) reduce NO to N2O; and nitrous oxide reductases (NosZ) reduce N2O to N2. A variety of bacteria and some archaea perform denitrification, including members of the genera Pseudomonas, Paracoccus, Bacillus, and Thiobacillus.

Denitrification occurs in soils, freshwater and marine sediments, wetlands, and groundwater, and is important in engineered systems such as wastewater treatment where anoxic zones and carbon sources drive nitrate removal. It requires sufficient nitrate and an available electron donor, with low oxygen levels and neutral to mildly alkaline pH favoring the activity of denitrifiers.

Ecologically, denitrification helps prevent nitrate accumulation and eutrophication but can release nitrous oxide, a potent greenhouse gas, when the pathway is incomplete. It is distinct from DNRA, in which nitrate is reduced to ammonium and nitrogen is retained in the ecosystem. Rates are measured via methods such as 15N tracing or acetylene inhibition to quantify N2O formation.