Denitrification is a microbially facilitated process of dissimilatory nitrate reduction that may ultimately produce molecular nitrogen (N2) through a series of intermediate gaseous nitrogen oxide products. This respiratory process reduces oxidized forms of nitrogen in response to the oxidation of an electron donor such as organic matter. The preferred nitrogen electron acceptors in order of most to least thermodynamically favourable include: nitrate (NO3-), nitrite (NO2-), nitric oxide (NO), and nitrous oxide (N2O). In terms of the general nitrogen cycle, denitrification completes the cycle by returning N2 to the atmosphere. The process is performed primarily by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads), although autotrophic denitrifiers have also been identified (e.g.,Thiobacillus denitrificans). Denitrifiers are represented in all main proteolytic groups. Generally several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway have been identified in the reduction process.
Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs where oxygen, a more energetically favourable electron acceptor, is depleted, and bacteria respire nitrate as a substitute terminal electron acceptor. Due to the high concentration of oxygen in our atmosphere, denitrification only takes place in environments where oxygen consumption exceeds the rate of oxygen supply, such as in some soils and groundwater, wetlands, poorly ventilated corners of the ocean, and in seafloor sediments.
Denitrification generally proceeds through some combination of the following intermediate forms:
NO3- → NO2- → NO → N2O → N2 gas
The complete denitrification process can be expressed as a redox reaction:
2NO3- + 10e- + 12H+ → N2 + 6H2O
This reaction shows a fractionation in isotope composition. Lighter isotopes of nitrogen are preferred in the reaction, leaving the heavier nitrogen isotopes in the residual matter. The process can cause delta-values of up to -40, where delta is a representation of the difference in isotopic composition. This can be used to identify denitrification processes in nature.
Denitrification is commonly used to remove nitrogen from sewage and municipal wastewater. It is also an instrumental process in wetlands and riparian zones for the removal of excess nitrate from groundwater resulting from excessive agricultural or residential fertiliser usage.
Direct reduction from nitrate to ammonium, a process known as dissimilatory nitrate reduction to ammonium or DNRA, is also possible for organisms that have the nrf-gene. This is less common than denitrification in most ecosystems as a means of nitrate reduction.
Reduction under anoxic conditions can also occur through process called anaerobic ammonia oxidation (Anammox), this reaction is expressed as the following:
NH4+ + NO2- → N2 + 2H2O
In some wastewater treatment plants, small amounts of methanol are added to the wastewater to provide a carbon source for the denitrification bacteria.