Denitrification is the process of reducing nitrate, a form of nitrogen available for consumption by many groups of organisms, into gaseous nitrogen, which is far less accessible to life forms but makes up the bulk of our atmosphere. It can be thought of as the opposite of nitrogen fixation, which converts gaseous nitrogen into more biologically useful forms. The process is performed by heterotrophic bacteria (such as Pseudomonas fluorescens) from all main proteolitic groups. Denitrification and nitrification are parts of the nitrogen cycle.
Denitrification takes place under special conditions in both terrestrial and marine ecosystems. In general, it occurs when oxygen (which is a more favourable electron acceptor) is depleted, and bacteria turn to nitrate in order to respire organic matter. Because our atmosphere is rich with oxygen, denitrification only takes place in some soils and groundwater, poorly ventilated corners of the ocean, and in seafloor sediments.
Denitrification proceeds through some combination of the following steps:
- nitrate → nitrite → nitric oxide → nitrous oxide → dinitrogen gas
Or expressed as a redox reaction:
- 2NO3- + 10e- + 12H+ → N2 + 6H2O
Denitrification is the second step in the nitrification-denitrification process: the conventional way to remove nitrogen from sewage and municipal wastewater.
Direct reduction from nitrate to ammonium is also possible for organisms that have the nrf-gene.