Denitrification
Overview
Denitrification is a microbially facilitated process of nitrate reduction (and ultimately producing molecular nitrogen) that may ultimately produce molecular nitrogen 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 favorable include nitrate (NO3−), nitrite (NO2−), nitric oxide (NO), nitrous oxide (N2O) finally resulting in the production of dinitrogen (N2) completing the nitrogen cycle.
Microbial Mechanisms
Denitrification is performed by a wide range of heterotrophic, autotrophic, and mixotrophic microorganisms. The process is primarily performed by heterotrophic bacteria while autotrophic denitrifiers have also been identified. Denitrifiers are represented in all main phylogenetic groups including bacteria, archaea, and fungi. Generally, the process of denitrification can occur in environments where oxygen, a more energetically favorable electron acceptor, is depleted, and a source of nitrate is present.
Biochemistry of Denitrification
Denitrification generally occurs in anoxic environments, where the concentration of dissolved and freely available oxygen is depleted. Under these conditions, nitrate (NO3−) or nitrite (NO2−) can be utilized as a substitute for oxygen as an electron acceptor in the process of respiration. This leads to the reduction of nitrate or nitrite to gaseous nitrogen compounds, a process known as denitrification. The overall process of denitrification can be represented by the net reaction: 2NO3− + 10e− + 12H+ → N2 + 6H2O.
Environmental Implications
Denitrification plays a critical role in the nitrogen cycle, a biogeochemical cycle that describes the transformations of nitrogen and nitrogen-containing compounds in nature. It is the primary pathway by which fixed nitrogen in the biosphere is returned to the atmosphere as N2 or N2O. Denitrification occurs in various types of environment including soil, freshwater, saltwater, and wetlands, and it is carried out by a diverse group of microorganisms.
Agricultural Implications
In agricultural systems, denitrification can lead to a significant loss of nitrate from soil, through the conversion of nitrate to gaseous nitrogen compounds that are released to the atmosphere. This can result in decreased soil fertility and reduced agricultural yield. However, denitrification can also be beneficial in removing excess nitrate from soil or water, thereby preventing nitrate pollution.
Industrial Applications
Denitrification is also used in wastewater treatment to remove nitrate and nitrite from effluent. The process is commonly used in treatment plants that employ the activated sludge process. It is a critical step in the nitrogen removal process in wastewater treatment, as it allows for the conversion of nitrate or nitrite to nitrogen gas, which can then be vented off to the atmosphere.