Nitrification

Nitrification is the step in the nitrogen cycle that links the oxidation of ammonia (produced from the degradation of organic matter) to the loss of fixed nitrogen in the form of dinitrogen gas. It is performed by a few different groups of microorganisms, including the ammonia-oxidizing bacteria, the ammonia-oxidizing archaea, and the nitrite-oxidizing bacteria. These microbes are all aerobes and are predominantly autotrophic. Their specialized metabolism provides them a unique niche, but results in slow and inefficient growth. Another group of microbes, the anammox organisms, performs the direct anaerobic oxidation of ammonia to nitrogen gas. Conventional nitrification occurs in soils, sediments, and aquatic environments. It is very important in agriculture, where it determines the availability of fertilizer nitrogen, and in wastewater treatment systems, where it participates in the removal of excess nitrogen. In the marine environment, nitrification determines the form of nitrogen available for primary production in the surface layer. Nitrification is coupled to denitrification in low-oxygen waters and in sediments, where it is an important oxygen sink. In both terrestrial and aquatic systems, nitrifying bacteria are involved in the production of the greenhouse gas, nitrous oxide, probably via a reductive pathway known as nitrifier denitrification. In natural systems, nitrification rates are determined by environmental factors such as salinity, temperature, oxygen, and pH.