<P> The nitrogen cycle is of particular interest to ecologists because nitrogen availability can affect the rate of key ecosystem processes, including primary production and decomposition . Human activities such as fossil fuel combustion, use of artificial nitrogen fertilizers, and release of nitrogen in wastewater have dramatically altered the global nitrogen cycle . </P> <P> Nitrogen is present in the environment in a wide variety of chemical forms including organic nitrogen, ammonium (NH), nitrite (NO), nitrate (NO), nitrous oxide (N O), nitric oxide (NO) or inorganic nitrogen gas (N). Organic nitrogen may be in the form of a living organism, humus or in the intermediate products of organic matter decomposition . The processes in the nitrogen cycle is to transform nitrogen from one form to another . Many of those processes are carried out by microbes, either in their effort to harvest energy or to accumulate nitrogen in a form needed for their growth . For example, the nitrogenous wastes in animal urine are broken down by nitrifying bacteria in the soil to be used as new . The diagram (besides) alongside shows how these processes fit together to form the nitrogen cycle . </P> <P> The conversion of nitrogen gas (N) into nitrates and nitrites through atmospheric, industrial and biological processes is called nitrogen fixation . Atmospheric nitrogen must be processed, or "fixed", into a usable form to be taken up by plants . Between 5x10 and 10x10 g per year are fixed by lightning strikes, but most fixation is done by free - living or symbiotic bacteria known as diazotrophs . These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is converted by the bacteria into other organic compounds . Most biological nitrogen fixation occurs by the activity of Mo - nitrogenase, found in a wide variety of bacteria and some Archaea . Mo - nitrogenase is a complex two - component enzyme that has multiple metal - containing prosthetic groups . An example of free - living bacteria is Azotobacter . Symbiotic nitrogen - fixing bacteria such as Rhizobium usually live in the root nodules of legumes (such as peas, alfalfa, and locust trees). Here they form a mutualistic relationship with the plant, producing ammonia in exchange for carbohydrates . Because of this relationship, legumes will often increase the nitrogen content of nitrogen - poor soils . A few non-legumes can also form such symbioses . Today, about 30% of the total fixed nitrogen is produced industrially using the Haber - Bosch process, which uses high temperatures and pressures to convert nitrogen gas and a hydrogen source (natural gas or petroleum) into ammonia . </P> <P> Plants can absorb nitrate or ammonium from the soil via their root hairs . If nitrate is absorbed, it is first reduced to nitrite ions and then ammonium ions for incorporation into amino acids, nucleic acids, and chlorophyll . In plants that have a symbiotic relationship with rhizobia, some nitrogen is assimilated in the form of ammonium ions directly from the nodules . It is now known that there is a more complex cycling of amino acids between Rhizobia bacteroids and plants . The plant provides amino acids to the bacteroids so ammonia assimilation is not required and the bacteroids pass amino acids (with the newly fixed nitrogen) back to the plant, thus forming an interdependent relationship . While many animals, fungi, and other heterotrophic organisms obtain nitrogen by ingestion of amino acids, nucleotides, and other small organic molecules, other heterotrophs (including many bacteria) are able to utilize inorganic compounds, such as ammonium as sole N sources . Utilization of various N sources is carefully regulated in all organisms . </P>

The microbes or bacteria which converts atmospheric nitrogen into nitrate in the soil is