Nitrogen Cycle: In soil, bacteria and fungi decompose organic matter (contains carbon), cycle nitrogen, and transfer nutrients to plants. Nitrogen is essential to synthesis of protein. In its gaseous state nitrogen is so stable it will not readily combine with other elements.
Nitrifying bacteria are essential in getting nitrogen into a form that can be used by plants and animals. Nitrogen is converted to ammonia or nitrate by nitrogen fixing bacteria (anaerobic bacteria that lives in soil, the roots of some plants, and wetland mud), and cyanobacteria that live in water.
An enzyme (nitrogenase) in the bacteria breaks the nitrogen molecule and combines it with hydrogen. This enzyme functions only in a low oxygen environment or anaerobic conditions. Filament cyanobacteria have specialized cells, called heterocysts, that exclude oxygen allowing for nitrogen fixing. (Solomon & Berg & Martin & Villee, 1993; Gould.& Keeton, with Grant, 1996)
The nitrogen, as ammonia and nitrate, is used by the plants and then passed on up the food net. One group of bacteria convert ammonium ions into nitrite, and a second group convert the nitrite to nitrate that can be used by the plants.
Cyanobacteria are the most important natural nitrogen fixing bacteria living in soil or water. They fix between 10 and 20 Kg of nitrogen per hectare releasing ammonia. This cycle may be stressed as there has been an increase by over 100% the amount of nitrogen fixed above that of the preindustrialized world. (Gould & Keeton with Grant, 1996, p.p. 1173, 1174) Returning the nitrogen to the atmosphere is done by other bacteria.
Denitrificaiton, reducing nitrate back to nitrite and gaseous nitrogen is done by anaerobic bacteria living in mud. (Solomon & Berg & Martin & Villee, 1993)