Atmospheric nitrogen must be processed, or "fixed" to be used by plants. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. These bacteria have an enzyme that combines gaseous nitrogen with hydrogen to produce ammonia, which is then further converted by the bacteria to make their own organic compounds.
Conversion of N2 The conversion of nitrogen (N2) from the atmosphere into a form readily available to plants and hence to animals is an important step in the nitrogen cycle, which distributes the supply of this essential nutrient. There are four ways to convert N2 (atmospheric nitrogen gas) into more chemically reactive forms: 1.Biological fixation: some symbiotic bacteria (most often associated with leguminous plants) and some free-living bacteria are able to fix nitrogen as organic nitrogen. An example of mutualistic nitrogen fixing bacteria are the Rhizobium bacteria, which live in legume root nodules. These species are diazotrophs. An example of the free-living bacteria is Azotobacter. 2.Industrial N-fixation: Under great pressure, at a temperature of 600 C, and with the use of an iron catalyst, hydrogen (usually derived from natural gas or petroleum) and atmospheric nitrogen can be combined to form ammonia (NH3) in the Haber-Bosch process which is used to make fertilizer and explosives. 3.Combustion of fossil fuels: automobile engines and thermal power plants, which release various nitrogen oxides (NOx). 4.Other processes: In addition, the formation of NO from N2 and O2 due to photons and especially lightning, can fix nitrogen. Assimilation
Plants take nitrogen from the soil, by absorption through their roots in the form of either nitrate ions or ammonium ions. All nitrogen obtained by animals can be traced back to the eating of plants at some stage of the food chain.
Ammonification When a plant or animal dies, or an animal expels waste, the