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Nitrogen Cycle

 Nitrogen occurs prominently in all the spheres of the environment. The atmosphere is 78% elemental nitrogen, N2, by volume and comprises an inexhaustible reservoir of this essential element. Nitrogen, although constituting much less of biomass than carbon or oxygenoxygen, is an essential constituent of proteins. The N2 molecule is very stable so that breaking it down into atoms that can be incorporated with inorganic and organic chemical forms of nitrogen is the limit- ing step in the nitrogen cycle. This does occur by highly energetic processes in lightning discharges that produce nitrogen oxides. Elemental nitrogen is also incorporated into chemically bound forms. or fixed by biochemical processes mediated by microorganisms. The biological nitrogen is mineral- ized to the inorganic form during the decay of biomass. Large quantities of nitrogen are fixed synthetically under high-temperature and high-pressure conditions according to the following over- all reaction:


             N2+3H2         2NH3

The production of gaseous N2 and N2O by microorganisms and the evolution of these gases to the atmosphere complete the nitrogen cycle through a process called denitrification. The nitrogen cycle is discussed from the viewpoint of microbial processes. 



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