The conversion of atmospheric nitrogen into the nitrogenous compounds through the agency of living organisms is called biological nitrogen fixation. The process is carried out by two main types of microorganism: those which live in close symbiotic association with other plants and those which are “free living” or non-symbiotic.
Biological nitrogen fixation (BNF) is the process whereby atmospheric nitrogen is reduced to ammonia in the presence of nitrogenize. Nitrogenize is a biological catalyst found naturally only in certain microorganisms such as the symbiotic Rhizobium and Frankia, or the free-living Azospirillum and Azotobacter and BGA.
Details of biological nitrogen fixation follow.
Nearly 80% of Earths atmosphere contains nitrogen in the form of a highly inert di-nitrogen (N = N) which most plants cannot utilize as such. The atmospheric di-nitrogen (N2) consists of two nitrogen atoms linked by a triple-covalent bond. About 225 kcal of energy is required to break this triple bond which is difficult to achieve.
The phenomenon of reduction of inert gaseous di-nitrogen (N2) into ammonia (NH3) through the agency of some microorganisms so that it can be made available to the plants is called as biological nitrogen fixation or diazotrophy.
