Although molecular nitrogen (N2) is abundant (i.e 78-80 % by volume) in the earth’s atmosphere, but it is chemically inert and therefore, can not be utilized by most living organisms and plants. Plants, animals and most microorganisms, depend – on a source of combined or fixed nitrogen (eg. ammonia, nitrate) or organic nitrogen compounds for their nutrition and growth. Plants require fixed nitrogen (ammonia, nitrate) provided by microorganisms, but about 95 to 98 % soil nitrogen is in organic form (unavailable) which restrict the development of living organisms including plants and microorganisms. Therefore, cycling/transformation of nitrogen and nitrogenous compounds mediated by soil microorganisms is of paramount importance in supplying required forms of nitrogen to the plants and various nutritional classes of organisms in the biosphere. In nature, nitrogen exists in three different forms viz. gaseous / gas (78 to 80 % in atmosphere), organic (proteins and amino acids, chitins, nucleic acids and amino sugars) and inorganic (ammonia and nitrates).
Biological N2 Fixation:
A. Symbiotic: Eg. Rhizobium (Eubacteria) legumes, Frankia (Actinomycete) and Anabaena (cyanobacteria) non – legumes
B. Non Symbiotic:
Free Living: eg. Azobacter, Derxia, Bejerinkia, Rhodospirillum and BGA.
Associative: eg. Azospirillum, Acetobacter, Herbaspirillim.
Nutritional categories of N2 fixing Bacteria
Nitrogen cycle is the sequence of biochemical changes form free atmospheric N2 to complex organic compounds in plant and animal tissues and further to simple inorganic compounds (ammonia, nitrate) and eventual release of molecular nitrogen (N2) back to the atmosphere is called "nitrogen cycle".
In this cycle a part of atmospheric nitrogen (N2) is converted into ammonia and then to amino acids (by soil microorganisms and plant-microbe associations) which are used for the biosynthesis of complex nitrogen-containing organic compound such as proteins, nucleic acids, amino sugars etc. The proteins are then degraded to simpler organic compounds viz. peptones and peptides into amino acids which are further degraded to inorganic nitrogen compounds like ammonia, nitrites and nitrates. The nitrate form of nitrogen is mostly used by plants or may be lost through leaching or reduced to gaseous nitrogen and subsequently goes into the atmosphere, thus completing the nitrogen cycle. Thus, the process of mineralization (conversion of organic form of nutrients to its mineral /inorganic form) and immobilization (process of conversion of mineral / inorganic form of nutrient elements into organic form) are continuously and simultaneously going on in the soil.