Index >> Nitrification and Denitrification >> Nitrification

Nitrification

As stated earlier, mineralization of nitrogen in organic materials results in the formation of ammonium which is the most reduced form of inorganic nitrogen. This ammonium in soils is the starting point for a series of reac­tions resulting in the formation of nitrites and nitrates mediated by specialised bacteria.

The biological oxidation of ammonium salts in soil to nitrites and the subsequent oxidation of nitrites to nitrates or to put it in general terms, the biological conversion of nitrogen in soil from a reduced to a more oxidized state may be taken as definitions for nitrification processes in soil.

Several microorganisms belonging to the genera Pseudomonas, Corynebacterium, Nocardia, Aspergillus, Streptomyces. Mycobacterium, Bacillus and Vibrio have also been shown to produce either nitrite or nitrate from ammonia or other reduced forms of nitrogen. The reduced forms of nitrogen which have been used by several investigators in laboratory ex­periments are nitrophenols, nitrobenzoates, and oximes of a number of organic acids such as pyruvic, oxalacetic and a-ketoglutaric 'acids.

The reactions mediated by these microorganisms may at best produce not more than 5 ppm equivalents of nitrite-nitrogen as against the 2000 ppm trans­formed by Nitrosomonas and Nitrobacter. Experiments with Aspergillus flavus to find out its significance as a soil nitrifier have shown that addition of such cultures to sterilzied soils and quartz resulted in no nitrate produc­tion. Therefore, the role of such heterotrophic microorganisms in nitrifica­tion processes in soil is still rather uncertain.