Denitrifying bacteria like Pseudomonas denitrificans, Thiobacillus denitrificans utilizes nitrates and other oxidised ion as source of oxygen. They undergo denitrification in which nitrates are 'reduced to gaseous compounds of nitrogen and depletion of an important nutrient occurs from the soil. These microbes functions under anaerobic condition. Nitrogen fixing bacteria both free living (Azotobacter) and symbiotic (Rhizobium, Azolla etc.) undergo conversion of inert atmospheric nitrogen or dinitrogen (N2) into utilizable compounds of nitrogen like nitrate, ammonia, amino acids etc. Ammonifying bacteria like Bacillus ramosus, B. Vulgaris are the decay causing micro-organisms. They act upon nitrogenous excretions and proteins ohdead bodies of living organism. They breaks the proteins into amino acids, deaminating them into organic acid and ammonia. Nitrifying bacteria like Nitrosbmonas and Nitrobacter, Aspergillus flavus converts ammonium nitrogen to nitrate via nitrite formation.