Nitrogen is very important to human and living beings. It is chiefly found in amino acids and proteins. There are compounds continuously built and broken down in metabolic activities. Nitrogen has to be continuously taken up by the plants. 78% of the earth atmosphere is nitrogen but plants cannot taken up nitrogen in free gaseous state. It has to be absorbed by plants in the form of soluble salts. Nitrogen may be reduced oxidized, or hydrolyzed.
Nitrogen Fixation:
As nitrogen is not available in free gaseous state, it has to be fixed by either physiochemical processes or by biological process. The nitrogen fixation first requires the activation of molecular nitrogen by spitting it into two atoms of free nitrogen (N2–2N) combination of free nitrogen with hydrogen to form ammonia (N–NH3). This is an endothermic reaction. This is regulated by nitrogenase and hydrogenase. Bacteria causing this are symbiotic nitrogen fixers and free living nitrogen fixers.
Ammonification:
Many heterotrophic bacteria and fungi in both soil and water utilize organic nitrogen-rich material, converting it and releasing it as inorganic ammonia in a process is called ammonification.
Nitrification:
Ammonia is converted to nitrite by the bacterial genus Nitrosomonas, and the nitrite is then converted to nitrate by another genus Nitrobactor. This two step process termed as Nitrification. First the conversion of ammonia to ammonium salts or nitrite and second conversion of nitrite to nitrate takes place.
Denitrification:
Denitrification to molecular or gaseous nitrogen (N2) as well as to nitrous oxide (NO) and nitric oxide (N2O) is effected by bacteria such as pseudomonas and fungi. Most denitrifies reduce nitrate only to nitrite, and others reduce it to ammonia. Because denitrification to molecular nitrogen is known to occur under anaerobic or partially anaerobic conditions , it would be expected to occur in soils that are poorly aerated. Thus the cycle is completed and again ammonifiation and nitrification follows.
Nitrogen is introduced into the aquatic environment through the discharge of domestic sewage and organic industrial wastes. Organic nitrogen (Proteins) and ammonia are the main constituents. These may be partially oxidized in the treatment process to nitrites and nitrates. The discharge of excessive quantities of nitrogenous compounds to rivers and lakes can result in excessive nuisance growth of algae and macrophytic plants.