Nitrogen cycle refers to the series of complex reactions/ processes which nitrogen atoms undergo in nature through organic and inorganic forms. When plants and animals die, they undergo a process known as ammonisation. This is an enzymatic digestion which changes organic matter or proteins into complex amino acids. These amino acids will undergo another stage in the cycle known as ammonification. which involves the breaking down of complex amino acid into ammonia nitrogens. Some of this ammonia nitrogen may be used up directly by plants or by microorganism for humus formation and Amino acid ammonification R-OH-NI-1%. The remaining ammonia nitrogen then undergoes the third stage called nitrification which is an oxidative process where ammonial nitrogen is converted to nitrite with the help of two specific bacteria in nitrosomones and nitroecoccus.
The nitrite so formed undergoes further oxidative process, changing to nitrate with the help of the bacterium, Nitrobacter. Plants then absorb the nitrate and once more use it to build up plant protein. Animals which eat the plant die later and decay to add back organic matter to the soil. Some bacteria the soil take up nitrogen from the air and fix it to nitrate ready for plant use. Some are the symbiotic Rhizobiun) spp, which live in the root nodules of legumes.They fix large quantities of Nitrogen.
Some non-symbiotc bacteria also exist-Azo tobecter and clostridium live freely in the soil and fix atmospheric Nitrogen into the soil. Nitrogen is also fixed through lightening during rain storm Atmospheric nitrogen combines with oxygen during lightening to form nitric oxide and the nitric oxide is oxidised to form nitrogen dioxide by atmospheric oxygen, Nitrogen dioxide then dissolves in rain water to form nitrous acid and nitric acid. Nitrous acid is quickly oxidised and washed down the soil.
The nitrates so formed from these processes are changed back to atmospheric nitrogen by denitrifying bacteria. Some of the nitrates are lost through leaching and erosion. This process of addition and removal of nitrogen ensures balance in nature.
