The Nitrogen Cycle
Nitrogen is constantly taken, or fixed, from the atmosphere, oxidised to a form able to be utilised by plants and some bacteria, to be subsumed into metabolic pathways, and through the various routes described above is then excreted into the environment as reduced nitrogen where it may be reoxidised by bacteria or released back into the atmosphere as nitrogen gas. These combined processes are known collectively as the nitrogen cycle. The previous discussions have referred to the release of nitrogen during degradation of proteins and nucleic acid bases, either in the form of ammonia, the ammonium ion, urea or uric acid. The fate of all these nitrogen species is to be oxidised to nitrite ion by Nitrosomas, a family of nitrifying bacteria. The nitrite ion may be reduced and released as atmospheric nitrogen, or further oxidised to nitrate by a different group of nitrifying bacteria, Nitrobacter. The process of conversion from ammonia to nitrate is sometimesfound as a tertiary treatment in sewage works to enable the nitrate consent to be reached. The process typically occurs in trickling bed filters which have, over time, become populated with a Nitrosomas and Nitrobacter along with the usual flora and fauna which balance this ecosystem. Denitrification may then occur to release atmospheric nitrogen or the nitrate ion, released by Nitrobacter, may be taken up by plants or some species of anaerobic bacteria where it is reduced to ammonium ion and incorporated into amino acids and other nitrogen – carbon containing compounds. To complete the cycle, atmospheric nitrogen is then fixed by nitrifying bacteria, either free living in the soil or in close harmony with plants as described earlier.