Nitrogen cycle

Nitrogen cycle

Among the bio-geo chemical cycle,Nitrogen cycle is very important. In Earth's atmosphere about 78% is nitrogen, making it the largest pool of nitrogen. Nitrogen is essential for many biological processes; and is crucial for any life here on Earth. It is in all amino acids, is incorporated into proteins, and is present in the bases that make up nucleic acids, such as DNA and RNA.In plants, much of the nitrogen is used in chlorophyll molecules which are essential for photosynthesis and further growth.Although, most organisms live in a nitrogen rich atmosphere but it can be used by very few organisms. However the requirement of nitrogen is excess in almost all organism. The nitrogen cycle is initiated from the atmospheric nitrogen by microbes and it is maintained by all organisms and abiotic factoars of environment.

Processing, or fixation, is necessary to convert gaseous nitrogen into forms usable by living organisms. Some fixation occurs in lightning strikes, but most fixation is done by free-living or symbiotic bacteria. These bacteria have the nitrogenase enzyme that combines gaseous nitrogen with hydrogen to produce ammonia,which is then further converted by the bacteria to make its own organic compounds.The overall biological involvement in nitrogen cycle is for more extensive, complicated and ordered.The distinctive process of Nitrogen cycle are as follows -----

(i)Nitrogen fixation : Molecular nitrogen gets into the biological pathways of the nitrogen cycle through a process of nitrogen fixation. It involves oxidation or reduction of atmospheric nitrogen to nitrates and ammonia which can be used by living organisms. Nitrogen fixation occurs by the following important processes-

(a) Atmospheric fixation : Atmospheric fixation is accomplished by a physio- chemical process that occurs when lightning converts atmospheric nitrogen in nitric acid. This acid is dissolved by rain waters and carrying to the earth. During lighting dischange , oxides of nitrogen are farmed which are subsequently hydrated by water and carried to earth as nitrites and nitrates.

N₂+O₂ à2NO à2NO₂

2NO₂+H₂O à HNO₂+HNO₃

3NO₂+H₂Oà 2HNO₃+NO

(b) Industrial nitrogen fixation:There are few man made process where nitrogen is fixed but they required rather drastic condition temperature and pressure.Some of these are Haber process, Berkland-Eyed process, cynamide process etc.

(c) Biological nitrogen fixation: Few biological organism help in nitrogen fixation.

Some specialised biological nitrogen fixation are as follows----

(i) Symbiotic nitrogen fixation: It has been widely recognised from centuries that most crops decrease the fertility of the soli but leguminous plant increase it. Legumes thus restore or renew the soli. This is because legumes fix nitrogen from the air by certain symbiotic bacteria living in root nodule of them. Since, neither the plant nor the bacteria can fix the atmospheric nitrogen independently. The process is there fare called symbiotic nitrogen fixation. Reknown scientist Beijerinck in 1888 was isolated and identified the bacterium of root nodule is Rhizobium

Rhizobium sp. invades the root hairs of leguminous plant.The bacteria aggregate as thread and penetrate the plant root cells. The presence of bacteria stimulates the multiplication of infected celled resulting the formation of nodules. The legume, the bacteria and the nodule constitute the system far symbiotic N₂ fixation where both bacteria and the plant benefited by the association.

(ii) Non-Symbiotic nitrogen fixation: Some non symbiotic N₂ fixing bacteria can fix nitrogen without the involvement of host. Some gram negative non-spore farming aerobic pleomarphic coccoid or rod shaped bacteria belonging to genus Azotobacter are able to fix nitrogen non-symbiotically.The mechanism of nitrogen fixation is not fully understood but the first product detectable by isotopic studies in ammonia.The over all non symbiotic nitrogen fixation includes following steps

(a) Ammonification: Release of nitrogen from organic nitrogenous compounds is termed as ammonification. Proteins and other organic compounds of living and dead organism are other organic compounds of living and dead organism are decomposed by microorganism.These compound are hydrolysed by various proteolytic enzymes to amino acid and similar compounds. Proteolytic enzymes are elaborated by some species of Clostroidium, many fungi, Actinomcetes etc.The ultimate product of proteolysis is amino acid.Amino acids are deaminated under aerobic condition with the liberation of ammonia.The amino groups are split off to form ammonia. Reduction of nitrogen to ammonia requires a valency change (0 to -3).Six electrons are there fare required to reduce one molecule of nitrogen to two molecules of ammonia.

N₂+6H⁺+6e^- à 2NH₃

(b) Nitrification: The oxidation of ammonia to nitrate is called nitrification. This process consists of two steps and is carried out by two specific groups of organisms of family Nitrobacteriaceac.These are strict autotrophs and obtain their energy from the oxidation of ammonia to nitrate and of nitrites to nitrates.In the first step, oxidation of ammonia to nitrate takes place,which is called nitrosification and it is accomplished by bacteria of genera Nitrosomonas , Nitrosococcus , Nitrosospira etc. This nitrates are formed by those organisms which are toxic to plant as well as to organism forming it. Fortunately nitrates are removed by further oxidation of nitrate to nitrite by bacteria of the genus Nitrobacter.

NH₃+3/2O₂à NO₂^-+H₂O+H⁺(4G=66,500 cal)

NO₂^-+1/2O₂à NO₃^-(4G=17,500 cal)

The nitrification process is reversed by many micro organisms which are capable of reducing nitrate to nitrite and then to ammonia. This process is called Nitrate reduction.

HNO₃+4H₂-------------à NH₃+3H₂O

NH₃+H₂O -------------à NH₄

NO₃^-+2e^-+2H⁺---------------à NO₂^-+H₂O

Nitrite is ultimately reduced to molecular nitrogen through several reactions which are not clearly underatood. The overall reaction is -----

2NO₃^-+10e^-+12H⁺--------------à N₂+6 H₂O

When the nitrate is used as a source of electron acceptors, there is a net loss of nitrogen from the soil. This process is there fare called Dissimilatory nitrate reduction. Except that, nitrogen is also released by denitrification during seasonal flooding of the land or as a result of over irrigation of poorly drained land. Some anaerobic microbes also helps in denitrification.

eg.Thiobacillus denitricans,Micrococcus denitrifican, Serratia sp. Pseudomnus.

In the ecosystem , nitrogen is incorporated and gradually released from the biological organism to the environment. Nitrates, formed by microbes can enter groundwater due to their very high solubility. Plants get nitrogen from the soil, and by absorption of their roots in the form of either nitrate ions or ammonium ions.They convert it into amino acids and protein. Harbivores may eat plants and utilize the amino acids from the plant proteins in the synthesis of their own proteins and other cellular constituents. When carnovores use harbivores as food, they get nitrogen in the form of animal protein and so on. After the death and decay of plant and animal body, microbes convert nitrogen of proteins and other compounds to ammonis which farther reduce into nitrogen by bacteria. The movement of nitrogen is thus bidirectional and regulated both biologically and non- biologically.