In this article we will discuss about the nitrogen cycle.
Ultimate source of nitrogen to all the living organisms is molecular or atmospheric nitrogen which constitutes about 78% of the air. This nitrogen after being fixed into inorganic nitrogenous compounds is consumed by the plants where it is converted into proteins etc., i.e., organic nitrogen and is then eaten by the animals directly or indirectly.
Combustion and decay of the dead plants and animals result in the release of free molecular nitrogen which goes into the air and the formation of simple inorganic compounds which are again taken up by the plants or are converted into free molecular nitrogen by de-nitrification. Thus, the existence of the nitrogen cycle maintains equilibrium of nitrogen in nature.
Some details of the various steps of the nitrogen cycle which is shown in Fig. 9.29 are as follows:
(1) Some of the free molecular nitrogen of the atmosphere is fixed into nitrates non-bio- logically by lightening when oxygen of the air combines with nitrogen. The nitrogen so fixed is brought down into the soil along with the rains from where it is absorbed by the plants and is reduced to ammonia.
Atmospheric nitrogen is also fixed biologically by certain organisms like legumes, some blue-green algae, and some bacteria etc., which convert it into ammonia.
Nitrates, nitrites, and ammonia are also supplied to the soil artificially as fertilizers.
(2) Ammonia in plants is converted into organic form and utilized in the synthesis of proteins and other organic nitrogenous compounds.
(3) Plant proteins are eaten by animals directly or indirectly through other smaller animals which they eat.
(4) Combustion of dead plant and animal bodies results in the production of free molecular nitrogen which again goes back into the atmosphere.
(5) Excreta of animals and dead organic nitrogenous matter produced as a result of the death of plants and animals are decomposed by certain micro-organisms in the soil through the processes of ammonification and nitrification to produce ammonia, nitrates and nitrites which may again be taken up by other plants.
(6) The nitrates and nitrites so produced may also be converted into molecular nitrogen by certain micro-organisms (denitrifying bacteria) e.g., Bacillus denitrificans present in the soil through a process called as de-nitrification.
Conversion of dead organic compounds in the soil into ammonia by ammonifying bacteria e.g. Bacillus mycoides, B. ramosus and B. valgaris is called as ammonification.
Oxidation of ammonia produced by ammonification to nitrates by nitrifying bacteria is called as nitrification.
It takes place in two steps:
(i) In the first step the ammonia is oxidised to nitrites by nitrifying bacteria called as Nitrosomonas and Nitrosococcus.
2NH3 + 3O2 → 2HN2O +2H2O
(ii) In the next step the nitrites are further oxidised to nitrates by another group of nitrifying bacteria called as Nitrobacter.
2HNO2 + O2 2HNO3