Ans:Nitrogen Cycle
Nitrogen is an important component of many biomolecules, like proteins and nucleic acids (DNA and RNA). Atmosphere is the reservoir of free gaseous nitrogen. Living organisms cannot pickup this gaseous nitrogen directly from atmosphere (except for nitrogen fixing bacteria). It has to be converted into nitrates to be utilized by plants. Nitrogen cycling involves several stages.
a) Formation of Nitrates
It is done in the following ways:
I) Nitrogen Fixation
Conversion of nitrogen gas into nitrates is called nitrogen fixation. It occurs in the following ways.
• Thunderstorms and lightning convert atmospheric gaseous nitrogen to oxides of nitrogen to oxides of nitrogen. These oxides dissolve in water and form nitrous acid and nitric acid. The acids in turn combine with other salts to produce 'nitrates', It is called as atmospheric nitrogen fixation.
• Some bacteria also have the ability to transform gaseous nitrogen into nitrates. Jt is called biological nitrogen fixation. Some of these nitrogen fixing bacteria live as symbionts and many are free-living.
• Nitrogen fixation is also done in industries. In Industrial nitrogen fixation, hydrogen is combined with atmospheric nitrogen under high pressure and temperature. It produces ammonia which is further converted into ammonium nitrates. . r
It) Ammonification and Nitrification .
Ammonification is the breakdown of the proteins of dead organisms and nitrogenous wastes (urea, uric acid etc.) to ammonia. It is done by ammonifying bacteria. After the formation of ammonia, it is converted into nitrates. It is called nitrification and is done by nitrifying bacteria. First, ammonia is converted into nitrates by bacteria (e.g. Nitosomonas). The nitrates are then converted into nitrates by other bacteria (e.g. Nitrobacter).
b) Assimilation .
The nitrates formed by the above processes, are absorbed by plants and are utilized for making proteins etc. Animals take nitrogenous compounds from plants. The utilization of nitrates by organisms is called assimilation.
c) Denitrification
It is biological processes in which nitrates and nitrites are reduced to nitrogen gas by denitrifying bacteria. By this process, nitrogen is returned to atmosphere.
Excessive denitrification reduces soil fertility and is stimulated by water logging, lack of aeration and accumulation of organic matter in the soil Figure 16 8