The absorption of nitrogen by plants from the soil, then consumption of plants by animals and when the animals and bacteria decompose them, returning back of nitrogen to the soil, the whole cycle is called as nitrogen cycle. In this cycle, nitrogen converts into its various chemical forms. Both biological and non biological methods are involved in this cycle.
Nitrogen is the most important component of the air. Among all the gases present in the air, nitrogen is most abundantly present. It is an odorless and colorless gas. It is the most important gas for all organisms on earth. Nitrogen is present in DNA, RNA and other proteins which are the building blocks of human body as well as all other organisms. Nitrogen present in the air is not in the usable form. There are certain bacteria which convert the atmospheric nitrogen into nitrogen compounds so that it can be used by all living beings. There is a process for this purpose called as nitrogen cycle.
Steps involved in Nitrogen Cycle:-
To convert the atmospheric nitrogen into usable nitrogen, five steps are involved.
1) Nitrogen fixation:-
Nitrogen present in the air is in the static form N2 that is it cannot be used by any organism until it is fixed by a process called nitrogen cycle. Through precipitation, nitrogen enters the soil and surface waters. When it settles down in the soil or water, it converts into two separate atoms by undergoing some changes which combine nitrogen with Hydrogen. The resulting compound is called as ammonium. Microorganisms present in the soil are responsible for carrying out this conversion. They are placed into three groups such as bacteria live in symbiotic relationship with legume plants, free aerobic bacteria and algae. All the microorganisms are beneficial for the soil. There are certain bonds between atmospheric nitrogen which make it inert. Bacteria and other microbes break these bonds with the help of an enzyme nitrogenase. When the bond breaks, nitrogen converts into two forms each combing with hydrogen and other chemicals. Sometimes light also plays an important role in breaking down the bond in atmospheric nitrogen and converts it into ammonia and nitrates. When there is no oxygen, only then the enzyme nitrogenase acts. Therefore, the microbes involved in nitrogen cycle live at places where there is no oxygen like root nodules. Rhizobium is the example.
It is a process of conversion of ammonia into nitrites and then nitrates. Special kinds of bacteria are involved in this process which occurs naturally in the environment. The bacteria nitrosomonas and nitrococcus convert the ammonia into nitrite and then nitrobacter convert the nitrites into nitrates by oxidizing NO2 to NO3. All these bacteria reside in soil and are called as nitrifying bacteria. When the bacteria convert nitrogen into usable form, they gain energy from this process and need oxygen so that the process can be carried out.
In this step, all the nitrites and nitrates produced in the whole nitrogen cycle are consumed by plants and animals and use it in their cellular processes. For example, NO3 and NH4 both are consumed by plants which take them through their roots into various plant parts to make proteins and other nucleic acids. When animals eat these plants, the nitrates and nitrites automatically transfer into their body.
When n animal or plant dies or hen they release wastes from their bodies, nitrogen is released in the organic form. This organic nitrogen is converted into ammonium by fungi and bacteria through the process Ammonification. The ammonia produced can be used in other biological processes of plants.
When the ammonia is converted back into inert nitrogen, the process is called as denitrification. Bacteria are involved in this process which takes place in anaerobic conditions. Places like deep soils and deep water are the places without oxygen. Pseudomonas and Clostridium are responsible for the step of denitrification. These bacteria can also live in the places where there is availability of oxygen.
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