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Nitrogen is the important constituents that build up the structural and functional component of living organisms. The ultimate source of nitrogen for the living organisms is the atmospheric nitrogen. The process of conversion of fixing the nitrogen present in atmosphere into the living organisms and liberating it back to the atmosphere is called as the nitrogen cycle. The nitrogen cycle is completed by the 5 main steps.

1. Nitrogen fixation: There two natural means of nitrogen fixation; they are non-biological fixation and biological fixation.

a. Non biological nitrogen fixation: It also known as electro chemical or photochemical nitrogen fixation. The conversion of atmospheric nitrogen into NO2, NO3, NH3 with help of lightning and radiation.

b. Biological nitrogen fixation: The atmospheric nitrogen can only be directly used up by the nitrogen fixing bacteria. These nitrogen fixers utilize nitrogen to synthesize NH3. Some of the nitrogen fixing bacteria are, blue green algae, fungi and lichens, they are commonly known as nitrogen fixers. These microorganisms maybe either free living or symbiotic. Bacteria Azotobacter, Clostridium, Rhodospirillum and the cyanobacterium Nostoc are the free living nitrogen fixers. The symbiotic nitrogen fixers are the lichen Collema, the bacterium Rhizobium which is found in the root nodules of leguminous plants.

2. Nitrification: The flowering plants absorbs nitrate than ammonium ions from the soil, so these ammonia has to be converted into nitrate. This process of conversion of the ammonia into nitrate is called as nitrification, the bacterium which brings about this conversion is known as nitrifying bacteria. Nitrification is completed by two main steps;

a. ammonia is oxidized to nitrite: In the first step the ammonia is oxidized by free living nitrogen fixing bacteria such as Nitrosomonas and Nitrosococcus.

b. Nitrite is oxidized to nitrate: This process occurs by another type nitrogen fixing bacteria known as Nitrobacter.

3. Nitrate assimilation: Now the higher plants can absorb nitrate from the soil, it then get reduced to ammonia in plants with the help of two enzymes. They are the nitrate reductase and nitrite reductase, the former reduces nitrate into nitrite and later reduces nitrite into ammonia From the plants the nitrogen will pass on to the animals through the food chain by amino acids.

4. Ammonification: The process of conversion of protein present in the dead plants and animals into ammonia is called ammonification or mineralization. Some nitrifying bacteria transforms soil ammonia to soil nitrates also.

5. Denitrification: The process of releasing the nitrogen back to the atmosphere is known as denitrification. The bacteria which mediated this reaction is known as the denitrifying bacteria e.g. Pseudomonas. This step of the cycle replenishes back the nitrogen to the atmosphere and completes the cycle.

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