We know that air consists of 80% nitrogen. Most living forms including plants depend on nitrogen for their growth. The cycle in which nitrogen is converted into organic matter and then returned to nature as nitrogen is called nitrogen cycle. It has four important reactions – Nitrogen fixation, ammonification, nitrification and denitrification. Micro organisms play a very important role in all these processes.
The first step is all about converting nitrogen into more usable nitrogenous compounds. Most plants have nitrogen fixing bacteria called rhizobium present in the soil that helps the plant use nitrogen in a more usable form called ammonia. This process is called nitrogen fixation. When nitrogen combines with hydrogen, ammonia is formed. Green plants use this to make proteins. If we pull up a plant and look closely at its roots, we may spot little nodules. Rhizobium converts nitrogen from the air into ammonia and stores in the form of lumps on the nodules.
Nitrogen fixation can happen in 3 ways – atmospheric fixation (small amount fixed by lightning), industrial fixation (Haber’s process used to make nitrogen fertilizers) and biological fixation (Rhizobium).
The second step is ammonification. Certain bacteria and fungi in the soil act as decomposers breaking down the proteins in dead living organisms and animal wastes releasing ammonium ions.
Then the nitrification process involves two processes – converting ammonium ions first into nitrites and then into nitrates which is most usable by the plants.
The final step is the denitrification process in which nitrates are reduced to nitrogen gas. Thus nitrogen gas is returned to the atmosphere completing the nitrogen cycle.
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