Question

What are the various steps involved in biological nitrogen fixation in plants?

Answer 1

Hint: The conversion of molecular nitrogen present in the atmosphere to ammonia or ammonium compounds in the soil is known as nitrogen fixation. The molecular nitrogen is mostly converted into utilisable form by the help of microorganisms.

Complete answer:
The atmosphere contains about 78% of the molecular nitrogen. This nitrogen is non-reactive and cannot be easily absorbed by the plants. This nitrogen is converted into ammonia or ammonium salts by the help of bacteria. This is known as nitrogen fixation. The cyanobacteria convert nitrogen into ammonia by the help of nitrogenase enzyme.

The steps involved in the fixation are:
1)Nitrification: It is the process of conversion of ammonia into nitrates. The ammonia is converted into nitrites by Nitrosomonas and nitrococcus. The nitrites are converted into nitrates by Nitrobacter.

2)Assimilation: In this the ammonia and nitrates are utilised by the plants. The roots help in absorbing these nutrients which are taken up by plants.

3)Ammonification: By this process, nitrogen present in the living matter is converted into ammonia. The ammonia is further released into the environment and is reused.

4)Denitrification: In this process, the nitrates are broken down back to nitrogen gas by the help of anaerobic bacteria.

Note: Leguminous plants help in fixing free nitrogen present in the air. The Rhizobium bacteria present in the roots of these plants help in converting the free nitrogen into nitrates, nitrites and ammonia. This relationship is known as symbiosis. Through this both the organism and the plants are benefitted. This symbiosis helps in increasing the fertility of the soil.