Question

What is the role of Nitrogen fixing bacteria in the Nitrogen cycle?

Answer 1

Nitrogen cycle is one of the biogeochemical cycles in which elementary nitrogen is circulated between the atmosphere and living organisms in various forms. There are various steps involved in the nitrogen cycle. These are- nitrogen fixation, ammonification, nitrification, assimilation and finally denitrification.

Complete answer:
The nitrogen fixation is the first step of the nitrogen cycle. During nitrogen fixation, the inert, unstable nitrogen is converted into ammonia or other forms, which is the usable form of nitrogen. In nature, nitrogen fixation occurs mainly by two processes. First is atmospheric nitrogen fixation and the other one is biological nitrogen fixation. However, most of the nitrogen fixation is done by bacteria.
In atmospheric nitrogen fixation, the inert nitrogen is converted into nitrous oxide by lightning. The high temperature at the time of lightning the nitrogen is broken down and reacts with oxygen to form nitrous oxide. It is converted into nitrates when on reaction with alkalis present on the earth surface.
The bacteria play an important role in the nitrogen fixation during the nitrogen cycle. In biological nitrogen fixation, the nitrogen fixing bacteria present in soil converts the nitrogen into ammonia. This ammonia is later converted into nitrites and then nitrates by other soil living bacteria. Biological nitrogen fixation is done by free-living bacteria and symbiotic bacteria. Example of free living bacteria that helps in nitrogen fixation is- Azotobacter. The symbiotic bacteria present in the roots of leguminous plants also helps in nitrogen fixation. Rhizobium is the most common bacteria which are known for nitrogen fixation.
The nitrogen fixation by bacteria is important as it is the primary step in the nitrogen cycle. The conversion of nitrogen to plant usable form is essential for the growth and survival of plants.

Note:
All the nitrates are not used by the plants. Some of the nitrates are converted back into elementary nitrogen and released into the atmosphere. This process is called denitrification. The bacteria Pseudomonas and Clostridium help in the denitrification process. This is the last step of the nitrogen cycle.