Nitrogen Fixation

Nitrogen fixation is the process by which elemental nitrogen is converted into other nitrogen compounds which are useful for chemical processes. Nitrogen is fixed in two ways: one during lightning and by nitrogen-fixing bacteria, and blue-green algae.

Rhizobium bacteria: -

Rhizobium bacteria living in the root nodules of legume plants fix nitrogen from the atmosphere. Microorganisms in the soil convert atmospheric nitrogen into ammonium compounds, thereby enabling the survival of plants and animals.

What is the mechanism of each of these steps?

This is the first step of the nitrogen cycle. This step is characterized by the conversion of atmospheric N2 into ammonia (NH3). Bacteria like Azotobacter and Rhizobium have a major role in this process. They are harbored in the roots of the leguminous plants and help convert inert nitrogen to ammonia. Nitrogen fixation can occur in any of the following ways: atmospheric fixation (involves lightening), industrial fixation (manufacturing ammonia under high temperature and pressure conditions)


Once the nitrogen has been fixed in the soil, plants can absorb nitrogen through their roots. This process of absorption is known as assimilation.


This is another process by which ammonia can be generated. Organic remains of plants and animals are broken down in the soil by some bacteria to release ammonia into the soil. These dead and waste matters are used by these microorganisms as food and they release ammonia into the soil.


This occurs in two steps. The first step is in which NH3/NH$+ is converted to NO3- (nitrates). The bacteria Nitrosomonas and Nitro coccus present in the soil convert NH3 to NO2-, and another bacterium, Nitrobacter converts NO2- to NO3-. These bacteria gain energy through these conversions.


Is the reverse of nitrification that occurs in the deep layers of soil where the bacteria convert NO3- is converted into N2 and other gaseous compounds like NO2. This occurs because in deep layers of soil, oxygen is not available and the soil bacteria use these nitrogen compounds instead of oxygen.

What is the importance of the nitrogen cycle?

  1. As we all know by now, the nitrogen cycle helps bring in the inert nitrogen from the air into the biochemical process in plants and then to animals.
  2. Plants need nitrogen to synthesize chlorophyll and so the nitrogen cycle is absolutely essential for them.
  3. During the process of ammonification, the bacteria help degrade decomposing animal and plant matter. This helps in naturally cleaning up the environment.
  4. Due to the nitrogen cycle, nitrates and nitrites are released into the soil which helps in enriching the soil with nutrients needed for cultivation.
  5. As plants use nitrogen for their biochemical processes, animals obtain nitrogen and nitrogen compounds from plants. Nitrogen is needed as it is an integral part of the cell composition