Explain: Biological Nitrogen Fixation.

(N/A) Definition: The reduction of nitrogen to ammonia by living organisms is called biological nitrogen fixation.
Explanation: Very few living organisms can utilize the nitrogen in the form of $N_{2}$,which is available abundantly in the air.
Only certain prokaryotic species are capable of fixing nitrogen. Thus,nitrogen is reduced into ammonia. This process is exclusively found in prokaryotic organisms. These microbes are called $N_{2}$ fixers.
$N \equiv N \xrightarrow{\text{Nitrogenase}} NH_{3}$
The nitrogen-fixing microbes could be free-living or symbiotic. Examples of free-living nitrogen-fixing aerobic microbes are $Azotobacter$ and $Beijerinckia$. While $Rhodospirillum$ is anaerobic and $Bacillus$ is free-living.
$A$ number of cyanobacteria such as $Anabaena$ and $Nostoc$ are also free-living nitrogen fixers.
Symbiotic Biological Nitrogen Fixation: Several types of symbiotic biological nitrogen-fixing associations are known. The most prominent among them is the legume-bacteria relationship.
Species of rod-shaped $Rhizobium$ have such a relationship with the roots of several legumes such as alfalfa,sweet clover,sweet pea,lentils,garden pea,broad bean,clover beans,etc.
The most common association on roots is in the form of nodules. These nodules are small outgrowths on roots. The microbe,$Frankia$,also produces nitrogen-fixing nodules on the roots of non-leguminous plants (e.g.,$Alnus$).
Both $Rhizobium$ and $Frankia$ are free-living in the soil,but as symbionts,they can fix atmospheric nitrogen.