Theory:

Nitrogen fixation
Rhizobium is a genus of bacteria that lives in the root nodules of leguminous plants, such as beans and peas, with which it has a symbiotic relationship. The atmospheric nitrogen gets fixed through these microbes.

Nitrogen cycle
Our atmosphere has $$78$$$$\%$$ of nitrogen. All living species including humans, animals, and plants require nitrogen to survive. Plants and animals cannot take nitrogen directly from the atmosphere. Nitrogen cycle takes place in the following steps:
1. Bacteria like Rhizobium and blue-green algae are used to fix nitrogen in the soil from the atmosphere to increase the fertility of the soil.

2. Once nitrogen is converted into these usable compounds, it can be utilised by plants from the soil through their root system. Nitrogen is then used for the synthesis of plant proteins and other compounds. Animals feeding on plants get these proteins and other nitrogen compounds.

3. When these plants die, the same bacteria and fungi will convert the nitrogen present in the dead plants into usable compounds, so that other plants and animals can make use of this compound.
This process of releasing the nitrogen back into the atmosphere from the dead plants and animals by microorganisms is called, Denitrification.
Majority of microorganisms involved in the entire nitrogen cycle are bacteria.