Legume plants such as peas, beans and clover contain nitrogen-fixing bacteria. These bacteria live in swellings in the plant roots called nodules. Nitrogen-fixing bacteria convert nitrogen gas from air into a form that plants can use to make proteins.
Free-living nitrogen-fixing bacteria are also found in the soil. When they die the nitrogen they have fixed into their biomass is converted into ammonium.
Animals consume plant protein, digest it using specific enzymes and absorb the free amino acids.
Animals cannot store excess protein in their bodies. They break it down and turn it into waste products and excrete them from their bodies.
Decomposers (some free-living bacteria and fungi) break down animal and plant proteins (from dead organisms) and nitrogenous waste products to release energy. As a result of decomposition nitrogen is released into the soil in the form of ammonium.
A group of free-living soil bacteria called nitrifying bacteria convert ammonium into nitrates in order to obtain energy.
Non-legume plants absorb nitrates from the soil into their roots and use the nitrates to produce their proteins.
This is when bacteria in the soil convert the nitrate back into nitrogen gas which then gets released back into the atmosphere.