The Nitrogen Cycle.

We breath nitrogen in air which is 4/5 and 1/5 oxygen is in small quantity.
The nitrogen cycle is the process whereby nitrogen passes from the atmosphere into living things and ultimately back into the atmosphere.
In the process, it is converted to nitrates and nitrites, compounds of nitrogen and oxygen that are absorbed by plants in the process of forming plant proteins.
These plant proteins, in turn, are converted to animal proteins in the bodies of animals who eat the plants, and when the animal dies, the proteins are returned to the soil.
Denitrifying bacteria break down these organic compounds, returning original nitrogen to the atmosphere.
Nitrogen, is a component of proteins and nucleic acids, and it is essential to life on Earth. Although 78 percent by volume of the atmosphere is nitrogen gas, this abundant reservoir exists in a form unusable by most organisms. Through a series of microbial transformations, nitrogen is made available to plants, which in turn ultimately sustains all animal life. The steps, which are not altogether sequential, fall into the following classifications.
Nitrogen fixation, Nitrogen assimilation, Ammoni-fication, Nitri-fication, and De-nitrification.
Nitrogen fixation, in which nitrogen gas is converted into inorganic nitrogen compounds, is accomplished by certain bacteria. A much smaller amount of free nitrogen is fixed by means of lightning, ultraviolet radiation, electrical equipment and by conversion to ammonia.
Nitrates and ammonia resulting from nitrogen fixation are assimilated into the specific tissue compounds of algae and higher plants. Animals then ingest these algae and plants, converting them into their own body compounds.
The remains of all living things--and their waste products--are decomposed by micro-organisms in the process of ammoni-fication, which yields ammonia.
Ammonia can leave the soil or be converted into other nitrogen compounds, depending in part on soil conditions.
Nitri-fication, a process carried out by nitrifying bacteria, transforms soil ammonia into nitrates, which plants can incorporate into their own tissues.
Nitrates also are metabolized by de-nitri-fying bacteria, which are especially active in water-logged, anaerobic soils. The action of these bacteria tends to deplete soil nitrates, forming free atmospheric nitrogen.