The nitrogen cycle contains several stages:

Nitrogen fixation

Atmospheric nitrogen occurs primarily in an inert form (N2) that few organisms can use; therefore it must be converted to an organic – or fixed – form in a process called nitrogen fixation. Most atmospheric nitrogen is ‘fixed’ through biological processes.

Nitrification
While ammonia can be used by some plants, most of the nitrogen taken up by plants is converted by bacteria from ammonia – which is highly toxic to many organisms – into nitrite (NO2-), and then into nitrate (NO3-). This process is called nitrification, and these bacteria are known as nitrifying bacteria.

Assimilation
Nitrogen compounds in various forms, such as nitrate, nitrite, ammonia, and ammonium are taken up from soils by plants which are then used in the formation of plant and animal proteins.

Ammonification
when plants and animals die, or when animals emit wastes, the nitrogen in the organic matter reenters the soil where it is broken down by other microorganisms, known as decomposers. This decomposition produces ammonia which is then available for other biological processes.

Denitrification
Nitrogen makes its way back into the atmosphere through a process called denitrification, in which nitrate (NO3-) is converted back to gaseous nitrogen (N2). Denitrification occurs primarily in wet soils where the water makes it difficult for microorganisms to get oxygen. Under these conditions, certain organisms – known as denitrifiying bacteria – will process nitrate to gain oxygen, leaving free nitrogen gas as a byproduct.

Source: Environment by Shankar IAS