The nitrogen cycle is the cycling of nitrogen between organisms and the environment. Without nitrogen, proteins and nucleic acids cannot be produced. Nitrogen gas, reacts chemically only under unique conditions. Only organisms capable of making their own food can use nitrates to generate proteins, nucleic acids, and other nitrogen compounds. Atmospheric nitrogen can be converted into the nitrates, the usable form of nitrogen.
All organisms produce wastes and eventually die.
In bacterial decay, proteins are slowly broken broken down into ammonia compounds, then to nitrites, and finally back to nitrates. The nitrates can then reenter the cycle.