The return of nitrogen from living organisms back into the atmosphere is achieved through a process called denitrification. Denitrification is carried out by certain microorganisms, such as bacteria and archaea, that use nitrates as an electron acceptor during respiration. These microorganisms convert nitrates into nitrogen gas, which is released into the atmosphere.

Denitrification typically occurs in environments with low oxygen levels, such as waterlogged soils, wetlands, and compost heaps. It is an important process in the nitrogen cycle, as it helps to maintain the balance of nitrogen in the ecosystem and ensures that nitrogen is available for plant growth.