How does nitrogen eventually return to the atmosphere?
The Nitrogen cycle.
Nitrogen fixation: Nitrogen fixing bacteria in root nodules of legumes fix nitrogen from the atmosphere into ammonia. This is the natural equivalent of the Haber process.
Nitrification: Nitrites and Nitrate are produced from ammonium by Nitrifying bacteria. Nitrates are required by plants for the production of proteins.
Ammonification: The production of ammonia by microorganisms from dead plant and animals.
These process cycle Nitrogen from the air throughout the ground. Nitrogen is returned to the amosphere by:
Denitrification: Produces Nitrogen gas. The complete opposite of nitrification, ie Nitrates to Nitrites to Ammonia to Nitrogen gas.