What is the one way nitrogen may be returned to the air?

Nitrogen in its gaseous form (N2) can't be used by most living things. It has to be converted or 'fixed' to a more usable form through a process called fixation. Biologically: Nitrogen gas (N2) diffuses into the soil from the atmosphere, and species of bacteria convert this nitrogen to ammonium ions (NH4+), which can be used by plants.

Question - What is the one way nitrogen may be returned to the air?

Answer - Nitrogen fixation

Further Explanation On Answer - 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.