Final answer:
Nitrogen fixation is not involved in the conversion of organic nitrogen to nitrogen gas; it's the process where atmospheric nitrogen is converted to ammonia. Ammonification, nitrification, and denitrification are the other processes that contribute to the nitrogen cycle.
Step-by-step explanation:
The process not involved in the conversion of organic nitrogen to nitrogen gas is nitrogen fixation. Nitrogen fixation is the initial step in the nitrogen cycle, where atmospheric nitrogen (N₂) is converted by bacteria into ammonia (NH₃), making it available for other organisms. The other options, nitrification, denitrification, and ammonification, are all processes that contribute to the cycling of nitrogen. Ammonification is the conversion of organic nitrogenous matter from living organisms into ammonium (NH₄+). Nitrification is a two-step process where ammonium is first oxidized to nitrites (NO₂-) and then to nitrates (NO₃-). Finally, denitrification is the process where nitrates are reduced and eventually converted back into nitrogen gas (N₂), completing the cycle.