What process converts ammonia to nitrogen gas?

The correct choice is denitrification, which is the process that converts ammonia and other nitrogen compounds into nitrogen gas (N2), resulting in its release into the atmosphere. This microbial process is critical in the nitrogen cycle, as it helps to reduce excess nitrogen in soil and water, thus preventing issues such as eutrophication.

While nitrification is involved in the transformation of ammonia into nitrite and then into nitrate, it does not complete the conversion to nitrogen gas. In contrast, denitrification takes these nitrate compounds further and ultimately converts them into nitrogen gas, which is integral for balancing the nitrogen in ecosystems. This means that denitrification plays a vital role in sustaining the health of water bodies and terrestrial environments by removing excess nitrogen, which can cause pollution and other ecological problems.