The process you are referring to, where certain bacteria living in the roots of plants convert nitrogen gas (N₂) into ammonia (NH₃), is called nitrogen fixation. The bacteria involved in this process are often associated with leguminous plants, such as peas and beans, and include genera like Rhizobium.
To clarify the other terms:
- Nitrification is the process where ammonia is converted into nitrites (NO₂⁻) and then into nitrates (NO₃⁻) by different types of bacteria.
- Denitrification is the process where nitrates are converted back into nitrogen gas (N₂), thus returning it to the atmosphere.
So, the correct term for the conversion of nitrogen gas to ammonia is nitrogen fixation, and the bacteria involved would be categorized as bacteria (specifically, nitrogen-fixing bacteria).