Which process in the nitrogen cycle involves the conversion of atmospheric nitrogen into a usable form for plants?

Nitrogen fixation is the process in the nitrogen cycle where atmospheric nitrogen (N₂), which is inaccessible to plants, is converted into a form that plants can utilize, such as ammonia (NH₃) or related compounds. This conversion is primarily facilitated by specific bacteria, including those associated with the roots of leguminous plants (e.g., clover and beans) and free-living bacteria in the soil. These microorganisms possess the enzyme nitrogenase, which allows them to break the strong triple bond in molecular nitrogen and convert it into ammonia.

Once nitrogen is fixed, it can be further transformed into nitrates and nitrites through subsequent processes such as nitrification, which is the conversion of ammonia into these forms. However, the initial step of transforming atmospheric nitrogen into a usable form for plants—wherein plants can then absorb it—is nitrogen fixation, making it the essential process for integrating nitrogen into the ecosystem.

Other processes like denitrification and ammonification play different roles in the nitrogen cycle, such as returning nitrogen to the atmosphere or converting organic material back into ammonia, respectively, and do not directly contribute to the conversion of atmospheric nitrogen for plant use.