What process in the nitrogen cycle converts atmospheric nitrogen into usable forms for living organisms?

The process that converts atmospheric nitrogen into usable forms for living organisms is known as fixation. During nitrogen fixation, atmospheric nitrogen gas (N₂) is transformed into ammonia (NH₃) or related compounds, which can then be utilized by plants. This critical step is primarily carried out by certain bacteria, including those in the root nodules of leguminous plants, and some free-living bacteria present in the soil and water.

Nitrogen fixation is essential because most living organisms cannot use atmospheric nitrogen directly. Instead, they rely on these fixed forms of nitrogen, which play a vital role in the production of amino acids, nucleic acids, and other important biological molecules. Without nitrogen fixation, the availability of nitrogen in forms that plants can assimilate would be limited, hindering the entire ecosystem's productivity and growth.

Other processes like ammonification and nitrification contribute to the nitrogen cycle but serve different roles; they involve the breakdown of organic nitrogen compounds and the transformation of ammonia into nitrates, respectively. Denitrification, on the other hand, returns nitrogen to the atmosphere. These processes highlight the dynamic nature of the nitrogen cycle, but it is fixation that initiates the cycle by making nitrogen available in a form that living organisms can utilize.