Which process primarily involves the conversion of atmospheric nitrogen to a usable form for plants?

Nitrogen fixation is the process that primarily converts atmospheric nitrogen gas (N₂), which is not usable by most living organisms, into forms of nitrogen that plants can absorb and utilize, such as ammonia (NH₃) or nitrates (NO₃⁻). This transformation is crucial for the growth of plants, as nitrogen is an essential nutrient needed for the synthesis of amino acids, proteins, and nucleic acids.

In nature, this process is often carried out by specific bacteria, some of which live in symbiotic relationships with the roots of leguminous plants (like peas and beans). These nitrogen-fixing bacteria can take nitrogen gas from the atmosphere and convert it into a form that is accessible for the plants, ultimately benefiting the entire ecosystem by enriching the soil with usable nitrogen.

Understanding nitrogen fixation highlights its fundamental role in the nitrogen cycle, where nitrogen is circulated through the environment and made available for various organisms, thereby maintaining ecosystem health. Other processes listed, such as evaporation, photosynthesis, and transpiration, do not involve the conversion of atmospheric nitrogen and are related to different biological or physical processes.