During which process does nitrogen convert from its gaseous state to a form usable by plants?

Nitrogen fixation is the process through which atmospheric nitrogen (N₂), which is in a gaseous state, is converted into a form that plants can utilize, such as ammonia (NH₃) or related compounds. This transformation is crucial because atmospheric nitrogen is relatively inert and cannot be directly used by plants.

During nitrogen fixation, specific bacteria, often found in the root nodules of legumes or in the soil, convert the gaseous nitrogen into ammonia through various biochemical reactions. This ammonia can then be used by plants to synthesize essential compounds like amino acids and nucleotides, which are vital for their growth and development.

To provide additional context, nitrification refers to the conversion of ammonia into nitrites and then nitrates, forms which are also usable by plants but do not involve the initial conversion from gas to a usable form. Ammonia volatilization describes the loss of ammonia gas from the soil into the atmosphere, not its conversion into a usable form. Denitrification involves the conversion of nitrates back into nitrogen gas, thereby releasing it back into the atmosphere and reducing the amount available for plant uptake.

Thus, nitrogen fixation is the critical step that directly enables plants to access nitrogen in a usable form.