Which of the following best describes nitrogen fixation?

Nitrogen fixation is the biochemical process through which atmospheric nitrogen gas (N₂) is converted into ammonia (NH₃) or related compounds, which can then be incorporated into organic molecules. This process is essential for life, as most organisms cannot use nitrogen gas directly. Instead, they rely on the fixed forms of nitrogen produced during this process to synthesize proteins and nucleic acids.

The correct choice describes nitrogen fixation accurately by highlighting the conversion of nitrogen gas into organic compounds. This conversion is primarily carried out by certain microorganisms, including bacteria in the genera Rhizobium and Frankia, which form symbiotic relationships with plants.

The other options do not accurately capture the essence of nitrogen fixation. Option B refers to the uptake of nitrogen already present in the soil, which typically involves forms of nitrogen like nitrates or ammonium rather than the fixation of atmospheric nitrogen. Option C describes a process that would occur during denitrification, where nitrogen is returned to the atmosphere; this is the opposite of fixation. Option D discusses the conversion of ammonia back into nitrogen gas, which is also part of the nitrogen cycle but is not related to the initial fixation process.