Which statement defines nitrogen fixation?

• A. Conversion of atmospheric N2 into ammonia by bacteria or industrial processes.
• B. Conversion of ammonia into nitrate.
• C. Nitrification.
• D. Assimilation of nitrate into plant tissue.

Answer: (A)

Nitrogen fixation is the process of converting inert atmospheric nitrogen (N2) into a form that living organisms can use, typically ammonia (NH3) or ammonium (NH4+). Biologically, specialized bacteria and cyanobacteria carry out this reduction of N2 to NH3 using the enzyme nitrogenase, often in symbiotic relationships with plants (like legume root nodules) or as free-living soil bacteria. Industrially, nitrogen fixation also occurs through the Haber-Bosch process, which converts N2 into ammonia for fertilizer production.

This differs from other nitrogen-cycle steps. Nitrification is the successive oxidation of ammonia to nitrite and then to nitrate, which is a separate transformation. Assimilation of nitrate into plant tissue refers to taking up nitrate and incorporating it into organic molecules, not creating usable nitrogen from atmospheric N2.