How is atmospheric nitrogen gas converted into forms usable by plants?

• A. Nitrogen fixation converts N2 to ammonia or nitrates, mainly by bacteria and some industrial processes.
• B. Denitrification converts ammonia to nitrates.
• C. Nitrification converts N2 to nitrates.
• D. Photosynthesis converts N2 into glucose.

Answer: (A)

Atmospheric nitrogen gas is very unreactive, so plants can’t use it directly. The process that makes nitrogen available to plants is nitrogen fixation: it converts N2 into ammonia or related forms like ammonium, and nitrates, mainly carried out by certain bacteria (such as those in legume roots or free-living in soil) and also by some industrial fertilizer production. Once nitrogen is in those forms, other steps of the nitrogen cycle can further transform it for plant uptake.

Denitrification is the reverse: it turns nitrates back into gaseous N2, removing usable nitrogen from the soil. Nitrification does convert ammonia to nitrite and then to nitrate, but it starts from ammonia already formed, not directly from atmospheric N2. Photosynthesis uses carbon dioxide to make sugars and does not fix nitrogen gas.