Atmospheric nitrogen is converted to ammonia or nitrates in a process known as what?

• A. Nitrification
• B. Ammonification
• C. Denitrification
• D. Nitrogen fixation

Answer: (D)

The main idea is nitrogen fixation—the process that makes atmospheric nitrogen usable by living organisms. N2 gas is very stable and cannot be taken up directly by most plants, so it must be converted into forms like ammonia or nitrate. This happens mainly through nitrogen-fixing bacteria (in soil and in the root nodules of legumes) that use the enzyme nitrogenase to reduce N2 to ammonia, which becomes ammonium in the soil and can be absorbed by plants. Some fixation also occurs abiotically, for example through lightning, which converts N2 to nitrates that wash into the soil. Once nitrogen is fixed, other steps in the cycle transform it further (ammonification releases ammonia from organic material; nitrification converts ammonia to nitrate; denitrification returns nitrogen to the atmosphere), but the key conversion from atmospheric N2 to usable forms is nitrogen fixation.