Which of the following best describes what happens during nitrogen fixation?

• A. Atmospheric nitrogen (\(N_2\)) is converted into ammonia (\(NH_3\)) by certain bacteria.
• B. Ammonia (\(NH_3\)) is converted into nitrate (\(NO_3^-\)) by soil bacteria.
• C. Nitrate (\(NO_3^-\)) is converted back into nitrogen gas (\(N_2\)) by denitrifying bacteria.
• D. Plants absorb nitrogen directly from the atmosphere.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the concept of nitrogen fixation. Nitrogen fixation is a process where atmospheric nitrogen (\(N_2\)), which is inert and not directly usable by most organisms, is converted into a biologically usable form, such as ammonia (\(NH_3\)). This process is primarily carried out by certain bacteria, including free-living bacteria and those in symbiotic relationships with plants (e.g., Rhizobium in legumes).
2. Step 2: Differentiate nitrogen fixation from other processes in the nitrogen cycle. Nitrogen fixation specifically refers to the conversion of \(N_2\) into \(NH_3\). Other processes, such as nitrification (conversion of ammonia to nitrate, \(NO_3^-\)) and denitrification (conversion of nitrate back into nitrogen gas, \(N_2\)), are distinct steps in the nitrogen cycle.
3. Step 3: Eliminate incorrect options. For example, plants do not absorb nitrogen directly from the atmosphere; they rely on nitrogen compounds like nitrate (\(NO_3^-\)) or ammonium (\(NH_4^+\)) in the soil, which are products of nitrogen fixation and other processes.
4. Step 4: Focus on the correct description of nitrogen fixation. The correct answer should describe the conversion of atmospheric nitrogen (\(N_2\)) into ammonia (\(NH_3\)) by certain bacteria, as this is the defining characteristic of nitrogen fixation.
5. Step 5: Review the role of bacteria in nitrogen fixation. Certain bacteria, such as those in the genus Rhizobium, use specialized enzymes like nitrogenase to break the strong triple bond in \(N_2\) molecules and convert it into ammonia (\(NH_3\)), which can then be used by plants and other organisms.