Which process is responsible for converting atmospheric nitrogen gas (\(N_2\)) into biologically usable forms such as nitrite (\(NO_2^-\)) and nitrate (\(NO_3^-\))?

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

Answer: (D)

Verified step by step guidance

1. Understand the problem: The question asks which process converts atmospheric nitrogen gas (\(N_2\)) into biologically usable forms such as nitrite (\(NO_2^-\)) and nitrate (\(NO_3^-\)). This is a key concept in the nitrogen cycle.
2. Clarify the term 'Nitrogen fixation': Nitrogen fixation is the process by which certain microorganisms, such as nitrogen-fixing bacteria (e.g., Rhizobium), convert atmospheric nitrogen gas (\(N_2\)) into ammonia (\(NH_3\)). Ammonia can then be further processed into nitrite (\(NO_2^-\)) and nitrate (\(NO_3^-\)) by other bacteria in the soil.
3. Differentiate from other processes: Ammonification refers to the conversion of organic nitrogen into ammonia (\(NH_3\)). Assimilation involves the uptake of nitrate (\(NO_3^-\)) or ammonia (\(NH_3\)) by plants to form organic molecules. Denitrification is the reduction of nitrate (\(NO_3^-\)) back into nitrogen gas (\(N_2\)), completing the nitrogen cycle.
4. Identify the correct process: Nitrogen fixation is the only process that directly converts atmospheric nitrogen gas (\(N_2\)) into biologically usable forms, making it the correct answer.
5. Summarize the nitrogen fixation process: Nitrogen-fixing bacteria use the enzyme nitrogenase to break the strong triple bond in \(N_2\), enabling its conversion into ammonia (\(NH_3\)). This ammonia can then be transformed into nitrite (\(NO_2^-\)) and nitrate (\(NO_3^-\)) by nitrifying bacteria.