RAS question
The nitrogen cycle includes which process that converts atmospheric N2 to ammonia?
Correct answer: (C) Nitrogen fixation.
Nitrogen fixation is the nitrogen-cycle process that converts atmospheric N2 into ammonia.
Explanation
Nitrogen fixation is the step that makes atmospheric nitrogen usable. e-PG Pathshala explains that atmospheric nitrogen exists mainly as N2 and is chemically inert, so it must be fixed before plants and animals can use it. In this cycle, fixation can occur through biological agents such as Rhizobium, Azotobacter and cyanobacteria, through lightning, and through the Haber-Bosch industrial process. Later transformations are different: ammonification converts organic nitrogen back to ammonium, nitrification oxidises ammonium to nitrite and then nitrate, and denitrification returns nitrate-derived nitrogen to the atmosphere as N2. Therefore, the process is nitrogen fixation, not a downstream conversion.
Why the other options are wrong
- (A) Nitrification is wrong because it oxidises ammonium or ammonia into nitrite and then nitrate; it does not convert atmospheric N2 into ammonia.
- (B) Denitrification is wrong because it reduces nitrates back to molecular nitrogen and releases it to the atmosphere, the reverse direction of nitrogen fixation.
- (D) Ammonification is wrong because it converts organic nitrogen from dead matter and wastes back into ammonium or ammonia, not atmospheric N2 into ammonia.
Concept
This tests the biogeochemical cycles portion of Environment and basic Science, especially the direction of nitrogen transformations. It recurs in RAS because options often swap closely named processes such as fixation, nitrification, ammonification and denitrification.