RAS question
The nitrogen cycle involves biological nitrogen fixation primarily by:
Correct answer: (D) Rhizobium bacteria in root nodules of legumes and free-living bacteria like Azotobacter.
Biological nitrogen fixation in the nitrogen cycle is carried out mainly by bacteria such as Rhizobium in legume root nodules and free-living bacteria such as Azotobacter.
Explanation
Biological nitrogen fixation is the conversion of atmospheric nitrogen (N2) into ammonia (NH3), a form that can enter biological use. The key organisms are nitrogen-fixing bacteria, not soil animals or ordinary algae. Rhizobium works symbiotically in the root nodules of legumes, while free-living bacteria such as Azotobacter and Clostridium can also fix nitrogen; cyanobacteria such as Nostoc and Anabaena are part of the same broad biological route. The nitrogenase enzyme drives this process and it contributes about 65% of biologically available nitrogen. The Food and Agriculture Organization of the United Nations describes earthworms as agents of vermicomposting and soil improvement, not as nitrogen-fixing organisms.
Why the other options are wrong
- (A) Earthworms improve soil structure and composting activity, but the Food and Agriculture Organization of the United Nations treats them as vermicomposting organisms, not as agents that fix atmospheric nitrogen.
- (B) Green algae in oceans are not the primary biological nitrogen fixers; the nitrogen-fixing photosynthetic group is cyanobacteria such as Nostoc and Anabaena.
- (C) Mycorrhizal fungi are associated with nutrient uptake, especially phosphorus, but the organisms responsible for biological nitrogen fixation are bacteria and cyanobacteria.
Concept
The nitrogen cycle includes biological nitrogen fixation and the organisms responsible for it. RAS ecology commonly distinguishes nutrient cycling roles that sound similar but belong to different organisms.