Discussion
Which one of the following nutrients cycle have biological fixation ?
Answer: D
The cycle with biological fixation is Nitrogen.
Atmospheric nitrogen (N2) is inert, and most organisms cannot use it. Certain prokaryotes, Rhizobium in legume root nodules, free-living Azotobacter and cyanobacteria such as Anabaena, carry the enzyme nitrogenase and convert it to ammonia. This step, biological nitrogen fixation, is what 'biological fixation' means in the nutrient-cycle sense.
The phosphorus cycle is sedimentary, with no gaseous form to fix; phosphate is released by weathering. Oxygen is returned by photosynthesis rather than fixed. Plants do take up carbon dioxide, a process sometimes also called carbon fixation, but the standard textbook usage the question follows reserves biological fixation for nitrogen.
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