Answer: oxygen.
- A oxygen
- B carbon dioxide
- C water
- D sunlight
Correct answer: A. oxygen
Explanation: Nitrogenase is oxygen-sensitive, so nitrogen fixation needs anaerobic or protected conditions.
Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.