Answer: ammonia.
- A ammonia
- B nitrogen gas
- C oxygen
- D carbon dioxide
Correct answer: A. ammonia
Explanation: Nitrogen-fixing organisms reduce N₂ to ammonia (NH₃), a usable form.
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.