Answer: Ammonia or nitrates usable by plants.
- A Nitrogen gas released back into the atmosphere
- B Ammonia or nitrates usable by plants
- C Nitrogen dioxide formed during combustion
- D Pure nitrogen metal deposited in soil
Correct answer: B. Ammonia or nitrates usable by plants
Explanation: Nitrogen fixation converts N<sub>2</sub> gas (inert) into ammonia (NH<sub>3</sub>) or nitrates that plants can absorb. Without this process, atmospheric N<sub>2</sub> is unavailable to most organisms.
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.