Answer: As nitrate ions.
- A As nitrogen gas
- B As ammonia gas
- C As nitrate ions
- D As urea only
Correct answer: C. As nitrate ions
Explanation: Plants take up nitrogen chiefly as nitrate (NO<sub>3</sub><sup>-</sup>) ions, and also as ammonium.
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.