Answer: Denitrification.
- A Nitrification
- B Ammonification
- C Nitrogen fixation
- D Denitrification
Correct answer: D. Denitrification
Explanation: Denitrification is the conversion of nitrates (NO3-) to molecular nitrogen (N2) by denitrifying bacteria (Pseudomonas, Thiobacillus). It occurs in waterlogged anaerobic soil and removes nitrogen from the soil.
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.