Answer: Converting NH 3 to nitrites and then nitrates by bacteria.
- A Converting N<sub>2</sub> to NH<sub>3</sub> using nitrogenase enzyme
- B Converting NH<sub>3</sub> to nitrites and then nitrates by bacteria
- C Converting nitrates back to N<sub>2</sub> gas by denitrifiers
- D Converting dead organic proteins to ammonia
Correct answer: B. Converting NH<sub>3</sub> to nitrites and then nitrates by bacteria
Explanation: Nitrification is the two-step oxidation of ammonia: NH<sub>3</sub> to NO<sub>2</sub><sup>-</sup> (by Nitrosomonas) and then NO<sub>2</sub><sup>-</sup> to NO<sub>3</sub><sup>-</sup> (by Nitrobacter). It increases the availability of nitrogen in a form most plants prefer to absorb.
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.