Answer: Converting NH3 to nitrites and then nitrates by bacteria.
- A Converting N2 to NH3 using nitrogenase enzyme
- B Converting NH3 to nitrites and then nitrates by bacteria
- C Converting nitrates back to N2 gas by denitrifiers
- D Converting dead organic proteins to ammonia
Correct answer: B. Converting NH3 to nitrites and then nitrates by bacteria
Explanation: Nitrification is the two-step oxidation of ammonia: NH3 to NO2- (by Nitrosomonas) and then NO2- to NO3- (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.