Answer: NH3 to NO2- and then NO3- (by Nitrosomonas and Nitrobacter).
- A N2 gas directly to NH3 using the nitrogenase enzyme complex
- B NH3 to NO2- and then NO3- (by Nitrosomonas and Nitrobacter)
- C NO3- back to N2 gas through denitrifying bacteria
- D Organic nitrogen compounds to NH3 through decomposition
Correct answer: B. NH3 to NO2- and then NO3- (by Nitrosomonas and Nitrobacter)
Explanation: Nitrification: Nitrosomonas converts NH3 to NO2- (nitrite); Nitrobacter converts NO2- to NO3- (nitrate). Both are chemoautotrophs.
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.