Answer: NH 3 to NO 2 - and then NO 3 - (by Nitrosomonas and Nitrobacter).
- A N<sub>2</sub> gas directly to NH<sub>3</sub> using the nitrogenase enzyme complex
- B NH<sub>3</sub> to NO<sub>2</sub><sup>-</sup> and then NO<sub>3</sub><sup>-</sup> (by Nitrosomonas and Nitrobacter)
- C NO<sub>3</sub><sup>-</sup> back to N<sub>2</sub> gas through denitrifying bacteria
- D Organic nitrogen compounds to NH<sub>3</sub> through decomposition
Correct answer: B. NH<sub>3</sub> to NO<sub>2</sub><sup>-</sup> and then NO<sub>3</sub><sup>-</sup> (by Nitrosomonas and Nitrobacter)
Explanation: Nitrification: Nitrosomonas converts NH<sub>3</sub> to NO<sub>2</sub><sup>-</sup> (nitrite); Nitrobacter converts NO<sub>2</sub><sup>-</sup> to NO<sub>3</sub><sup>-</sup> (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.