Answer: Rhizobium and other nitrogen-fixing bacteria.
- A Plant roots alone through active mineral uptake
- B Rhizobium and other nitrogen-fixing bacteria
- C Mycorrhizal fungi that colonize root cortex
- D Earthworms that aerate and till the soil
Correct answer: B. Rhizobium and other nitrogen-fixing bacteria
Explanation: Nitrogen fixation: N<sub>2</sub> gas → NH<sub>3</sub> (ammonia) by nitrogen-fixing bacteria like Rhizobium (in legume root nodules) and free-living Azotobacter.
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.