Answer: Rhizobium bacteria (nitrogen fixers).
- A Viruses that trigger nodule formation
- B Rhizobium bacteria (nitrogen fixers)
- C Mycorrhizal fungi that enhance phosphorus uptake
- D Actinomycetes that decompose dead root tissue
Correct answer: B. Rhizobium bacteria (nitrogen fixers)
Explanation: Rhizobium bacteria live symbiotically in root nodules of legumes. They fix atmospheric N<sub>2</sub> into ammonia used by the plant.
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.