Answer: Azotobacter.
- A Rhizobium
- B Nitrosomonas
- C Azotobacter
- D Salmonella
Correct answer: C. Azotobacter
Explanation: Azotobacter is a free-living (non-symbiotic) aerobic nitrogen-fixing bacterium in soil. Other free-living fixers: Clostridium (anaerobic), Beijerinckia. Unlike Rhizobium, it does not need a plant host.
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.