- 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.
Concept context
Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.