Which one of the following is an aerobic non-symbiotic nitrogen-fixing bacteria?

Correct option is A

The correct answer is (A) Azotobacter

Explanation:
• Nitrogen-fixing bacteria are broadly categorized into symbiotic (living in association with plants) and non-symbiotic (free-living in the soil).
• Azotobacter is a genus of free-living (non-symbiotic), aerobic bacteria. They live independently in the soil and utilize organic matter for energy while fixing atmospheric $N_2$ into ammonia.
• They are commonly used as bio-fertilizers for non-leguminous crops like wheat, maize, and vegetables.
• They require oxygen for metabolism (aerobic) but have specialized mechanisms to protect the nitrogenase enzyme from oxygen damage.

Information Booster:
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• Non-symbiotic nitrogen fixation generally contributes less nitrogen to the soil (approx. 20-30 kg/ha/year) compared to symbiotic systems like Rhizobium (up to 200+ kg/ha/year).
• Beijerinckia is also an aerobic free-fixer but is mostly restricted to tropical acidic soils.

Additional Knowledge:
• Clostridium (Option B): A free-living but anaerobic nitrogen-fixing bacterium.
• Rhizobium (Option C): A symbiotic nitrogen-fixing bacterium that forms nodules on the roots of legumes.
• Beijerinckia (Option D): While aerobic and non-symbiotic, Azotobacter is the more standard and widespread answer for general agricultural contexts.