Question

Free living nitrogen-fixing bacteria are found in

• A. Air
• B. Soil
• C. Root nodules
• D. None of above

Answer 1

The correct option is B Soil

Earth’s atmosphere contains 78% nitrogen but it cannot be used by living organisms due to its inert form. This inert nitrogen is fixed into biologically active ammonia by free living or symbiotic bacteria and other microorganisms. The free living bacteria are present in soil where they perform nitrogen fixation; example: Azotobacter, Azospirillum and Clostridium. This makes option B correct. The finger like projections on roots of legumes, termed as root nodules, carries Rhizobium bacteria which in turn are responsible for nitrogen fixation. Legume get ammonia from Rhizobium and provide carbohydrates as an energy source to them; a symbiotic relation. This makes option C incorrect as root nodules harbour symbiotic bacteria. Nitrogen fixation does not occur in air as none of nitrogen fixing microbes are present in air; option A is incorrect. Thus, the correct answer is option B.