Question

Which one of the following conversions belongs to nonsymbiotic nitrogen fixation?

• A. Atmospheric nitrogen to ammonia by Rhizobium bacteria in nodules attached to roots of legumes
• B. Atmospheric nitrogen to ammonia by Azotobacter species
• C. Nitrate to gaseous nitrogen under anaerobic conditions
• D. Nitrate to ammonia under aerobic conditions

Answer 1

The Correct Option is B

Step 1: Understand nitrogen fixation. Nitrogen fixation refers to the process of converting atmospheric nitrogen (\( N_2 \)) into ammonia (\( NH_3 \)), a form usable by plants. 

This process can be classified into: 
Symbiotic nitrogen fixation: Performed by bacteria like \textit{Rhizobium}, which form symbiotic relationships with legumes. 
Nonsymbiotic nitrogen fixation: Performed by free-living bacteria like Azotobacter species, independent of a host plant. 

Step 2: Analyze the options. (1) Atmospheric nitrogen to ammonia by \textit{Rhizobium}: This is symbiotic nitrogen fixation. (2) Atmospheric nitrogen to ammonia by Azotobacter: This is nonsymbiotic nitrogen fixation. (3) Nitrate to gaseous nitrogen under anaerobic conditions: This is denitrification, not nitrogen fixation. (4) Nitrate to ammonia under aerobic conditions: This is ammonification, not nitrogen fixation. 

Step 3: Select the correct option. The conversion of atmospheric nitrogen to ammonia by \textit{Azotobacter} species is an example of nonsymbiotic nitrogen fixation.