Nitrogen Fixation

Step 1: Understand nitrogen fixation. Nitrogen fixation refers to the process of converting atmospheric nitrogen ( ) into ammonia ( ), 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.