Which one of the following bacteria is responsible for fixation of atmospheric nitrogen while free living in the soil?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Azospirillum
(1) Azospirillum — Correct. Azospirillum and Azotobacter are bacteria that fix atmospheric nitrogen while living freely in the soil. They add to the nitrogen content of the soil, so they are used as biofertilisers.
(2) Rhizobium — Incorrect. Rhizobium fixes nitrogen in a symbiotic way. It lives in the root nodules of legumes and gives the plant the nitrogen it fixes.
(3) Methanobacterium — Incorrect. This is a methanogen. It lives without oxygen, for example in sewage sludge and in the gut of cattle, and makes methane in biogas. It is not used for nitrogen fixation.
(4) Acetobacter aceti — Incorrect. This bacterium turns ethanol into acetic acid, which is vinegar.
Note: A similar question was also asked in CUET (UG) 2026 on 14-05-2026 (Slot 2), in CUET (UG) 2025 on 22-05-2025 (Slot 2), and in CUET (UG) 2023 on 30-05-2023 (Slot 1).
NCERT Extract:
Microbes as Biofertilisers – …
