Which one of the following microbes can fix atmospheric nitrogen whil…

The correct answer is Option (1) → Azospirillum

Note 1: The given answer is as per the official NTA Answer Key. However, the question can have more than one correct answer, i.e. both Option (1) and Option (2), because NCERT also describes cyanobacteria such as Oscillatoria as microbes of aquatic and terrestrial environments that fix atmospheric nitrogen, as explained below.

Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources are bacteria, fungi and cyanobacteria. Some bacteria fix nitrogen inside the root nodules of legumes (Rhizobium, symbiotic). Others fix nitrogen while free-living in the soil.

(1) Azospirillum — Correct. Azospirillum and Azotobacter are the bacteria given as examples of free-living nitrogen fixers in the soil. They enrich the nitrogen content of the soil. This is the answer given by NTA.

(2) Oscillatoria — Also correct as per NCERT. Oscillatoria is a cyanobacterium (blue-green alga). Cyanobacteria are autotrophic microbes found in aquatic and terrestrial (land) environments, and many of them, such as Anabaena, Nostoc and Oscillatoria, fix atmospheric nitrogen. They are important biofertilisers in paddy fields.

(3) Mycorrhiza — Incorrect. Mycorrhiza is a symbiotic association of a fungus with the roots of a plant, not a free-living microbe. The fungus absorbs phosphorus from the soil for the plant; it does not fix nitrogen.

(4) Glomus — Incorrect. Glomus is the fungus that forms mycorrhiza. It helps the plant take up phosphorus.

Note 2: A similar question was also asked in CUET (UG) 2026 on 29-05-2026 (Slot 2), and in CUET (UG) 2025 on 22-05-2025 (Slot 2).

NCERT Extract:
Microbes as Biofertilisers – …