Which of the following acts as biofertilizer in paddy field?

The correct answer is Option (3) → Azospirillum

Note: The given answer is as per the official NTA Answer Key. However, as per NCERT the correct answer should be Option (1) Nostoc, as explained below.

Why the answer given by NTA is not as per NCERT

Option (1) Nostoc — Correct as per NCERT. A nitrogen-fixing cyanobacterium that grows in the standing water of paddy fields.

Option (2) Rhizobium — Incorrect. It fixes nitrogen only in the root nodules of leguminous plants; rice is not a legume.

Option (3) Azospirillum — The answer given by NTA, but not as per NCERT. NCERT lists it only as a free-living soil bacterium that fixes nitrogen; for paddy fields it names cyanobacteria such as Nostoc.

Option (4) Azotobacter — Incorrect. Also a free-living nitrogen-fixing soil bacterium; NCERT does not link it to paddy fields.

NCERT Extract: “Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g. Anabaena, Nostoc, Oscillatoria, etc. In paddy fields, cyanobacteria serve as an important biofertiliser. Blue green algae also add organic matter to the soil and increase its fertility.”
Microbes as Biofertilisers – “… Other bacteria can fix atmospheric nitrogen while free-living in the soil (examples Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.”