Which one among the following biofertilizers does not fix atmospheric nitrogen ?

The correct option is Glomus.

To understand why this is the correct answer, let's look at the biological roles of each of the options provided:

1. Oscillatoria: This is a genus of filamentous cyanobacteria (blue-green algae). Many cyanobacteria, including species of Oscillatoria, are capable of fixing atmospheric nitrogen under certain conditions (often in anaerobic environments or through specialized cellular configurations), thereby acting as nitrogen-fixing biofertilizers.

2. Rhizobium: This is a well-known genus of symbiotic bacteria that associate with the root nodules of leguminous plants. They actively fix atmospheric nitrogen ($N_2$) and convert it into ammonia, which the host plant can readily assimilate.

3. Azospirillum: This is a genus of free-living or associative nitrogen-fixing bacteria. They live in close association with the roots of grasses and other agricultural crops, fixing atmospheric nitrogen and promoting plant growth.

4. Glomus: This is a genus of arbuscular mycorrhizal (AM) fungi. Unlike the bacteria and cyanobacteria mentioned above, fungi do not possess the nitrogenase enzyme complex required to fix atmospheric nitrogen. Instead, Glomus forms symbiotic associations with plant roots to primarily facilitate the absorption of phosphorus, water, and other essential minerals from the soil. It acts as a biofertilizer by improving nutrient uptake rather than fixing atmospheric nitrogen.

Therefore, among the given choices, Glomus is the only biofertilizer that does not fix atmospheric nitrogen.