Which of the following biofertilizers is most effective in increasing nitrogen content in paddy fields?
Nostoc, a cyanobacterium, fixes atmospheric nitrogen, enriching soil fertility in paddy fields.
Ref: NCERT Class 12 Biology Chapter 8: Microbes in Human Welfare Microbes as Biofertilizers - Rhizobium Azospirillum Mycorrhiza Cyanobacteria