Assessment of microbial biomass under integrated nutrient management in soybean-winter maize cropping sequence

Field experiments were conducted during 1999–2000 and 2000–2001 at Agricultural Research Station, Banswara on a clay loam soil to assess the improvement in microbial biomass to applied bioinoculants by integrating them with farmyard manure (FYM) and fertilizers. Microbial biomass C, N and P under soybean (Glycine max L.) and winter maize (Zea mays L.) were significantly higher (r = 0.623−0.716, p < 0.01) when a combination of Bradyrhizobium japonicum or Azotobacter chroococcum, Bacillus megaterium and Glomus fasciculatum were used, and was maximum with 50% recommended fertilizers (10 kg N ha⁻¹ and 20 kg P ha⁻¹ for soybean; 75 kg N ha⁻¹ and 30 kg P ha⁻¹ for winter maize) along with bio-inoculants. Microbial biomass C, N and P contents in the rhizosphere soil were maximum at 30 days and decreased subsequently (at 60 days or harvest) in soybean-winter maize sequence. Population of Azotobacter in the rhizosphere increased significantly from 73.5 cfu g⁻¹ under recommended fertilizer to 146.5 cfu g⁻¹ under FYM and bio-inoculants. Acetylene reduction assay (ARA) and nodule dry weight were significantly higher in the treatments receiving Bradyrhizobium inoculation. Vesicular arbuscular mycorrhizal (VAM) infectivity was also enhanced significantly with combined inoculants and FYM compared to sole application of FYM or fertilizers (N and P). Application of fertilizer nutrients along with FYM, use of nitrogen fixers, phosphate solubilizers and VAM increased grain (r = 0.614, p < 0.05) and straw yields (r = 0.604, p < 0.05) and, in general, decreased C:N and C:P ratio. The results suggested that for maximum crop yield only 50% of the required fertilizer might be supplied along with bio-inoculants.