Microbial population in rice rhizopheric soil as influenced by Azotobacter and PSB inoculation with graded levels of fertilizer nutrients (NP)

A field experiment was conducted to study the effect of microbial inoculants at different levels of fertilizer nutrients (NP) on microbial population in the rhizosphere of rice. Four treatments of microbial inoculants viz., uninoculation, Azotobacter, PSB and Azotobacter + PSB were interacted with four levels of NP fertilizers viz., N0P0, N60P20, N90P40 and N120P60 in Randomized Block Design with three replications. Microbial population of total bacteria, fungi, actinomycetes, phosphate solubilizing micro-organism (PSB) and Azotobacter in the rhizospheric soil of rice significantly increased during both the years due to application of bioinoculants (biofertilizers). Among bioinoculant treatments, the maximum population of these microbes in the rhizosphere of rice were observed in under the dual inoculation of Azotobacter + PSB at different stages of growth of the crop. Application of NP fertilizers to rice crop also significantly increased the population of total bacteria, fungi, actinomycetes and phosphate solubilizing micro-organism (PSB) over without fertilizer application. Among fertilizer levels, the highest level of N120P60 had the maximum population of total bacteria, fungi, actinomycetes and PSB during both years in the rhizospheric soil of rice. On contrary to other microbes, population of Azotobacter decreased due to higher application of fertilizers i.e. N90P40 and N120P60. However, the application of N60P20 increased the Azotobacter counts marginally in most of the cases. The maximum population of total bacteria, fungi, actinomycetes and PSB in the root zone of rice were noted under the treatment combination of Azotobacter + PSB + N120P60, whereas Azotobacter population was recorded under the treatment combination of Azotobacter + PSB + N60P20. Among the crop growth stages, maximum and minimum population of all the microbes were found at 80 DAT and harvest stage, respectively.