Effect of residue placement and chemical fertilizer on soil microbial biomass under tropical dryland cultivation

Four treatments (control, chemical fertilizer, wheat straw, and wheat straw+fertilizer) were established on the dryland experimental farm of the Institute of Agricultural Sciences, Banaras Hindu University. Organic in C in the different treatments ranged from 0.69 to 0.93%, total N from 0.08 to 0.11%, and total P from 0.018 to 0.021. The application of straw significantly increased the soil water-holding capacity. The maximum effect on the microbial biomass was realized with the straw+fertilizer treatment, followed by straw and then by the fertilizer treatment. During the study microbial biomass C ranged from 144 to 491 g g^-1 dry soil, biomass N from 14.6 to 50.1 g g^-1, and biomass P from 7.2 to 17.6 g g^-1 soil. Microbial biomass C, N and P represented 3.2–4.6% of total C, 2.6–3.8% of total N, and 5.8–8.2% of total P in the soil, respectively, in all cases the highest proportion occurred in the straw+fertilizer treatment and the lowest in the control. Microbial biomass C, N, and P were positively correlated with each other. Microbial biomass C and N increased by 77% in straw+fertilizer-treated plots relative to the control. The increase in microbial biomass P in the straw+fertilizer treatment over the control was 81%. The increase in the microbial biomass is expected to enhance nutrient availability in the soil, as the microbial biomass acts both as a sink and a source of plant nutrients.