Effect of systemic herbicides on microbial biomass in relation to availability of some plant nutrients in an alluvial soil of West Bengal.

An experiment was conducted under laboratory conditions to investigate the effect of three herbicides, viz., fenoxaprop, pendimethalin and paraquat, at their recommended field application rates (50 g, 1.0 kg and 1.0 kg a.i. ha⁻¹, respectively), on the changes of microbial biomass C, N and P in relation to transformations and availability of plant nutrients in an alluvial soil (Typic Orchraqualf) from West Bengal, India. Application of herbicides, in general, significantly increased the microbial biomass C, N and P, resulting in greater availability of these plant nutrients in soil. The microbial biomass C was highly increased due to application of fenoxaprop (39.8%) followed by paraquat (28.2%). Application of pendimethalin led to the maximum stimulation of microbial biomass N (37.1%), while microbial biomass P was increased by 15.2% following the incorporation of paraquat into the soil. Compared to untreated control soil, the soil retained the highest amount of organic C (19.8%) when it was treated with pendimethalin. A similar trend was recorded with fenoxaprop for total N (19.6%) and exchangeable NH₄⁺ (21.3%) in soil. The stimulation of soluble NO₃⁻ was highest under fenoxaprop (22.3%) followed by paraquat (20.7%). Regarding the availability of P in soil, paraquat manifested greater stimulation (17.8%) than fenoxaprop (15.4%) or pendimethalin (13.1%). Application of paraquat also gave the highest amount of total P (17.8%) in the soil solution.