Residual effect of pre-emergence herbicides on microbial activities in relation to mineralization of C, N and P in the Gangetic alluvial soil of West Bengal, India

An experiment has been conducted under laboratory conditions to investigate the residual effect of three pre-emergence herbicides (thiobencarb, pendimethalin and pretilachlor) at fivefold field application rates (7.5, 10.0 and 2.5 kg a.i. ha−1, respectively), on the changes of microbial activities and some biochemical processes in the Gangetic alluvial soil of West Bengal. Application of herbicides in general significantly increased microbial biomass resulting in greater mineralization of C, N and P in soil. The highest stimulation of microbial biomass C was recorded with thiobencarb (24.4 %) followed by pendimethalin (23.4 %). Microbial biomass N was highly induced under pretilachlor (54.5 %) and thiobencarb (52.7 %), while the stimulation of microbial biomass P was at par in the herbicide-treated soils. Compared to untreated control, the highest amount of organic C was retained with thiobencarb followed by pendimethalin. A similar trend was recorded with thiobencarb for total N, while pendimethalin induced exchangeable NH4+ and soluble NO3− to the highest extent (42.2 and 34.5 %, respectively). Regarding the availability of P in soil, pretilachlor manifested greater stimulation (33.1 %) than thiobencarb (21.6 %) and pendimethalin (11.4 %). As compared to untreated control, thiobencarb harboured maximum number of bacteria (107.9 %), while pretilachlor exerted the highest stimulations towards the proliferations of actinomycetes (132.6 %) and fungi (149.5 %) in soil.