Impact of quinalphos on microbial biomass and activities in tropical clay loam soil

Clay loam soil from agricultural field of Gangetic alluvial of West Bengal was investigated for the effect of quinalphos application at field rate and two times field rate on soil microbial variables under laboratory conditions. Microbial biomass carbon (MBC) content in soil was observed to increase significantly with time upto 30 d for all the treatment doses indicating adoption of microbes to use the pesticide as carbon or, energy source. Thereafter the value was found to decrease and reached the initial level by 90 d. There is no significant change of basal soil respiration (BSR) rate at field rate of application of quinalphos with control but on the other hand, decreased upto 30 d followed by an increasing pattern upto 90 d of incubation for higher treatment dose. Quinalphos spiking at field rate had no effect on fluorescein diacetate hydrolysing activity (FDA) upto d 7 compared to the control treatment followed by a sharp and significant (P <0.05) increase at d 15 and became stabilized at d 30. Thereafter, FDA showed a steady decrease till d 60 followed by an increase at the end of the incubation period. At two times the field application rate of quinalphos there was a sharp decrease in FDA compared to the control till the end of incubation period. The decrease in FDA due to quinalphos application could be due to the reduction in soil MBC as well as the inhibitory effect of quinalphos on the synthesis of the enzymes involved in fluorescein diacetate hydrolysis.