Effect of chlorpyrifos on microbial biomass and activities in tropical clay loam soil.

Clay loam soil from agricultural field of Gangetic alluvial zone of West Bengal was investigated to evaluate the effect of chlorpyrifos application at field rate (0.5 mg kg(-1) soil) and 100 times of the field rate (50 mg kg(-1) soil) on soil microbial variables under laboratory conditions. Acetone-induced stress on soil microorganisms was evident in the initial stages in terms of microbial biomass carbon (MBC) content in soil and basal soil respiration (BSR) in control soil samples which received acetone only as compared to control soil without acetone. The soil MBC content increased significantly by application of chlorpyrifos. The BSR and the fluorescein diacetate hydrolysing activity (FDHA) were not adversely affected by chlorpyrifos at field rate, whilst the chemical at higher dosage significantly decreased the metabolic activities of soil microbes in terms of BSR and FDHA.

Metabolism of chlorpyrifos in relation to its effect on the availability of some plant nutrients in soil.

A laboratory experiment was conducted to study the persistence and metabolism of chlorpyrifos in Gangetic Alluvial soil of West Bengal and also to evaluate their effect on the availability of the major plant nutrients (N, P and K) in soil following the application of chlorpyrifos @ 1 kg (T1), 10 kg (T2) and 100 kg (T3) a.i.ha(-1). The dissipation followed first order kinetics and the calculated half-life (T1/2) values ranged from 20 to 37 days. The primary metabolite of chlorpyrifos, 3,5,6-trichloropyridinol (TCP) was detected from 3rd day after application and was at maximum on 30th day which decreased progressively to non-detectable level (NDL) on 120th day for all the treatment doses. The secondary metabolite 3,5,6-trichloro-2-methoxy pyridine (TMP) was detected on 30th, 15th and 7th day in T1, T2 and T3 doses respectively which decreased to NDL during 90-120th day. ANOVA study revealed significant decrease in the available N and P content in soil treated with chlorpyrifos in comparison to the control set. The inhibitory effect on available N was attributable to TMP and for P it was due to the presence of TCP and TMP rather than chlorpyrifos itself as revealed by the step wise multiple regression technique. In the later stage of incubation, however the average N and P status was recovered significantly at 120 days which might be due to the disappearance of the metabolites. The variation due to time of observations or treatment doses was minimum in case of available K in soil.