SOIL RETENTION AND BIOAVAILABILITY OF CHLORPYRIFOS TO MAIZE IN SOIL RECEIVING DIFFERENT ORGANIC AMENDMENTS

Abstract

Chlorpyrifos (CP) is a toxic organophosphorus pesticide. Owing to its large scale field application, its residual contents have been reported in different fruits, vegetables, crop plants, soils and waters. Due to its persistency it can enter in food chain and prove harmful for humans and animals. Therefore it has become imperative to restrict its entry in agricultural products for food safety. Therefore, two laboratory and one greenhouse pot experiment were conducted to test the effect of biochar and compost amendments on the sorption, persistence and bioavailability of CP using five different CP concentrations (5, 10, 50, 100 and 200 mg L-1) and two levels 0.25 and 0.50 % of compost and biochar. The sorption of CP on amended and unamended soil was tested using batch equilibrium method. Freundlich model fitted well and explained the sorption behavior of CP. Both compost and biochar significantly increased the sorption of CP and the maximum sorption capacities achieved at 0.50 % levels. However, biochar at both levels showed high sorption capacities for CP compared to compost. The degradation kinetics of CP at two initial concentrations (100 and 200 mg kg-1) was tested in controlled conditions in a laboratory incubation trial in unamended (sterilized and nonsterilized) and amended (sterilized and non-sterilized) soil with biochar and compost (at 0.25% and 0.50% of each). The degradation data of CP in amended and unamended soil was subjected to first order kinetic model. CP at 200 mg kg-1 showed less degradation rate compared to 100 mg kg-1. The CP half-lives of 30 and 60 days were recorded at 100 and 200 mg kg-1 respectively in non-sterilized soil which were increased to 94 and 125 days in sterilized soil. Incorporation of compost and biochar in CP contaminated soil affected differently to the CP degradation and higher degradation of CP was recorded in compost amended treatments than unamended as well as biochar amended treatments CP significantly reduced the soil dehydrogenase, urease and phosphatase activities at the initial stage of incubation. Both amendments significantly alleviated the negative effect of CP on all studied enzymes activities and compost showed even higher enzyme activities compared to control even in the presence of CP. xv A greenhouse pot experiment was conducted to evaluate the influence of biochar and compost amendments (at 0.25% and 0.50% of each) on the uptake of CP (at 100 and 200 mg kg-1) to maize plants. The CP was toxic at both loadings and significantly reduced the shoot and root fresh biomass as well as all tested physiological parameters. Maize plants showed increased residual concentration of CP in both shoots and roots with increasing level of CP. Maize plants induced variations in antioxidant enzymes activities in response to CP stress. Application of both biochar and compost amendments alleviated the adverse effects of CP in all studied parameters as manifested by the improvement in maize fresh biomass and physiological parameters, recovered antioxidant enzymes activities and decreased residual CP concentration in both roots and shoots of maize. However, biochar at 0.50% level was more effective in reducing uptake of CP by maize plants compared to compost.