Comparative toxicity of chlorpyrifos and its oxon derivatives to soil microbial activity by combined methods

The inhibitory effects of the pesticide Chlorpyrifos (CPF) and its oxon derivative (CPO) on soil microbial activity were evaluated through the measurement of metabolic parameters and the microbial urease enzyme. The thermodynamic parameters related to microbial activity were measured and recorded as power-time curves. Microbial growth rate constant k, total heat evolution Q(T), metabolic enthalpy DeltaH(met), mass specific heat rate J(Q/S), microbial biomass C and inhibitory ratio I were calculated. They showed the linear relationship with doses of CPF and CPO. Thereinto, the linear correlations, k versus biomass C and DeltaH(met) versus biomass C, elucidated that k and DeltaH(met) were growth yield dependent. In this work, 20% inhibitory ratio IC(20) was obtained with 9.8 microg g(-1) for CPF and 0.37 microg g(-1) CPO, meaning that the acute toxicity of CPO was 26 times that of CPF, since the CPO had more potent toxicity to living organism due to its active functional group. Comparing the change tendency of DeltaH(met) and other parameter, the values almost kept constant when exposure to CPF (<5.0 microg g(-1)). It illustrates that individual reacted to stress resulted from environment change by shifting resources from other biological activities (such as reproduction or growth) toward survival to some extent. Urease activity responses in relation to the CPF and CPO exposure were observed and consistent with above thermodynamic parameters.