Little information is available on the interactive effects of inorganic and organic pollutants on carbon utilization by soil microorganisms.This study examined the effects of two common soil pollutants,lead(Pb) and bensulfuron-methyl herbicide(BSM),on decomposition of an adscititious carbon source(14C-glucose).Two contrasting paddy soils,a silty clay soil and a clay loam soil,were incubated with different concentrations and combinations of pollutants for 60 days.Orthogonal rotatable central composite design was adopted to design the combinations of the pollutant concentrations so that rate response curves could be derived.Rapid decomposition of 14C-glucose occurred in the first three days for both soils where no Pb or BSM was added(control).Overall,63%-64% of the added 14C-glucose was decomposed in the control over the 60-day incubation.The addition of Pb or BSM significantly decreased the decomposition of 14C-glucose during the first week but increased the decomposition thereafter;as a result,the percentages of 14C-glucose decomposed(57%-77%) over the 60-day period were similar to or higher than those of the control.Application of the pollutants in combination did not further inhibit decomposition compared with the control.Overall,decomposition rates were lower in the silty clay soil than in the clay loam soil,which was related to the soil texture,cation exchange capacity,and pH.The relationship between the decomposition rates and the pollutants could be well characterized by the quadratic regression orthogonal rotation model.The initial antagonistic effects of the pollutants followed by the synergistic effects on microbial activity might result from changes of the concentrations of the pollutants.
