Enantioselective Degradation And Enantiomerization Of The Chiral Dichlorprop Herbicide In Soils

Chiral phenoxyalkanoic acid herbicides are widely used in agriculture and lawn care to control broad-leaved weeds.But the excessive use could result in the environmental pollution,for example they frequent occur in ground water,more than the specified value in some European countries.And they have potential risk to human health.Chiral phenoxyalkanoic acid herbicides possess asymmetrically substituted C atom and exist in one pair of enantiomeric forms or more.There are many differences for different enantiomers in herbicidal activity,environmental behavior,ecotoxicity,microbial uptake and degradation,so we should study the information on stereochemistry to assessing the environmental safety.In recent years,many scientists have carried out a large number of researches on enantioselective degradation of chiral phenoxyalkanoic acid herbicides in soils.However,there were few studies about the change of abundance of enantioselective rdpA and sdpA dioxygenase genes when chiral phenoxyalkanoic acid herbicides degrading in soils.In this study,the kinetic data for degradation,enantioselective degradation and enantiomerization of dichlorprop in four screening soils was investigated.The influence of soil properties on enantioselective degradation of dichlorprop was also studied.Furthermore,the alteration of relative abundance of enantioselective rdpA and sdpA dioxygenase genes during degradation of the dichlorprop and its pure R and S-enantiomer in soils.The main results are as follows:(1)The degradation of(R,S)-dichlorprop and its pure R and S-enantiomer accorded with the first order reaction model,and is mainly mediated by biological activity since the degradation rates of dichlorprop in sterilized soils were much lower than those in unsterilized soils.In forest soil(Anhui)and vegetable soil(Hangzhou),R-dichlorprop was preferentially degraded,S-dichlorprop degraded more rapidly than the R-dichlorprop in rice soil(Ningbo),and the degradation rates of R and S-dichlorprop were almost identical in field soil(Hangzhou).There was no enantioselective degradation when dichlorprop incubated in sterilized soils,suggests the abiotic degradation is no enantioselectivity.The increasing organic matter in soils could effect the enantioselective degradation by improve the degradation rate of R-dichlorprop or reduce the degradation rate of S-dichlorprop.(2)In forest soil(Anhui),the enantiomerization process was observed when incubated with R-dichlorprop but not with S-dichlorprop.And in other three soils,we all could detect the other enantiomer when incubated with R-dichlorprop or S-dichlorprop,which induced by enantiomerization.The K=kRs/ksR in vegetable soil(Hangzhou)and field soil(Hangzhou)were 0.54 and 0.53,favoring an inversion of S into R enantiomer,and in rice soil(Ningbo)was 1.60,favoring an inversion of R into S enantiomer.(3)We had not detected the qPCR product with the rdpA primers in DNA extracts recovered from all four soil incubated with Rac-dichlorprop and its pure R and S-enantiomer.In contrast,sdpA had been detected in all situations and the peak values of the relative abundance of sdpA in four soils all appear in the cases which incubated with the racemic.The correlation between R-,S-and Rac-dichlorprop residue and sdpA gene suggested that different types of enantiomer of dichlorprop could induce the proliferation of sdpA gene.