Enantioselectivity Degradation Of Chiral Phenoxypropionic Acid Herbicides In Soils Under Different Incubation Conditions

2,4-dichlorophenoxypropionic acid(DCPP)and 2-methyl-4-chloropropionic acid(MCPP)are typical chiral phenoxypropionic acid herbicides.Although they relatively rapidly degrade in soil,they are frequently detected in surface water and groundwater in many countries.Their enantiomers generally have different environmental behaviors,so it is necessary to study their degradation and transformation in the environment at the chiral level.Many literatures reported the enantioselectivity degradation of chiral phenoxypropionic acid herbicides in environmental media,but the effect of environmental factors on enantioselective degradation is still limited.In this paper,under different initial concentration(1 mg/kg and 10 mg/kg),moisture content(30%,60% and 125% of water holding capacity(WHC))and temperature(4 ?,25 ? and 40 ?)conditions,the enantioselectivity degradation,stable carbon isotope fractation,microbial community composition and diversity in Hefei forest soil and Ningbo vegetable soil when incubated with Rac-DCPP and RacMCPP respectively were studied.The main results are as follows:(1)When initial concentration decreased,moisture content increased and temperature increased,the degradation rates of Rac-DCPP and Rac-MCPP in the two soils increased.Except for 4?,1 mg/kg,60% WHC,under other incubation conditions,the degradation of Rac-DCPP and Rac-MCPP in the two soils fitted the first-order kinetic reaction model,and the degradation half-lives were 3.4 ? 17.7 d and 3.3 ? 28.7 d,respectively.(2)Under different initial concentration,moisture content and temperature conditions,S-DCPP was preferentially degraded in the two soils during Rac-DCPP degradation,but its degree of enantioselectivity was different: when initial concentration increased,moisture content decreased and temperature increased,the enantioselectivity of Rac-DCPP in Hefei forest soil both enhanced,while it weakened in Ningbo vegetable soil.(3)Under different initial concentration,moisture content or temperature conditions,the enantioselectivity of Rac-MCPP during degradation in soils was different: the two soils were preferential degradation of R-MCPP at 1 mg/kg,60% WHC,25?,while were preferential degradation of S-MCPP at 10 mg/kg,60% WHC,25?;in Hefei forest soil,S-MCPP was preferentially degraded at 30% WHC,1 mg/kg,25?,and R-MCPP was preferentially degraded at 125% WHC,1 mg/kg,25?;in Ningbo vegetable soil,S-MCPP and R-MCPP were preferentially degraded at 4?,1 mg/kg,60% WHC and 40?,1 mg/kg,60% WHC,respectively.(4)Except for Hefei forest soil when incubated with Rac-MCPP at 10 mg/kg,60% WHC,25? and Ningbo vegetable soil when incubated with Rac-DCPP and RacMCPP at 10 mg/kg,60% WHC,25? and 30% WHC,1 mg/kg,25? respectively,significant stable carbon isotope fractionation occurred under other incubation conditions,the carbon isotope enrichment factor ? =-1.68‰ ?-4.25‰,and the apparent carbon isotope effect AKIE = 1.017 ? 1.040,which indicated that oxidative cleavage of C-H bonds occurred during the degradation process.(5)Compared with controls,the relative abundances of Sphingomonadaceae and Xanthomonadaceae both significantly increased in Ningbo vegetable soil when incubated with Rac-DCPP and Rac-MCPP;in Heifei forest soil,the relative abundance of Sphingomonadaceae significantly increased after Rac-DCPP addition except for 10 mg/kg,60% WHC,25?,and after Rac-MCPP addition,the relative abundance of Sphingomonadaceae only significantly increased at 40?,1 mg/kg,and 60% WHC.(6)Under different initial concentration,moisture content or temperature conditions,the microbial community structure in Rac-DCPP or Rac-MCPP incubated soil was significantly different.When initial concentration and moisture content increased,in Hefei forest soil,the species diversity of Rac-DCPP group increased,and the Rac-MCPP group decreased;in Ningbo vegetable soil,the species diversity of RacDCPP group decreased,while the Rac-MCPP group decreased in the former and increased in the latter.The species diversity of the Rac-DCPP and Rac-MCPP groups in the two soils decreased with increasing temperature.