Selective Ecological Effects Of Typical Chiral Chloroacetamide Herbicides In Soil

The long-term and large-scale use of herbicides in modern agricultural cultivation has led to serious long-term pollution in the soil.The use of chiral herbicides is increasing year by year.A large number of studies have shown that chiral pesticides have obvious stereoselectivity in soil.The study on the residual accumulation characteristics of different enantiomers of chiral herbicides in long-term polluted soil and the changes of soil microecological effects,especially the changes of microbial community structure and degradation genes,is of great scientific significance for the study of microbial remediation of long-term polluted soil.Therefore,the target compounds of typical chiral chloroacetamide herbicide acetochlor and S-metolachlor,which are widely used in modern agricultural cultivation,were selected in this study.Firstly,the enantiomers of acetochlor and S-metolachlor were separated by chromatography,and then their residue analysis methods were established.Subsequently,the strategy of single and repeated treatment of enantiomers was adopted to study the changes of the accumulation characteristics,soil microbial community structure and composition and the diversity and abundance of biodegradation genes.The main research results are as follows:1.The enantiomers of acetochlor and S-metolachlor were resolved by high-performance liquid chromatography.And the QuEChERS and UPLC-MS/MS methods were combined to establish the method of enantiomer residue analysis of acetochlor and S-metolachlor.The enantiomers of acetochlor and S-metolachlor had good linear relationships in soil samples with the correlation coefficient of 0.9915-0.9999.And the method had good sensitivity and accuracy with the LOQ of 0.0014-0.0024 mg/kg,LOD of 0.0004-0.0007 mg/kg,the recovery rate of 91.2%-100.6% and the relative standard deviation of 0.7%-4.1%.The residue analysis methods met the requirements for pesticide residue analysis.2.The degradation of enantiomers of acetochlor and S-metolachlor in soil was selective,and their S enantiomers were preferentially degraded.What’s more,the degradation half-life was affected by the times of application and the concentration of application.The degradation half-lives were 13.9-96.7 days(Rac-acetochlor),59.5-101.0 days(R-acetochlor)and 13.0-98.6 days(S-acetochlor),respectively.The degradation of S-metolachlor was slow in the repeated soil.And the degradation half-lifves were 171.1-417.6 days(Rac-S-metolachlor),162.7-331.6 days(αR,1’S-metolachlor)and 157.9-284.1 days(αS,1’S-metolachlor),respectively.During the repeated treatment,the residues of acetochlor and S-metolachlor in the soil were significantly accumulated,and the accumulation of R enantiomer was significantly greater than that of S enantiomer.3.The repeated treatment of acetochlor and S-metolachlor significantly reduced the soil microbial diversity.Compared with the control group,the Simpson index,Shannon index and Chao1 index of microorganisms were significantly affected by pesticides in all treatment groups.And the effect of R enantiomers of acetochlor and S-metolachlor on soil microbial diversity index was greater than that of S enantiomers.At the microbial phylum level,the repeating of acetochlor and S-metolachlor significantly increased the relative abundance of Actinobacteria and Bacteroidetes in soil,and at the level of microbial genus,repeated treatment of acetochlor and S-metolachlor significantly increased the relative abundance of Streptoccus,Amycolatopsis,Myroides and other dominant microbial species in the soil.Meanwhile the relative abundance and increasing speed of Actinobacteria,Bacteroidetes and the above-mentioned dominant microorganisms in the soil treated with S-acetochlor and αS,1’S-metolachlor were significantly higher than that in the soil treated with R-acetochlor and αR,1’S-metolachlor.4.The results of metagenomic sequencing showed that acetochlor and S-metolachlor increased the diversity and relative abundance of biodegradation genes in soil.And the relative abundances and increasing speed of ppah,alkb,benA and p450 in the soil treated with S-acetochlor and αS,1’S-metolachlor were significantly higher than that in the soil treated with R-acetochlor and αR,1’S-metolachlor.Compared with the control soil,the relative abundances of ppah,alkb,benA and p450 increased significantly of 48.16%-12367.24%(Rac-acetochlor),43.25%-10407.66%(R-acetochlor)and 36.19%-13074.47%(S-acetochlor)in acetochlor repeated-treated soil,corresponding to the significant increase of 20.60%-6715.29%(Rac-S-metolachlor),0.20%-3258.69%(αR,1’S-metolachlor)and 28.58%-9069.50%(αR,1’S-metolachlor)in the S-metolachlor repeated-treated soil.