| Quizalofop-ethyl and quizalofop-p-ethyl are commonly used aryloxy-phenoxy-propionic acid herbicides for soybeans and other crops in Northeast China,and studies have confirmed that they have adverse effects on a variety of organisms.Due to their different enantiomeric ratios（quizalofop-ethyl is racemate,quizalofop-p-ethyl is R-enantiomer content>90%）,the degradation and transformation dynamics and toxic effects in soil are also different.Therefore,studies on the toxic effects of chiral herbicides quizalofop-ethyl and quizalofop-p-ethyl on soybean-mollisol system were carried out to clarify the environmental behavior and toxic effects of quizalofop-ethyl and quizalofop-p-ethyl,and to evaluate environmental risks are of great significance.In this study,a high performance liquid chromatography（HPLC）detection method for chiral herbicides quizalofop-ethyl and quizalofop-p-ethyl was established.The enantioselective degradation and transformation processes of quizalofop-ethyl and quizalofop-p-ethyl in mollisol,the effects of quizalofop-ethyl and quizalofop-p-ethyl on soil enzyme activities and soil physicochemical properties,and the effects on the physiological and biochemical properties of soybean seedlings were investigated in this study.The Alpha diversity analysis and Beta diversity analysis of soil bacterial communities were carried out by high-throughput sequencing.Species composition analysis and environmental factor association（RDA）analysis to analyze the toxic effects of quizalofop-ethyl and quizalofop-p-ethyl on soybean-mollisol system from the microbial community level.The main results and conclusions of this study are as follows:（1）A method for detecting the content of chiral enantiomers was established.The R-enantiomer and S-enantiomer of quizalofop-ethyl and quizalofop-p-ethyl were separated by high performance liquid chromatography LC-20A and（R,R）-Whelk-O1 chiral column.The average recoveries of the enantiomers of quizalofop-ethyl and quizalofop-p-ethyl with different concentrations in soil were all above 90%,and the coefficients of variation were all less than 10%.The detection method had good reproducibility and high accuracy.（2）The degradation of quizalofop-ethyl enantiomers and quizalofop acid enantiomers in mollisol showed enantioselectivity,and the residue of R-quizalofop acid in soil had long-term effects.The degradation rate of R-quizalofop-ethyl is higher than that of S-quizalofop-ethyl.In mollisol,the configurations of R-quizalofop-ethyl and S-quizalofop-ethyl are stable,and it is not easy to transform each other.The degradation of the secondary metabolites of R-quizalofop-ethyl and S-quizalofop-ethyl also showed great differences in mollisol.S-quizalofop acid was preferentially degraded in mollisol,while R-quizalofop acid remained in soil for a longer time.The degradation half-lives of R-quizalofop acid and S-quizalofop acid were 38.96±1.93 d and 7.65±0.49d,respectively.S-quizalofop acid can undergo enantiomeric transformation to R-quizalofop acid,while R-quizalofop acid cannot be converted to S-quizalofop acid.（3）Quizalofop-ethyl and quizalofop-p-ethyl affected the enzymatic activity and physicochemical properties of mollisol.When quizalofop-ethyl and quizalofop-p-ethyl were exposed to pollution for 7 days,the activities of soil invertase and acid phosphatase decreased significantly compared with the control group（P<0.05）.When quizalofop-ethyl and quizalofop-p-ethyl were exposed to pollution for 35 days,soil urease activity decreased significantly compared with the control group（P<0.05）.Correlation analysis showed that the enantiomer content of quizalofop acid was significantly negatively correlated with urease activity（P<0.01）.Under moderate(5 mg·kg-1)and high(10 mg·kg-1)concentrations of quizalofop-p-ethyl pollution exposure,soil dissoluble organic carbon content and soil ammonium nitrogen content were significantly reduced compared with the control group（P<0.05）,the content of dissoluble organic carbon decreased by 16.2%and 18.5%,and the content of ammonium nitrogen decreased by 19.4%and33.1%,respectively.（4）Quizalofop-ethyl and quizalofop-p-ethyl exhibited oxidative stress effects on soybean seedlings.Under the pollution exposure of quizalofop-ethyl and quizalofop-p-ethyl,the fresh weight,plant height and root length of soybean seedlings were significantly reduced（P<0.05）.Under the high concentration(10 mg kg-1)of quizalofop-p-ethyl,the contents of chlorophyll a and chlorophyll b in leaves decreased significantly（P<0.05）by 17.7%and 20.0%respectively compared with the control group.Under moderate(5 mg·kg-1)and high(10 mg·kg-1)concentrations of quizalofop-ethyl and quizalofop-p-ethyl pollution exposure,the net photosynthetic rate of leaves decreased and the intercellular carbon dioxide concentration increased.Under the pollution exposure of quizalofop-ethyl and quizalofop-p-ethyl from 2.5 mg·kg-1 to 10 mg·kg-1,the oxidative stress of soybean seedling leaf cells increased to varying degrees,which was manifested by the increase of MDA content and the increase of SOD,CAT,APX activities.（5）Quizalofop-ethyl and quizalofop-p-ethyl induced changes in soil bacterial community structure and composition.Alpha diversity index analysis showed that compared with quizalofop-ethyl treatment,quizalofop-p-ethyl treatment significantly reduced the richness and uniformity of bacterial community in soil（P<0.05）.Indicating that compared with racemate,R-enantiomers had a greater effect on soil bacterial community richness and uniformity.The stress of different pollutants was the dominant factor affecting the Beta diversity of rhizosphere soil bacteria.Species composition analysis showed that under moderate(5 mg·kg-1)and high(10 mg·kg-1)concentrations of quizalofop-ethyl and quizalofop-p-ethyl,the relative abundance of some bacteria with the function of degrading organic pollutants increased in soil.Quizalofop-p-ethyl stress increased the relative abundances of Sphingomonas,Bacillus and Pseudomonas in the rhizosphere soil,while quizalofop-ethyl increased the relative abundance of Luteibacter in the bulk soil.The results of RDA analysis showed that DOC and p H were the dominant environmental factors affecting the relative abundance and community composition of bulk soil bacteria under the stress of quizalofop-ethyl and quizalofop-p-ethyl. |
