The Effects Of Imazethapyr On Soil Microbial Community Structure,and The Chiral Differences Of Degradation In Soils

The abuse of herbicides has caused serious harm to the environment.Half of the commercial herbicides today are chiral racemic pesticides.Different optically active monomers of chiral herbicides have great differences in bioavailability.Therefore,many scholars suggest using bioactive optically active monomers in agricultural production to reduce environmental harm caused by the abuse of herbicides.However,we need to be clear about the environmental impact and degradation characteristics of each type of rotatory monomer herbicides after use,so as to avoid the serious harm to the environment caused by the blind use of bioefficient monomer herbicides.In this study,Imazethapyr(IM),a widely used herbicide,was used to study its effects on soil microbial community structure and its chiral chemical effects in soil degradation.The main research results are divided into four aspects:the influence of environmental factors on the degradation of IM enantiomers,the influence of biological factors on the degradation of IMenantiomers,the influence of IM enantiomers on the soil microbial community structure and composition,and the exploration of efficient degradation bacteria of IM enantiomers.The overview is as follows:Genetic markers(16S rRNA gene V3-V4 regions)were used to characterize and evaluate the variation of the bacterial communities potentially resulting from IM enantiomers.Globally,the bacterial community structures based on the OTU profiles in the soil treated with(-)-R-IM were significantly different from those in(+)-S-IM treated soil,and the difference were enlarged with the treatment dose increasing.Interestingly,the Rhizobiaceae family and several other beneficial bacteria,including Bradyrhizobium,Methylobacterium and Paenibacillus,were strongly enriched in(-)-R-IM treatment compared to(+)-S-IM treatment.In the contrast,the pathogenic bacteria,including Erwinia,Pseudomonas,Burkholderia,Streptomyces and Agrobacterium,were suppressed in the presence of(-)-R-IM compared to(+)-S-IM.Furthermore,we also observed that the soil treated with(-)-R-IM was more quickly restored to its original bacterial community structure compared with the(+)-S-IM treated soil.These findings unveil a new role of chiral herbicide in the development of soil microbial ecology and provide theoretical support for the application of low-persistence,high-efficiency,and eco-friendly optical rotatory(-)-R-IMThen,we studied the influence of environmental factors pH,temperature and humidity on the enantio-selective degradation of IM,and found that under different pH,temperature and humidity conditions,the residual rate of(+)-S-IM is always higher than that of(-)-R-IM.The enantio-selective degradation differences(EF=S-enantiomer/(S-enantiomer + R-enantiomer))in the four soil types increased with temperature rise ranged from 15? to 35?,and the best results reached between 30? and 35?;The soil humidity for the best enantioselective degradation effect of IM enantiomers was about 50%to 60%maximum water holding capacity(MWHC).(+)-S-IM tended to be degradated under the weakly acidic soil condition,whereas(-)-R-IM preferred to be degradated under the weakly alkaline soil condition,and the difference in selective degradation reaches the maximum under weak alkaline conditions.The content of soil organic matter was positively correlated with the selective degradation of optically active monomers.Therefore,we can control the action time and dose of IM enantiomers in soil by adjusting pH,temperature and humidity,providing a good theoretical basis for the practical application of IM racemate and enantiomers.Then,we studied the effect of biological factors on the degradation of IM enantiomers in soil.Compared with the sterilized soil,the residual rate of IM enantiomers in the soil was significantly reduced and the EF value was expanded.The addition of earthworms can further reduce the residual rate of IM enantiomers in soil and expand the difference of enantio-selective degradation.This indicated that soil microorganisms played an important role in the enantio-selective degradation of IM enantiomers,and the degradation of microorganisms on IM enantiomers were enhanced under the synergistic action of earthworm activities.Further analysis of the soil microbial phospholipid fatty acid profiles(PLFAs)found that 60 days'IM enantiomers appliecation did not have significant effects on total microbial communities PLFAs.However,the addition of earthworms significantly increased the Total PLFAs,the proportions of gram-positive bacteria(G+),the proportions of the bacteria(Total Bact)and anaerobic-aerobic bacteria in the process of degradation IM enantiomers,and significantly reduced the proportion of fungus(Total Fungi)as well as the nutrition pressure from environment on microbial community.This indicated the important combined action of microorganism and earthworm in the degradation of soil IM.Finally,three kinds of highly effective degradation bacteria S1,S2 and S3 isolated from contaminated soil belong to Kocuria rosea,Microbacterium oxydans and Alcaligene faecalis,respectively.The optimal inoculation amount of all three strains against(+)-S-IM and(-)-R-IM was 7%,and the optimal initial substrate concentration was 100 mg/L.The best degradation temperatures of the three bacteria for two enantiomers ranged from 30 ? to 35 ?.The optimal degradation temperature of S2 and S3 was higher than that of S1.S1 strain was suitable for degrading IM under acidic conditions,while S3 strain was most suitable under alkaline conditions;The degradation ability of S2 strain was the strongest in neutral environment.Under the optimal degradation condition,the degradation of(-)-R-IM by three bacteria was faster than that of(+)-S-IM,EF values were all greater than the initial value(EF0=0.5),and the number of bacterial cells(OD600)was negatively correlated with the residual rate.The competition of glucose for S1 ultilizing enantiomers was the greatest.Sodium citrate showed the greatest competition for S2 and S3 degradating enantiomers.Under soil conditions,degradation rate of IM enantiomers decreased(half-life increased),residual rate increased,and chiral degradation difference decreased(EF tended to initial value);And the most reduction for each index happened in the unsterilized soil.The results showed that the competition of external carbon source,the change of environmental media and the interference of other microorganisms could significantly affect the ability of these three highly efficient biodegradable bacteria to degrade IM enantiomers.