Mechanism And Effect Of Earthworm On Acetochlor Degradation In Soil

Acetochlor is a widely used chloroacetanilide herbicide and could be detected in many natural soil and water due to its widely usage and high leaching capacity.Acetochlor has posed environmental risks to ecological environment and human health.Earthworm represents the dominant invertebrate in soil and can promote the decomposition of organic pollutants.In this study,column and microcosm incubation experiments were carried out to determine the effect and mechanism of earthworm(Eisenia fetida)on acetochlor degradation in soil.The effect of earthworm on soil acetochlor degradation was studied by the degradation kinetics in soil column experiment;the degradation capacities of drilosphere and bulk soil to acetochlor were performed by microcosm experiments;soil chemical and microbiological characteristics were determined to explore the mechanism of the influence of earthworm on acetochlor degradation;high-throughput sequencing(HTS)was applied to find the functional microorganisms involved in the soil acetochlor degradation processes.The results and conclusions are shown as below:1.The degradation kinetics of acetochlor in soil column experiments suggested that both indigenous microorganisms and earthworm played important roles in acetochlor degradation.The degradation rates of acetochlor in natural soil treatments were significantly higher than sterile soil treatments(p<0.05).The presence of earthworm made the rates increase 51% and 9% in sterile and natural treatments.Earthworm cast was the degradation hotpot.The residual acetochlor in casts were lower than those in soils from the same beaker at the early stage of exposure.In the 14 th day of incubation,the acetochlor concentrations of casts in the natural/sterile groups were 47% and 27% lower than corresponding soils.2.Through the determination of physicochemical and microbiological properties in soil and cast collected from column experiment,we found that the quicker degradation in earthworm contain groups was related to the neutralized pH,higher organic matter and enhanced soil microbial activities.Cast provided a more suitable environment for acetochlor degradation than soil.Earthworm also can repair the ecological damage caused by acetochlor.Acetochlor posed inhibition effect on three soil enzymes(catalase,dehydrogenase and invertase),adding earthworm increased these activities to unpolluted status.Earthworm stimulated the carbon cycle in soil,while its effect on nitrogen and phosphorus cycle was not obvious.The inhibition of acetochlor on bacteria population was in a transient way(the third day of incubation),while the effect on fungi was in a long term(the whole thirty days of incubation),earthworm accelerated the population recovery of both bacteria and fungi.3.The degradation capacities of drilosphere(cast,burrow lining,gut content(replaced by earthworm tissue)),bulk soil and control soil to acetochlor were obtained from microcosm incubation experiment.After 20 days' incubation,the acetochlor degradation rates in cast,burrow lining and bulk soil were 74.7%,70.3% and 68.7% respectively,all higher than 61.4% in control soil.Cast presented higher acetochlor degradation capacity in anaerobic environment(83.4%).Earthworm tissues which contained gut content showed highest acetochlor degradation capacities in microcosm experiment,the degradation rate was higher than 90% at 4th day of incubation,indicating possible degraders in earthworm gut.However,the result of soil column experiment showed that earthworm digestion in gut slightly contributed to the acetochlor degradation,which might be attributed to the short stay time of soil in earthworm gut.4.Possible bacterial and fungal acetochlor degraders were identified by analyzing the high-throughput sequencing datas of column and microcosm experiments.From the results of column experiment,15 possible bacterial degraders and 15 possible fungal degraders were identified,among which the abundances of 8 bacteria and 10 fungi were enhanced by earthworm.Sphingomonas,Pseudomonas_fulva,Methylobacillus,Candidatus_Chlorothrix,Rhodococcus_ruber and Rhodococcus_jostii were suspected as acetochlor degraders in both column and microcosm experiments.Sphingomonas,Pseudomonas_fulva and Methylobacillus were evenly distributed in all samples of microcosm experiments,indicating they were indigenous acetochlor degraders.Candidatus_Chlorothrix only presented in cast samples,indicating that it was suspected intestinal degraders.For fungus,most of the degraders stimulated by earthworm belong to order Mortierellales and Microascales.Mortierellales was possible indigenous degraders which appeared in all samples of microcosm experiments;Microascales was suspected intestinal degraders because of the more abundance in cast samples.