Study On Response Of Eisenia Foetida And Its Bioremediation Mechanism Exposed To Cadmium

Heavy metal pollution of soil has become a major threat to human health,among which cadmium was widely considered to be the most toxic heavy metal because of it is hyperaccumulation,irreversibility,imperceptibility and surface-cohesion.As the lower Zoobenthos in soil,earthworm evinces a certain tolerance and high sensitivity to pollutants,and it is recognized as an important biomonitor of ecotoxicology experiment.The current studies usually focus on the response mechanism of earthworm to contamination at home and abroad,there are also a number of latest research progresses related to the improvement of soil structure and function by earthworm.However,few scholars pay attention to the effects of Earthworms on heavy metal contaminated soil,the data of microbial community composition in the bioremediation process of contaminated soil were insufficient as well.In this thesis,Eisenia foetida was used as an experimental material,the different concentrations of cadmium treatment and earthworm inoculation were taken as the experimental conditions.By determining the biomarkers of earthworm inoculation treatment groups before and after the experiment to study the response of biomarkers under cadmium exposure;the treatment without earthworms as the control group,by analyzing soil physicochemical properties and bacterial community composition to explore the mechanism of bioremediation by earthworm under cadmium stress.Based on the above study,we look forward to providing technical reference for monitoring and evaluating cadmium exposure risk of soil in field,offering theoretical support and data basis for soil contamination control and bioremediation either.The main conclusions of the experiment are as follows:(1)Earthworms could enrich cadmium in soil well,and there was a significant positive correlation between cadmium content in organism and that in soil(P<0.01).Bioaccumulation factor(BAF)indicated the enrichment ability of earthworm to cadmium.BAF increased linearly under low concentration of cadmium expose,while the bioaccumulation of cadmium gradually reached saturation under high concentration of cadmium stress(P<0.05).(2)By measuring different levels of biomarkers,the results showed that cadmium significantly inhibited the growth and reproduction of earthworm(P<0.05);cadmium stress could inhibit the glycogen and promote the production of soluble protein(P<0.05);CAT activity,GST activity and MT content increased significantly with the promotion of cadmium(P<0.05);SOD activity showed a trend of "first rising and then decreasing"(P<0.05).The content of MDA showed a trend of "first inhibition and then increase"(P<0.05).The cadmium content in earthworm,MT,CAT and daily proliferation rate were sensitive to cadmium.(3)Earthworm activities can improve soil structure,promote mineralization of organic matter and enhance soil fertility.Compared to control group,soil bulk density decreased(P<0.05)and porosity increased(P<0.05),pH and electrical conductivity elevated(P<0.05),the content of rapidly-available phosphorus,rapidly-available potassium,alkali-hydro nitrogen and nitrate nitrogen increased significantly(P<0.05),organic matter and ammonia nitrogen dropped(P<0.05).In addition,the impact of earthworms on soil bulk density,porosity,electrical conductivity,organic matter,rapidly-available phosphorus,alkali-hydro nitrogen and nitrate nitrogen were inhibited by high concentration of cadmium(P<0.05).(4)The results of soil dehydrogenase reflected that cadmium could inhibit the activity of microorganisms in artificial soil.However,earthworm treatment alleviated or even significantly enhanced the metabolic capacity of microorganisms.Under 0.1mg/kg cadmium stress,earthworm treatment had the greatest influence on the activity of soil dehydrogenase(P<0.05),while high concentrations stress inhibited this effect(P<0.05).(5)The next-generation sequencing technology was used for identifying the soil bacterial community.According to the results,earthworm inoculation introduced more unique species to the artificial soil,resulting in differences of soil bacterial community between treatment groups and control groups at any cadmium concentration set in this experiment,and significantly improved the richness and diversity of bacterial community(P<0.05).At phylum level,the relative abundances of Firmicutes and Spirochaetes in the experimental groups were higher than those in the control group(P<0.05),while the relative abundances of Actinobacteria,Deinococcus-Thermus and Gemmatimonadetes were lower than those in the control group(P<0.05).At genus level,the dominant bacterial species in soil changed after earthworm's inoculation.Exposed to low concentrations of cadmium(?0.1 mg/kg),Bacterial species using unique carbon sources such as cellulose and long-chain fatty acids dominate.Under the stress of high concentration of cadmium(?1 mg/kg),the dominant species in the control group were mainly bacteria species with strong tolerance to cadmium,while the dominant species in the experimental group were mostly anaerobic functional bacteria.