The Response And Remedial Role Of Soil Arbuscular Mycorrhizal Fungi To Heavy Metal Contamination At WEEE-recycling Sites

The electrical and electronic equipment(EEE)are continually replaced by new ones,and the quantity of waste EEE(WEEE)has increased dramatically.However,the informal WEEE-recycling has caused serious soil heavy metal(HM)contaminations,which pose a huge threat to human health.Arbuscular mycorrhizal(AM)fungi have strong tolerances to HMs and potential applications in phytoremediation.Therefore,a field investigation was carried out at a typical WEEE-recycling site to determine the concentrations and ecological risks of HMs in soils,and to study the response of soil AM fungal community to HM contaminations.Then,a pot experiment was conducted to investigate the growth and HM absorbtion of sunflower(Helianthus annuus L.)plants in response to two different AM fungal species on a heavily HM-contaminated(H)soil and a lightly HM-enriched(L)soil,both of which were collected from the WEEE-recycling site.Finally,the effects of intercropping sunflower and inoculating AM fungus on the growth of garlic chives(Allium tuberosum Rottl.ex Spreng.)on the H soil were also studied.The main results of this dissertation were listed as below.(1)A typical WEEE-recycling area in Luqiao,Taizhou,Zhejiang Province was selected,and HM concentrations in the soils collected from WEEE-dismantling sites,residue-dumping sites,and surrounding farmlands were analyzed.Most of the investigated samples had been contaminated by HMs(i.e.,Cd,Cu,Pb,Cr,Zn,and Ni).The contamination at WEEE-disassembling and-dumping sites reached high ecological hazard degrees,while the potential ecological risk levels of some surrounding farmlands were moderate.The contamination degrees of Cd,Cu,Pb,and Zn were all high,and the potential ecological risk levels of Cd,Pb,and Cu were very high,high,and moderate,respectively.The genesis analysis results showed that in WEEE-recycling area the diffusion pathway of HMs from WEEE-dismantling and disposing processes was mainly air transportation,supplemented by rainfall transportation.(2)The real-time polymerase chain reaction(PCR),the PCR-denaturing gradient gel electrophoresis(DGGE),and the Illumina high throughput sequencing techniques were used to systematically study the response of soil AM fungal community to HM contamination in the study area.The diversity and richness of AM fungi had no significant correlations with the single concentrations,the comprehensive factor pollution index,and the potential ecological risk index of the above six HMs in soil,respectively.The results obtained from Illumina high throughput sequencing showed that the relative abundance of Rhizophagus vesiculiferus(Rv)significantly correlated with the single concentrations,the comprehensive factor pollution index,and the potential ecological risk index of the above six HMs.(3)A pot experiment was conducted to compare the growth and HM absorbtion of sunflower plants in response to inoculating Funneliformis mosseae(Fm)and F.caledonium(Fc).Compared with the L soil,the H soil led to high HM accumulations in plants,inhibiting plant growth and damaging the organelles of leaves.On the H soil,AM fungal inoculations elevated root P acquisition,promoted plant growth,decreased shoot HM concentrations,increased leaf catalase(CAT)activity,and alleviated the toxicity symptoms of HMs in the organelles of leaves.It effectively alleviated the stress of HM contamination on plant growth of sunflower caused by WEEE-recycling.Additionally,inoculation with AM fungi also increased HM uptake due to the significant increases of sunflower biomass and tended to decrease the concentrations of HMs in the soil after harvest.However,only Fc induced significant elevations in soil phosphatase activity and DTPA-extractable HM concentrations.(4)The effects of intercropping sunflower and inoculating Fc on the growth of garlic chives on the H soil were studied via a pot experiment.Intercropping sunflower alleviated the HM stress on garlic chives partly through rhizosphere competition,and decreased most HM concentrations in the shoot of garlic chives.Inoculating AM fungus increased soil phosphatase activity,promoted the growth and HM acquisitions of sunflower,enhacing the rhizosphere competition of sunflower over garlic chives.Meanwhile,inoculating AM fungi increased the root and shoot P absorption and growth of garlic chives,increased the interception of absorbed HMs by root,and reduced the transfer of HM to the shoot,inducing significant decreased shoot HM concentrations of garlic chives compared with the monocultural control.As a result,the edible safety of garlic chives were greatly elevated by intercropping sunflower and inoculating Fc.In conclusion,WEEE-recycling sites and surrounding farmlands would be contaminated by HMs in most cases.The relative abundance of Rv in soil AM fungal community can be used as a potential indicator of soil HM contamination degrees.Inoculation with AM fungi(i.e.,Fm or Fc)was beneficial to sunflower for the phytoextraction of HM-contaminated soils at WEEE-recycling sites.Furthermore,intercropping sunflower and inoculating Fc could promote the growth of garlic chives and reduce shoot HM concentrations,suggesting the potential application for both soil remediation and crop production.