Effects Of Earthworms And Earthworm Casts On Phytoremediation Of Cu And Zn Polluted Soil

Recently, many researches have been focused on the effects of earthworms on thephytoremediation of heavy metal contaminated soil. Some earthworm species were provedto tolerate and even preferentially accumulate certain heavy metals in their tissues throughnutrient-enrichment processes (NEP) and gut-associated processes (GAP). Furthermore ithad been revealed that earthworms not only were able to enhance plant biomass byimproving soil physical and chemical properties, but also increase metal availability in soil,had a potential role in modifying the efficiency of phytoremediation. But in these cases,hyperaccumulator was not included, and the relationship between earthworms andhyperaccumulator is still to be elusive. Furthermore, the changes of microbial activity andstructure of microbial communities in polluted soil after introducing earthworm still remainto be dissected. In previous studies, the biomass and activity of earthworm in past studiesalways declined dramatically after inoculation because organic materials were not added.To further improve the earthworm-plant phytoremediation system. We had designed a seriesof cultivated and potted experiments, and mainly focused on: (1), Effects of earthworms(Pheretima sp.) on the phytoremediation of zinc-polluted soil by ryegrass (L. multiflorum)and Indian mustard (Brassica juncea); (2), Roles of earthworm-straw interactions onphytoremediation of Cu contaminated soil by ryegrass; (3) Effects of earthworm andearthworm-straw interactions on soil microflora and microbial activity in Zn and Cucontaminated soil; (4) Roles of earthworm casts on the growth and Cu accumulation ofryegrass in Cu mine tailing. We had identified the potential roles of earthworm and straw inimproving phytoremediation efficiency and its mechanisms, and the main results wereshown as below:1. The employed earthworm could tolerate heavy metals and survive in our Cu and Znpolluted soil, and their Cu and Zn enrichment coefficients were about 0.18～0.31, 0.21～0.29 respectively, and the coefficients decreased as soil heavy metal dose increased. In addition,metal contents in casts were lower than that in earthworm bodies. Which indicated thatearthworms might reduce the poisonous effects resulted in from heavy metal by excretingheavy metal out of their bodies. But the growth of earthworm was negatively affected byheavy metal pollution, the growth rate declined with soil polluted degree increased, and Cuseemed to be more harmful to earthworm than Zn. In addition, straw supplied food forearthworm, and contracted some negative influences of Cu on the earthwormsimultaneously, thus significantly enhanced earthworm growth and activity2. Soil had good structure and higher available N, P concentration in earthworm treatmentcompared with control. Earthworm also increased NO₃^--N contents in Cu and Zncontaminated soil significantly, but had no effect on soil available K and NH₄⁺-N contents.3. Earthworm could increase not only available nutrients of the soil, enhance ryegrass andIndian mustard growth, but also markedly increase available Zn such as DTPA-Zn, make iteasy for plants to accumulate more metals in their tissues. We also found total Zn uptake inryegrass and Indian mustard was respectively increased 57.8-131.6%, 51.4～150.5%respectively. After introducing earthworms, at the same Zn concentration treatments,although ryegrass averagely contents less Zn concentration in its shoot and root than that ofIndian mustard, but total Zn uptake of it was higher than the ryegrass due to its higherbiomass.4. Our results showed that soil only with earthworm (E) treatment increased ryegrass shootbiomass significantly, but had no effect on plant root growth. On the contrary, strawtreatment (M) enhanced root biomass significantly instead of shoot biomass. We also foundplant shoot and root Cu concentration as well as plant Cu uptake were all enhanced byearthworm (E) and straw (M), while the increased amount by straw was lower than that ofearthworm. Earthworm-straw combination (ME) also enhanced plant Cu concentration.However, the increased amount by straw was lower than that of E treatment and higher thanthat of M treatment. This observation might be attributed to that many available nutrientswere fixed during straw decomposed process. Shoot Cu uptake of plant followed thesequence, E＞ME＞M＞CK. In conclusion, earthworm, straw and their interaction may have apotential role in elevating phytoextraction efficiency in low to medium level Cu contaminated soil.5. After measuring soil microbial properties, we observed that the populations of microbesin Zn polluted soil were not decreased obviously, and some Zn dose even stimulated thegrowth of microbes. But the activities of sucrase and urease were restricted by Zn. While inCu contaminated soil, numbers of bacteria and actinomycetes were decreased, instead ofthe number of fungi. When the dose of Zn was higher than 300 mg/kg and Cu dose washigher than 200 mg/kg, soil MBC (soil microbial biomass carbon) was inhibited. Afterinoculation earthworm, numbers of soil bacteria, actinomycetes and contents of soil MBCwere all increased. Furthermore, earthworms also dramatically enhanced the activities ofsucrase, urease and phosphatase in Zn contaminated soils, thus attenuated the negativeeffects of heavy metal on these enzymes. Straw also weakened the poisonous effects ofheavy metal on soil microbes, and the interactions of earthworm and straw had a significantsynergistical cooperation on increasing the number and activity of soil microbes. Amongthree sources of variance of ANOVA (earthworm, straw and Cu contents), straw was themost important factor of affecting soil microbe population and soil MBC, while theearthworm factor was in the next place. These studies indicated that in contaminated soil,the food resource was still the key factor to affect soil microbes. We could deduce theorganic material might be very important in heavy metal remediation process. Thus organicmaterial should be introduced in earthworm-plant remediation system.6. Earthworm activity significantly increased concentration of DTPA extractable Cu and Znexcept at most of treatments. Plant heavy metal concentrations were positively correlatedwith DTPA extractable contents. Furthermore, we had evaluated different forms of heavymetals by BCR sequential extraction procedure. The results showed that in Zn pollutedsoil, earthworm markedly enhanced the amounts of extractable and Fe-Mn oxides-boundforms (p＜0.05^*) with decreased residual fraction contents. In Cu contaminated soil,earthworm made Cu fractions change from residual forms to Fe-Mn oxides-bound andextractable forms. After supplying straw, amounts of soil B1 (extractable fraction), B2(Fe-Mn oxides-bound fraction) and B4 (residual fraction) were decreased, while B3(organic sulfide bound fractions) amount was increased, which suggested a change inheavy metal formation. The interaction of earthworm and straw enhanced B4 fractiontransformed into B1, B2, and B3 fractions, the changed amounts by ME treatment were higher than that of earthworm treatment. In addition, Soil DOC (dissolved soil organiccarbon) could enhance soil heavy metal availability greatly. We found earthworm and strawboth contributed to increase soil DOC, and amounts of DOC were highest in MEtreatments.7. In Cu mine tailings experiment, application of earthworm cast or bulk soil both increasedthe yield and Cu uptake of plant, and earthworm cast showed more effective. Ratios ofearthworm cast to tailing affected plant growth dramatically: the height of plant reachedhighest when tailing mixing quota was 50 percent, while the plant biomass and shoot Cuconcentration were highest at treatment with 25% tailing. However, the highest Zn uptakewas reached at the treatment with 75% tailing. In general, the best phytoremediationefficiency was observed at the treatment with 75% tailing and 25% earthworm cast. Also,the plant shoot biomass and shoot Cu uptake of this treatment were significantly higherthan these of control treatments with bulk soil and these of pure Cu mine tailing treatment.In addition, the dosage of earthworm casts employed in this treatment was practical.8. The researches mentioned above had showed that the main reasons that earthworm couldenhance phytoremediation efficiency was these as below: firstly, earthworm could improvesoil physical, chemical properties, and increased the activities of soil microbe and enzymesthus speeding the nutrient cycling, and enhanced the amount of available nutrients, whichwas beneficial to plant growth; secondly, earthworm promoted Cu fraction distributingfrom residual forms to Fe-Mn oxides-bound fraction and organic sulfide bound fractionsthrough increasing soil DOC by secreting mucus, protein or other organic material anddecomposing soil organic; thirdly, earthworm improved the micro-ecology of polluted soil,and the microbial activities could affect soil organic decomposing rate and heavy metalfraction as well.