| Phytoremediation of heavy metals in contaminated soil has been widely accepted as a cost-effective, environmental-friendly technology. Metal-accumulating plants are usually planted to remove metals from soils by concentrating them in the harvestable parts, while the efficiency depends on the amount of aboveground biomass and the bioavailabililty of metals. Though addition of chelating agents has been shown to increase metal bioavailabililty and phytoremediation efficiency, it costs too much and shows environmental risk.Three soils, Red soil from Jiangxi Province, Orthic aquisols from Jiangsu Province and Podzol from Hongkong, were used in the experiments. Soils were amended respectively to contain 0, 100, 200, 400 mg Cu kg-1 and 0, 5, 10 and 20 mg Cd kg-1 by adding appropriate concentrations of CuSO4 and CdCl2. By comparing with treatments with and without earthworm (Pheretima sp.) in earthworm culture experiment ryegrass (Lolium multiflorum) planting experiment and mycorrhiza {Glomus mosseae + Glomus intraradices) inoculation experiment, the objective of this study was to investigate the effects of earthworm on the plant biomass and bioavailability of heavy metal in soils, to show the mechanisms of earthworm effect on chemical behavior of heavy metal in soil-plant system, and to provide theoretic base for the application of earthworm in phytoremediaton.The used earthworms can tolerate heavy metals, but their growth was negatively affected by heavy metal pollution in soil. Earthworm enriched Cd significantly but not for Cu, and the Cd enrichment coefficients in Orthic aquisols and Red soil were 2.78~3.41 and 3.08~3.84 respectively, while the Cu enrichment coefficients were only 0.13~0.16 and 0.18~0.28 respectively. In Orthic aquisols with Cu or Cd addition, earthworm activity increased ryegrass aboveground biomass by 33%~96%, but the increment decreased with the increasing heavy metal application in soil. And in Red soil with Cu addition, earthworm activity also increased ryegrass aboveground biomass by about 0~40%, but had little effect in Red soil with Cd addition. Earthworm activity markedly enhanced concentration of available N in Red soil and Orthic aquisols by 11%~56% and 24%~94% respectively, but had little effect on available P and K contents in two soils and P and K content in ryegrass.One of the mechanisms that earthworm activity enhanced plant biomass is by increasing soil available N and plant absorbing N, consequently improving plant growth.The pH of Red soil decreased by 0.03-0.18 units in the presence of earthworm, but pH of Orthic aquisols increased slightly. Earthworm activity increased concentrations of DTPA-Cu, CaCl2-Cu and H2O-Cd in Red soil, but had little effect on fractions of Cu and Cd in Orthic aquisols. Soil pH was negatively correlated with each fraction of Cd in soil, and the correlation between soil pH and CaCl2-extractable Cd content was the most significant, the coefficient was -0.950**. So by affecting soil pH, earthworm activity affects bioavailability of heavy metal in soil or in earthworm cast. Though concentration of heavy metal in plant tissue did not increased, total uptake of Cu by ryegrass was enhanced by earthworm activity for the biomass increment. This is important for enhancing efficiency of phytoremediation.The mean mycorrhiza infection rate of root was about 22%, and it was not affected by Cd addition in Podzol, but it increased by 9% in the presence of earthworm. But mycorrhiza inoculation had no regular effect on earthworm growth rate. Mycorrhiza inoculation enhanced concentration of available N P and dissoluble organic carbon in soil and increased uptake of N and P by ryegrass, but did not increased ryegrass aboveground biomass. The reason is that mycorrhiza infection improved uptake of Cd by ryegrass, consequently restrained ryegrass growth. Mycorrhiza inoculation improved not only uptake of Cd by ryegrass but also transfer of Cd from root to shoot. This is beneficial to phytoremediation efficiency.Except in enhancing uptake of N by ryegrass,... |
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