| Phytostabilization has been proposed as a cost-effective remediation strategy to immobilize metals in heavy metal polluted soils. Athyrium wardii (Hook.) is a common perennial fern with a fast growth rate and large amount of biomass. A survey of plant species during 2007 and 2008 had identified it as a new ecotype, the mining ecotype (ME) of A. wardii, with a greater tolerance to Pb and a higher Pb concentration in roots growing in an old Pb-Zn mine tailing in Yingjing County, Ya’an, Sichuan Province of China.In this study, the mining ecotype (ME) and non-mining ecotype (NME) of A. wardii as used as test materials. Pot experiments were conducted to evaluate the Pb phytostabilization ability and Pb detoxification mechanisms of A. wardii under different Pb treatments, and the effects of chelants on its Pb phytostabilization efficiency. The main results are as follows:(1) The shoot and root biomass of the two ecotypes of A. wardii reduced with increasing Pb treatments in soil, and the biomass reduction of MEwas greater than that of NME. Pb concentration and accumulation of two ecotypes of A. wardii significantly increased with increasing Pb treatments in soil, and Pb concentration and accumulation in shoot and root of ME were significantly higher than those of NME. Pb accumulation in total plant of ME reached the value 184.11 mg plant-1 at Pb800 treatment, while Pb accumulation in total plant of NME was 26.22 mg plant-1. In addition, Pb accumulation in root of ME occupied 95.3%～98.6% in whole plant, demonstrating a predominant capability in Pb accumulation and immobilization Pb in root. The bioaccumulation coefficient of ME were higher than those of the NME under different Pb treatments, and the translocation factors of ME ranging from 0.02～0.08 were far less thanl. Furthermore, Pb remediation factors of the ME were higher than those of NME, and plant effective number of ME were lower than those of NME, which indicated that the ME showed greater Pb uptake and phytostabilization potential than the NME.(2) 27.8～39.0% of the total Pb in ME was sodium chloride (NaCl) extractable and 38.0～48.5% was acetic acid (HAc) extractable, whereas only a minority of total Pb was in ethanol and H2O extractable. In subcellular level,77.4～88.8% of total Pb was stored in cell walls of ME and 9.0～18.9% in soluble fractions. Increasing Pb concentrations enhanced sequestration of Pb into cell walls and soluble fractions of ME tissues to protect organelles against Pb. Synthesis of non-protein thiols (NP-SH) and phytochelatins (PCs) in roots of ME significantly enhanced in response to Pb stress, and significantly increases in glutathione (GSH) were observed in shoots of ME. Higher levels of NP-SH, GSH and PCs were observed in roots of the ME comparing with NME, which was1.26～1.75 times,1.25～1.30 times 1.09～3.45 times higher than those of NME. The results indicated that Pb was localized mainly in cell wall and soluble fraction of ME plants with low biological activity by cell wall deposition and vacuolar compartmentalization, which might be the important adapted Pb detoxification mechanisms of ME.(3) The concentration of exchangeable Pb in the rhizosphere of two ecotypes of A.wardii was higher than that of bulk soil, and concentrations of Pb bound to carbonate and Pb bound to organic matter were lower than those of bulk soil. The concentrations of Pb forms in the rhizosphere of ME were lower than those of NME. pH values in the rhizosphere of the ME and NME decreased by 0.02-0.16 units AND 0.05～0.14 units compared to the bulk soil, and the dissolved organic matter (DOM) concentrations in rhizosphere of the ME and NME were 1.33 times and 1.11 times higher than those in the bulk soil, respectively. Microorganism numbers in rhizosphere of both ecotypes reduced with increasing Pb treatments, and the sensitivity order of microbial community to Pb was:bacteria>fungi>actinomycetes. Microorganism number in the rhizosphere of ME was substantially higher than that of NME and bulk soil.(4) There was little effect on biomass of ME when treated with low dosage of NTA and EDDS, while a greater impact on biomass of A. wardii treated with high dosages of EDDS was observed when compared with NTA. Pb concentration in shoot of ME was higher in the NTA treatment than in the EDDS treatment, while the opposite phenomenon was observed in shoot. Pb accumulations in shoot in NTA treatments were 1.53～2.09 times and 1.63～2.66 times higher than those in EDDS treatments on 7th day and 14th day, and Pb accumulations in root in NTA treatments were 1.13～1.15 times and 1.10～1.30 times higher than those in EDDS treatments on 7th day and 14th day, respectively. The maximum Pb accumulation in root was found at 2 mmol kg-1 of NTA on 14th day, which was 155.5 mg plant-1. Higher BCF values and lower TF values were observed in NTA treatments compared to those in EDDS treatments. The concentration of available Pb in the soil significantly increased on 7th day with increasing dosages of NTA and EDDS, and gradually decreased on 14th day. The available Pb in soil treated with EDDS was higher than that treated with NTA on 7th day and lower on 14th day. A similar tendency was observed in exchangeable Pb in soil. The concentrations of Pb bound to carbonate and Pb bound to Fe-Mn oxides reduced after treated with NTA and EDDS on 7th day and increased on 14th day when compared with the control. Therefore, chelant application was an effective way to enhance Pb bioavailability in soil and Pb uptake of ME, and NTA was more effective than EDDS on enhancing Pb accumulation in root of ME. |
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