| Microorganisms are one of the most importment components in micro-ecosystem of rhizosphere. Soil microbes are the active colloids in the soil, with charges, high specific surface areas and metabolic activities. For heavy metal hyperaccumulator plants,in the process of specific heavy metal uptake and accumulation, microorganisms play an important role in improving soil micro-environment, increasing metal dissolution. Some progresses have been made towards an understanding of plant-internal processes associated with metal hyperaccumulation, however, relatively few is known about the role of rhizosphere processes involved in the bioactivation and uptake of heavy metals. The assessment of microbial metabolites in rhizosphere soil helps to understand whether and how rhizosphere processes contribute to the excessive metal uptake by hyperaccumulators.Sedum alfredii Hance has been identified as a new Zn-hyperaccumulating plant species indigenously growing in China. Two ecotypes of Sedum alfredii Hance were found, one is a Zn/Cd hyperaccumulator defined as the hyperaccumulating ecotype (HE) and the other is a non-hyperaccumulator defined as non-hyperaccumulating ecotype (NHE). In this study, field sample collection and a rhizobag experiment in green house was carried out . The rhizosphere process of these two ecotypes of Sedum alfredii Hance in three different polluted soils was investigated, including the difference in rhizospheric and non-rhizospheric soil microbial profiles and the effect of the uptake and accumulation of Zn by rhizospheric,non-rhizospheric soil microbial and ecotype.The major results were summarized as follows:1. Pot experiments were conducted to study the effects of different heavy mental contaminated soil on soil microbial characteristics of two contrast ecotypes of Sedum alfredii Hance. The results show that in different soil, populations of bacteria, actinomycetes and fungi and microbial biomass C in rhizosphere of Sedum alfredii were much greater than that in non-rhizosphere. In slightly polluted soil, there were no significant difference on soil microflora and major microbial physiological groups between the two ecotypes of Sedum alfredii, whereas in mining soil and seriously polluted soil, populations of bacteria and fungi, physiological groups and microbial biomass C in the rhizosphere of the hyperaccumulating ecotypes of Sedum alfredii were much greater than that in non-hyperaccumulating ecotype. The order of soil microorganisms being sensitive to heavy metal pollution was actinomycetes>bacteria>fungus, and nitrobacteria > azotobacteria > ammonifying bacteria > phosphbacteria > cellulose decomposing bacteria.2. No significant difference on soil base respiration and soil microbial metabolic quotient between the two ecotypes were found in slightly polluted soil, however, in mining soil and seriously polluted soil, this two indexes in the rhizosphere of non-hyperaccumulating ecotype were much greater than that in hyperaccumulating ecotype.3. It was found that zinc metal ion biosorption and removal efficency by soil microbial and epiphyte extracted from rhizosphere and non-rhizosphere of Sedum alfredii Hance were both increased in Developed on quaternary red earths(REQ),Aquic cambisol(AC) and Mining soi(lMS)as the initial metal concentration increased.The adsorption data with respect to both metals provide an excellent fit to the Freundlich isotherm.The capability of zinc absorption in the three soils used was restrained in order of rhizosphere-microbia > non-rhizosphere-microbial > CK, while Zinc absorption in the same three soils used were prompted in order of non-rhizosphere-epiphyte > rhizosphere-epiphyte > CK .For the three soils with different treatments,the effect in aquic cambisol soil is especially obvious. |
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