Micro-Ecological Characteristics In Rhizosphere Soil Of Hyperaccumulator Sedum Alfredii Hance

Phytoremediation has emerged as an alternative technique for removing toxic metals from soil and offers the benefits of being in situ, cost-effective and environmentally sustainable. Hyperaccumulator plants can acquire high amounts of heavy metals even from soils containing low concentrations of soluble metals. Some progress has 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 investigation and a rhizobag experiment in green house was carried out .We investigated the rhizosphere process of these two ecotypes of Sedum alfredii Hance in three polluted soils, including the change of heavy metal bioavailability, pH, DOM, enzymes activity, microbial metabolic activity microbial community functional diversity and so on. The major results were summarized as follows:1. In Pb-Zn mine area, certain plants have evolved to heavy metal hyperaccumulating ecotype because of high concentration of heavy metals in mining soils. A host-symbiont system was formed between hyperaccumulator and rhizosphere microbes. In this system, microbial metabolic activities (eg. microbial biomass carbon, soil basal respiration rate, Rmic/Cmic etc.), soils enzymes activity and heavy metal accumulation trait are quite different from other areas. With heavy metal concentration increasing, microbial biomass carbon decreases while soil basal respiration rate, Rmic/Cmic increase. The microbial biomass carbon in mine center soil was 72% of control, but basal respiration and qCO₂ was 1.6 and 2.3 fold respectively. Each kind of soil enzyme activities was higher than that of the mine soil, among which dehydrogenase activity changed most remarkably, so it can be a better index in evaluation of heavy metal pollution in mine area.2. A pot experiment was carried out to study the physi-chemical properties of rhizosphere of two kinds of Sedum alfredii Hance, and the results showed: There is a significant difference between Zn and Pb concentration in shoots of hyperaccumulator ecotype of Sedum alfredii Hance (HE) and non-hyperaccumulating ecotype (NHE), with that of HE much higher. Hyperccumulation characteristics still appears even in soil containing low concentrations of soluble metals;After planting HE, the pH of rhizospheric soil was deceased by 0.1 unit, while pH in non-rhizospheric soil remained unchanged. When planted with HE, organic matter content in rhizosphere increased by 10.2 % . 7.5 % in mining soil and light polluted soil respectively, as compared with non-rhizospheric soil. As for DOM concentration, the increase trend appears more obvious .And as comparison, DOM concentration only changed a little between rhizospheric soil and non-rhizospheric soil. After growing HE, bioavailable concentration of Zn and Cd remarkably decreased, while Pb and Cu changed unconspicuously.3. Soil enzymes activities and their relation with liable heavy metal content in rhizosphere of Sedum alfredii Hance were studied. After planted with HE, dehydrogenase and urease activity were much higher in rhizospheric soil than in non-rhizospheric soil. As for NHE, the increasing trend is relatively vague. This indicates stronger microbial activities and more effective heavy metal bioacitiviation in rhizospheric soil of HE. Dehydrogenase activity changed most significantly among these three soils studied, which is consistent with field investigation result, confirmed it can be used as a sensitive index in evaluation of heavy metal pollution of soil. Correlation analysis of enzymes activity in rhizospheric soil and heavy metal bioavailibilty suggested that, dehydrogenase and urease activity are significantly negative associated with DTPA-extractable Zn concentration, while other DTPA-extractable metals have no significant effect on enzymes activity.4. In soil planted with HE, microbes number, basal respiration rate and microbial biomass carbon of rhizospheric soil were higher than that of non-rhizospheric soil, suggesting that microbe in the rhizospheric soil of HE could transform organic carbon into microbe carbon more effectively. Comparison of two ecotypes showed that microbes number, basal respiration rate in rhizospheric soil planted with HE were significantly higher than that planted with NHE in mine soil and heavy polluted soil. The possible explanation is that rhizosphere environment of HE is different from that of NHE, the former can alleviate heavy metal stress on microbes partly. In mining soil and heavy polluted soil, Rmic/Cmic in the rhizospheric soil of NHE was higher than in the rhizospheric soil of HE, especially in heavy polluted soil, by 30.6 %, which indicated that microbes in rhizosphere of NHE were inhibited more seriously than that of HE. Despiteheavy metal stress on microbes exists in rhizosphere of both ecotypes, there are more microbes in the rhizosphere of HE, which indicates a positive interaction between HE and microbes in rhizospheric soil.5. In polluted soils, composition and functional diversity of microbial community changed as indicated by Biolog data and principal component analysis of Biolog community metabolic profiles. The result of pot experiment showed that soil microbial metabolic profiles (AWCD) values, community richness and diversity index were all higher in HE rhizospheric soil than in NHE rhizospheric soil. As for the same ecotype, these three indexes were higher in rhizospheric soil than in non-rhizospheric soil. All of these indicated that the microbial number used energy carbon is more and utilization ability of microbial communities for carbon substrate is stronger in rhizosphere soil of HE. Principal component analysis of carbon sources utilization by soil microorganisms in rhizospheric soil of HE and NHE indicated: In mine soil planted with HE, there is no obvious difference of carbon sources utilization between rhizospheric and non-rhizospheric soil, suggesting a relatively stable microbial community. As contrast, carbon sources utilization by microorganisms in rhizosphere of NHE showed a quite different pattern from non-rhizosphere.