| Soil heavy metal pollution has become a serious environmental problem in our country. Remediation of heavy metal contaminated soil has attracted extensive attention. Phytoextraction, the use of metal hyperaccumulating plants to clean up contaminated soil, is being considered as a promising, cost-effective and non-intrusive technology for the remediation of metal polluted soils. In the present study, classification of plant species for metal accumulation and identification of potential species for phytoextraction was conducted using cluster analysis based on a practical survey. The rhizosphere soils of some representative tolerant plants were collected in the field condition. Microbial biomass and activities of these soils were studied to evaluate the rhizosphere microbial effects of the tolerant plants. Pre-stratified rhizobox experiments were conducted to determine the effects of phytoextraction on microbial properties through the measurement of soil microbial biomass, activities and community structure during remediation of artificial Cd contaminated soil and multiple heavy metals contaminated soil. The main results are summarized as follows:1. The results of the survey conduected in the ancient silver mining site showed that total metal concentrations in the soils ranged5.7-84.4mg kg-1for Cd,467.2-8326.7mg kg-1for Zn and898.8-7381.0mg kg-1for Pb. The heavy metal concentrations in plant shoots ranged39-1053.58mg kg-1for Cd,71.3-3508.8mg kg-1for Zn and11.5-1073.0mg kg-1for Pb, in the roots ranged2.54-222.02mg kg-1for Cd,71.3-3508.8mg kg-1for Zn and122.0-3437.7mg kg-1for Pb.The plant species for metal accumulation were classified by cluster analysis based on shoot metal concentration, BF and TF. Combining with the results of cluster analysis and metal hyperaccumulation thresholds, five groups such as hyperaccumulator, potential hyperaccumulator, accumulator, potential accumulator and normal accumulating plant were graded. For Cd accumulation, S. alfredii was treated as a Cd-hyperaccumulator, and the others were normal Cd-accumulating plants. For Zn accumulation, S. alfredii was considered as a potential Zn-hyperaccumulator, C. canadensis and A. lavandulaefolia were Zn-accumulators, and the others were normal Zn-accumulating plants. For Pb accumulation, S. alfredii and E. lineolatum were potential Pb-hyperaccumulators, R. hunanensis, A. decumbens and E. annuus were Pb accumulators, C. Canadensis and A. lavandulaefolia were potential Pb accumulators, and the others were normal Pb-accumulating plants. Plant species with the potential for phytoextraction were identified such as S. alfredii for Cd and Zn, C. canadensis and A. lavandulaefolia for Zn and Pb, and E. lineolatum, R. hunanensis, A. decumbens and E. annuus for Pb. Cluster analysis is effective in the classification of plant species for metal accumulation and identification of potential species for phytoextraction.2. The microbial properties of the rhizosphere soil of4different metal accumulation patterns tolerant plants were studied by collecting the rhizosphere soils in the field condition. Soil microbial biomass carbon content, basal respiration rate, urease, acid phosphatase and invertase activities in the rhizosphere of S. alfredii, R. hunanensis, L. christinae, C. gracile were significantly (P<0.05) higher than that of non-rhizosphere soils, so were soil microbial biomass carbon content, basal respiration rate, acid phosphatase and invertase activities in the rhizosphere of S. alfredii than that in rhizosphere of the other3tolerant plants. The numbers of the kinds of individual PLFAs detected in the rhizosphere of S. alfredii, R. hunanensis, L. christinae and C. gracile were70,60,58and54, while that in the non-rhizosphere of the four plants were49,49,48and39. Shannon-Weiner index (H) of soil microbial community in the rhizosphere of S. alfredii, R. hunanensis and C. gracile were significantly (P<0.05) higher than that of non-rhizosphere soils, so were the Shannon-Weiner index (H) of soil microbial community in the rhizosphere of S. alfredii than that in rhizosphere of the other3tolerant plants. The PLFA concentrations of total, bacterial, Gram-positive and Gram-negative bacterial, actinomycete, fungal, AM fungal and protozoan in the rhizosphere of S. alfredii, R. hunanensis, L. christinae and C. gracile were significantly(P<0.05) higher than that of non-rhizosphere soils, so were these PLFA concentrations in the rhizosphere of S. alfredii than that in rhizosphere of the other3tolerant plants. Soil microbial activities and community structure in the rhizosphere of S. alfredii were significant different from the rhizosphere of the other3tolerant plants and all the non-rhizosphere soils.3. A pre-stratified rhizobox experiment was conducted with hyperaccumulator Sedum alfredii to determine the effects of phytoextraction an artificial Cd contaminated soil on microbial properties. The results indicated that, in the plant-grown zone, basal respiration, acid phosphatase and invertase activities of the rhizospheric soil separated by the shaking method were significantly (P<0.01) higher than that of the near-rhizospheric soil and the unplanted soil after3months growth, so were microbial biomass carbon, urease, acid phosphatase and invertase activities of the rhizospheric soil after6months growth. In the pre-stratified area, acid phosphatase activity of the0-2mm sub-layer rhizospheric soil collected by the pre-stratified method after3months growth was significantly (P<0.05) higher than that of other sub-layer rhizospheric soils and bulk soil, and so were microbial biomass carbon, basal respiration, urease, invertase and acid phosphatase activities of the0-2mm sub-layer rhizosphere soil after6months growth. It was also observed that the soil, basal respiration rate, and urease activities were significantly negatively correlated to water soluble Cd. It was concluded that phytoextraction by Sedum alfredii could decrease the water-soluble Cd and improve soil microbial properties, especially in rhizosphere.4. A pre-stratified rhizobox experiment with a Cd-contaminated soil was conducted to assess the effects of phytoextraction by S.alfredii on soil microbial community. In the plant-grown zone, after12months growth,53kinds of PLFAs were identified in rhizospheric soil, while only32in unplanted soil. Shannon-Weiner index (H) of soil microbial community in the (near-) rhizospheric soils were significantly (P<0.05) higher than that in unplanted soil. The PLFA concentrations of total, bacterial, actinomycete, fungal, AM fungal and protozoan in (near-) rhizospheric soils were significantly (P<0.05) higher than that in unplanted soil. The utilization rates of15substrates by rhizospheric soil were significantly (P<0.05) higher than that by unplanted soil. In the pre-stratified area, from41to60kinds of individual PLFAs in sub-layer rhizospheric soils were identified, while only39in unplanted soil. Shannon-Weiner index (H) of soil microbial community in the0-8mm soil were significantly (P<0.05) higher than that in bulk soil. The PLFA concentrations of total, bacterial, actinomycete, fungal, AM fungal and protozoan in0-2mm soil were significantly (P<0.05) higher than that in bulk soil, so were the utilization rates for the15substrates. PLFAs concentrations and substrate utilization rate were negatively correlated with pH, total, HOAc and NH2OH·HCl-extractable Cd concentratoions and positively correlated with labile carbon. The increase in microbial diversity and activities of soil microbial community were attributed to the raise of labile carbon and reduction of available Cd concentrations.5. To study the effects of phytoextraction by Sedum alfredii on microbial property improvement of a multiple heavy metals contaminated soil, a rhizobox experiment was conducted under greenhouse conditions. In the plant-grown zone, microbial biomass carbon content, basal respiration, urease, acid phosphatase, invertase activities of the rhizospheric soils were significantly (P<0.05) higher than that of unplanted soils after3and6months growth. In the pre-stratified area, microbial biomass C and urease of0-4mm sub-layer rhizospheric soils were significantly (P<0.05) higher than that of bulk soil after3months growth. MBC of0-2mm and BR rate of0-8mm sub-layer rhizospheric soils were significantly higher than that of bulk soil after6months growth. So were the three enzyme activities of0-4mm sub-layer rhizospheric soils. BR rate and urease were significantly (P<0.05) negatively correlated with soluble Cd, so were microbial biomass C, acid phosphatase and intervase activities with soluble Zn, microbial biomass C, basal respiration rate and three enzyme activities with soluble Pb. Phytoextraction by S. alfredii could reduce water soluble Cd, Zn, Pb concentrations in the rhizospheric soils and increase microbial biomass C, basal respiration rate and enzyme activities of the metal polluted soil.6. A rhizobox experiment with a Cd-Zn-Pb multiple contaminated soil was conducted to investigate the effects of phytoextraction by S.alfredii on soil microbial community structure during the phytoextraction process. In the plant-grown zone, after12months growth,50kinds of PLFAs were identified in rhizospheric soil, while only29kinds of PLFAs were identified in unplanted soil. Shannon-Weiner index (H) of soil microbial community in the (near-) rhizospheric soils were significantly (P<0.05) higher than that in unplanted soil. The PLFA concentrations of total, bacterial, actinomycete, fungal, AM fungal and protozoan in (near-) rhizospheric soils were significantly (P<0.05) higher than that in unplanted soil. The utilization rates of14substrates by rhizospheric soil were significantly (P<0.05) higher than that by unplanted soil. In the pre-stratified area,41kinds of individual PLFAs in0-2mm sub-layer rhizospheric soil were identified, while only31in unplanted soil. Shannon-Weiner index (H) of soil microbial community in the0-10mm sub-layer rhizospheric soil were significantly (P<0.05) higher than that in bulk soil.The PLFA concentrations of total, bacterial, actinomycete, fungal, AM fungal and protozoan in0-2mm sub-layer rhizospheric soil soil were significantly (P<0.05) higher than that in the2-10mm sub-layer rhizospheric soils and bulk soil. The utilization rates of14substrates by microorganisms of0-8mm sub-layer rhizospheric soils were significantly (P<0.05) higher than that by bulk soil. PLFAs concentrations and substrate utilization rate were negatively correlated with total, HOAc, NH2OH·HC1, and NH4OAC-extractable Cd and Zn concentrations, and positively correlated with labile carbon. These indicated that the enhencement of microbial activities and changes of microbial community structure were attributed to the raise of labile carbon and reduction of Cd and Zn concentrations. |
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