Plant-microbe Remediation Of Cd-DDT Co-contaminated Soils And Its Mechanisms

Environmental pollution of the soil-crop system is very complex due to the coexistence of old and new pollutants, as well as inorganic and organic compounds. Especially soil pollution caused by heavy metals and persistent organic pollutants (POPs) is widespread concerned. In recent years, the frequent outbreak of environmental pollution accidents due to soil-water pollution puts forward urgent demand for remediation and restoration of contaminated soil.In this research, the plant-microbe remediation of Cd and DDT was carried out under pot and field experiments to identify bioremediation strategy of Cd-DDs co-contaminated soil and to investigate the mechanism for enhancing bioremediation efficiency. Primary results were summarized as follows:1. Cadmium (Cd) and dichlorodiphenyltrichloroethane (DDT) or its metabolite residues DDD/DDE (DDT, DDE, and DDD are collectively called DDs) are frequently detected in agricultural soils and agricultural products, posing a threat to human health. The objective of this study was to compare the ability of21genotypes of Cucurbita pepo ssp in mobilizing and uptake of Cd and DDs (p,p’-DDT, o,p-DDT, p,p’-DDD and p,p’-DDE) in the co-contaminated soil. The plant genotypes varied greatly in the uptake and accumulation of Cd and DDs, with mean concentrations of0.26-1.12、0.49-2.25、1.04-4.84and1.61-7.72mg kg-1DWfor Cd, and338.4-793.2,1619-1812,1273-3548and3396-12811ng g-1DW for DDs in leaf, stem and root, respectively. The TF and BAF values were0.79and1.68for Cd, and0.60and1.54for DDs, respectively. These results indicate that Cucurbita pepo cv."Tebiexuan mibenwang" has a low ability to absorb and accumulate Cd and DDs from the contaminated soils, but Cucurbita pepo cv."Riben Hongtianmi" has great potential for accumulating Cd and DDs from moderately co-contaminated soil (Cd≤1.50mg kg-1, DDs≤1.00mg kg-1)。2. The development of an integrated strategy for the remediation of soil co-contaminated by heavy metals and persistent organic pollutants is a major research priority for the decontamination of soil slated for use in agricultural production. The objective of this study was to develop a bioremediation strategy for fields co-contaminated with cadmium (Cd), dichlorodiphenyltrichloroethane (DDT), and its metabolites1,1-dichloro-2,2-bis (4-chlorophenyl) ethylene (DDE) and1,1-dichloro-2,2-bis (4-chlorophenyl) ethane (DDD) using an identified Cd-hyperaccumulator plant Sedum alfredii (SA) and DDT-degrading microbes (DDT-1). Initially, inoculation with DDT-1was shown to increase SA root biomass in a pot experiment. When SA was applied together with DDT-1, the levels of Cd and DDs in the co-contaminated soil decreased by32.1-40.3%and33.9-37.6%, respectively, in a pot experiment over18months compared to3.25%and3.76%decreases in soil Cd and DDs, respectively, in unplanted, untreated controls. A subsequent field study (18-month duration) in which the levels of Cd and DDs decreased by31.1%and53.6%, respectively, confirmed the beneficial results of this approach. This study demonstrates that the integrated bioremediation strategy is effective for the remediation of Cd-DDs co-contaminated soils.3. Co-cropping, a widely accepted agronomical practice in China for2000years, can increase total crop yields through increased resource use efficiency. In our present experiments, co-cropping of Sedum alfredii and B. campestris ssp chinensis associated with DDT-1increased the biomass and metal phytoextraction of S. alfredii (P＜0.05) and also enhanced the root growth of B. campestris ssp chinensis (P＜0.05), but had no significant effects on shoot biomass of B. campestris ssp chinensis. The combined remediation strategy decreased the Cd concentration by48.0-51.5%and54.7-71.4%in the roots and shoots of B. campestris ssp chinensis, respectively, and also corresponded to a decrease in root DDs concentration by37%. The removal efficiencies were30-46%for Cd and36.8-42.7%for DDs. Laser scanning confocal microscopy revealed that gfp-tagged DDT-1heavily colonized in rhizosphere soil, and on rhizoplane of Sedum alfredii. The soil diversity indices, such as richness index (S), evenness index (E), Shannon’s diversity (H’), Simpson’s index (D’), were higher in all of the planted treatments than in the unplanted control. Principal component analysis of bacterial T-RFLP data also revealed strong shifts in bacterial community composition with the planted treatments. The results of this study indicate that the co-cropping of Sedum alfredii and B. campestris ssp chinensis associated with DDT-1appears to be a promising approach for the bioremediation of soils co-contaminated by Cd and DDs, while simultaneously decreasing the pollutant concentration in edible part of B. campestris ssp Chinensis, and hence maintaining product safety and and reliability of this vegetable.4. In our present experiments, co-cropping Sedum alfredii and Cucurbita pepo cv."Tebiexuan mibenwang" associated with DDT-1increased the root (26.8%) and shoot biomass (21.7%) of "Tebiexuan mibenwang"(P＜0.05), but had no significant effects on the root and shoot biomass of Sedum alfredii. In general, the combined remediation strategy did not significantly decrease the Cd concentrations in roots and shoots of both plants. However, for "Tebiexuan mibenwang", the bioremediation strategy decreased the shoot DDs concentration by38.2-44.5%(P＜0.05), but had no significant effects on root DDs concentration; for Sedum alfredii, increased and decreased DDs concentration by34.7-67.7%and38.2-44.5%in roots and shoots of Sedum alfredii, respectively. The removal efficiencies were41.9-60.7%for Cd and37.5-45.2%for DDs. In the present study, for roots of "Tebiexuan mibenwang", Cd was largely retained in the cell walls and vessel tissue, DDs was largely retained in central cylinder, and was adsorbed on root hairs, but for shoots, Cd was largely retained in the collenchyma tissue and vacuoles, DDs was largely retained in vessel tissue, vacuoles and glandular trichomes. Laser scanning confocal microscopy revealed that gfp-tagged DDT-1heavily colonized on rhizoplane of Sedum alfredii and "Tebiexuan mibenwang". The soil diversity indices, such as richness index (S), evenness index (E), Shannon’s diversity (H’), Simpson’s index (D’), were higher in the treatment than in the unplanted control. The results of this study indicate that the co-cropping of Sedum alfredii and "Tebiexuan mibenwang" associated with DDT-1appears to be a promising approach for the bioremediation of soils co-contaminated by Cd and DDs, and while decreasing the DDs concentration in shoot of "Tebiexuan mibenwang", and hence ensuring product safety of the pumpkin.