| As one of the four major lead-zinc mines in China,Fengxian County of Shaanxi Province has a large number of lead-zinc smelting enterprises,which brings great ecological risks to local agricultural production and soil environment.At present,physical and chemical remediation is widely used in heavy metal contaminated soil in this area,but the potential problem of secondary pollution exists extensively.Because of its economic green and environmental friendliness,phytoremediation technology has become a hot spot for soil heavy metal pollution restoration in recent years.However,due to the small amount of plant biomass and limited tolerance,this technology has been restricted in its development.Plant growth promoting rhizobacteria?PGPR?can secrete growth hormones,promote plant growth,improve the bioavailability of heavy metals,increase plant resistance,and enhance the efficiency of phytoremediation.In this study,the characteristics of heavy metal pollution,dominant plant composition and soil microbial diversity were analyzed by field investigation in the lead-zinc smelting area of Fengxian County,Shaanxi Province.On this basis,a one-year greenhouse pot experiments were conducted to analyze the effect of phytoextraction efficiency of heavy metals.In the pot experiments,a local dominant plant Alfalfa and a Zn/Cd hyperaccumulator Sedum alfredii were used as phytoremediation plants cultivated in monoculture and intercropping,and the plant growth promoting rhizobacteria Phyllobacterium myrsinacearum?RC6b?was introduced to enhance phytoremediation.The effect of RC6b on enhanced phytoremediation and its mechanism were illuminated in terms of plant growth,heavy metal extraction efficiency,rhizosphere soil ecological environment and plant physiological and biochemical characteristics to provide an effective method for the treatment of heavy metal contaminated soil in lead and zinc smelting area.The main research conclusions are as follows:?1?The average concentrations of Pb,Cd,Cu and Zn in the soil of lead and zinc smelting area of Fengxian County were 380.47 mg·kg-1,11.72 mg·kg-1,56.07 mg·kg-1 and535.23 mg·kg-1 respectively,in which Cd reached heavy pollution level,Pb reached moderate pollution level,and Zn reached slight pollution level,while Cu is in normal range.The pollution characteristics are as follows:the closer it is to the lead and zinc smelter,the more serious the pollution would be.In addition,the east side of the smelter pollution level is much higher than the west.Smoke dust from smelters is the main source of soil heavy metal pollution.The results of speciation analysis indicated that the proportion of species of Pb,Cd,Cu and Zn generally declined in the order of:residual fraction?34.45%45.98%?>oxidizable fraction?27.45%30.57%?>reducible fraction?17.63%23.61%?>exchangeable fraction?6.80%14.41%?.Soil organic matter content and pH value have great influence on the speciation distribution of heavy metals in soil.?2?The dominant plants,mainly compositae and gramineae,had higher bioaccumulation and biotransportion abilities towards Cd and Zn,compared with Pb and Cu.Among them,Medicago sativa,Lolium perenne,Artemisia carvifolia and Bidens maxi-mowicziana has potential for phytoremediation with stronger bioaccumulation abilities of Pb,Cd and Zn.In this study,high-throughput sequencing technology was used to analyze the soil microbial community.The results showed that,Proteobacteria,Acidobacteria,Bacteroidetes and Actinobacteria were the dominant bacteria groups in the bacterial community at the phylum level.Ascomycota,Mortierellomycota and Chytridiomycota were the dominant fungi in the fungal community.Variation in soil heavy metal content has a greater effect on bacterial community diversity,compared with that on fungal community diversity.?3?The effect of PGPR strain RC6b on the growth and phytoextraction efficiency of heavy metals by greenhouse pot experiments were studied.Results showed that,with the inoculation of RC6b,shoot biomass yields of plants were significantly increased by18.8925.39%and 17.1819.93%forAlfalfa and S.alfredii,respectively,compared with the non-inoculated plants.RC6b inoculation increased metal concentrations by 14.0821.16%?Pb?,16.3326.47%?Cd?,and 25.3530.24%?Zn?in Alfalfa shoots,and by 23.2424.38%?Pb?,22.7527.20%?Cd?,and 33.3834.09%?Zn?in S.alfredii shoots,respectively.RC6b inoculation increased the phytoextraction efficiencies of Pb,Cd,and Zn by 19.64%,46.30%,and 60.94%,respectively through shoots of the inter-cropping system.Nevertheless,phytoextraction of Cu was not improved by RC6b inoculation.?4?The effects of PGPR strain RC6b on the ecological environment of rhizosphere soil were also studied.The findings suggested that the concentrations of available nitrogen and available phosphorus in rhizosphere soil increased significantly?P<0.05?,and the available potassium concentration increased slightly after RC6b inoculation.The average increase of the three was 19.73%.The activities of urease,alkaline phosphatase,sucrase and catalase in rhizosphere soil were significantly increased?P<0.05?,with an average increase of 33.26%.The soil nutrient and soil enzyme activity had significant positive correlation.Enhanced enzyme activity promoted the improvement of soil nutrients,increased the soil microbial activity and the carbon utilization ability of the soil microbial community.The concentrations of exchangeable fraction and reducible fraction of Cd and Zn increased significantly?P<0.05?in rhizosphere soil,while the oxidizable fraction and residual fraction decreased significantly?P<0.05?.The changes of heavy metal speciation were beneficial to improve bioavailability of heavy metal,which consequently promoted the absorption and accumulation of heavy metals in plants.?5?The effects of PGPR strain RC6b on physiological and biochemical characteristics of plants were studied.Measurements showed that the chlorophyll concentrations of Alfalfa and Sedum alfredii were both significantly increased after RC6b inoculation?P<0.05?,but the effect on enzymatic antioxidant activities of two plants were different,the concentrations of MDA,O2-.,H2O2 in shoots and roots of Sedum alfredii were significantly decreased?P<0.05?,and the concentrations of SOD,POD,CAT and APX were significantly increased?P<0.05?.RC6b inoculation can increase the resistance of Sedum alfredii to heavy metals through reduction of plant oxidative damage and increase of antioxidant activity.However,RC6b inoculation did not effectively increase the resistance of Alfalfa to heavy metals.the concentrations of O2-.,H2O2 in shoots and roots of Alfalfa were increased,and the concentrations of SOD,POD and CAT in shoots were significantly increased?P<0.05?,but APX in shoot was significantly decreased?P<0.05?.This difference seems due to their individual tolerances to heavy metals.?6?Through comparison of the monoculture treatment with the intercropping treatment of Alfalfa and Sedum alfredii,it has been found in intercropping treatment that the biomass of two plants both decreased by 3.128.20%,the concentrations of Cd and Zn in Alfalfa decreased by more than 40%in shoot,and more than 20%in roots,while the concentrations of Cd and Zn in Sedum alfredii did not change much.The concentrations of MDA,O2-,H2O2 in shoot and root of Alfalfa were significantly decreased?P<0.05?,and the concentrations of SOD,POD,CAT and APX in shoot and root of Alfalfa were also significantly decreased?P<0.05?.In the rhizosphere microenvironment,the bioaccumulation ability of Cd and Zn of Sedum alfredii was much higher than that of Alfalfa,Competitive absorption of Cd and Zn by Sedum alfredii effectively alleviated the threats of heavy metals to Alfalfa under the intercropping treatment. |
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