Study On Remediation Of Heavy Metals Contaminated Soil By Herb-tree Configuration Around Mining Area

Selecting suitable woody plants and herbs to construct herb-tree co-planting ecological remediation system,using the niche difference of herb and tree to enhance remediation efficiency and improve soil environmental quality,is the development trend of ecological remediation of heavy metal contaminated soil around mining area.In this study,aim at the As,Cd and Pb co-contaminated soil around a mining and smelting area of Hengyang,Hunan province,a pot experiment,co-planting the pioneer woody plants(Broussonetia papyrifera and Melia azedarach L.)with herbs(Pennisetum purpureum Schum and Pteris vittata L.),was conducted to study the effects of co-planting on plant growth,heavy metal uptake and accumulation,soil environmental quality,rhizosphere low molecular organic acids and microbial community.To screen the suitable configuration modes for the remediation of heavy-metal contaminated soil to provide technical support for soil ecological remediation around mining area.The main results are as follows:(1)Co-planting could promote the growth of P.purpureum,M.azedarach and B.papyrifera,and improve photosynthetic pigment synthesis and antioxidant enzyme activity in their leaves,and co-planting of three plants had more obvious effect.Compared with the monoculture,the contents of chlorophyll a,b and SOD activity in M.azedarach and P.purpureum leaves when co-planting of M.azedarach,P.vittata and P.purpureum(KWX)were significantly increased,and their total biomass was significantly increased by 51.1% and 129%,respectively(P < 0.05);The the chlorophyll b content of B.papyrifera and P.purpureum under the co-planting system of B.papyrifera,P.vittata and P.purpureum(GWX)was significantly increased,and their total biomass was significantly increased by 17.9% and 133%,respectively(P < 0.05).The content and accumulation of As in shoot and root of P.purpureum,M.azedarach and B.papyrifera were low,which mainly stabilized As in soil.Co-planting had no positive effect on the absorption and accumulation of As,but promoted the Cd and Pb accumulation in P.vittata rhizoid,and in the shoot and root of M.azedarach,Cd accumulation in P.purpureum shoot and Pb accumulation in P.purpureum root and in the shoot and root of B.papyrifera,and KWX and GWX treatment had more obvious effect,which could be used for remediation of heavy metal co-contaminated soil.(2)Phytoremediation could improve soil environmental quality,and decrease heavy metal availability and heavy metal content in soil solution.The contents of available Cd and Pb in soils under KWX and GWX treatment were lower than those of M.azedarach and P.vittata monoculture.The soil solution Cd content in under KWX treatment was significantly decreased by 44.2% and 33.0%,respectively,compared with P.vittata and P.purpureum monoculture(P < 0.05).Compared with corresponding monoculture,the soil solution DOC content and soil urease activities in KWX treatment was significantly increased by23.5%-30.0%and 33.9%-104%,respectively(P < 0.05),and the soil solution DOC content in GWX treatment was significantly increased by 35.0%,22.9%and 26.4%(P < 0.05).(3)Co-planting could promote tartaric acid secretion from plant roots,and co-planting of three plants was more beneficial to improve the microbial community richness and diversity in contaminated soil.Ace and Chao1 indices of rhizosphere microorganisms under KWX and GWX treatment were higher than those under corresponding monoculture and other co-planting treatments.The tartaric acid content of M.azedarach and B.papyrifera under KWX and GWX treatment was significantly increased by 55.6% and 86.7%(P < 0.05).The relative abundance of Proteobacteria and Chloroflexi in M.azedarach rhizosphere,Chloroflexi in P.vittata rhizosphere,and Chloroflexi and Acidobacteria in P.purpureum rhizosphere was increased under KWX treatment;The relative abundance of Proteobacteria and Acidobacteria in B.papyrifera rhizosphere and Actinobacteria and Gemmatimonadota in P.vittata and P.purpureum rhizosphere under GWX treatment.Correlation analysis showed that tartaric acid content in rhizosphere was positively correlated with the relative abundance of Proteobacteria,Chloroflexi and Acidobacteria,and the shoot and root biomass of P.purpureum and M.azedarach,and the shoot biomass of B.papyrifera were positively correlated with the relative abundance of Proteobacteria and Chloroflexi,Cd and Pb content in P.purpureum shoot and Pb content in B.papyrifera shoot were positively correlated with the relative abundance of Proteobacteria,Chloroflexi and Acidobacteria.Co-planting of three plants increased the relative abundance of bacterium associated with soil nutrient cycling and heavy metal tolerance,and improved the efficiency of phytoremediation.Thus,the co-planting of woody plants and herbs could promote plant growth and heavy metal accumulation,improve soil rhizosphere environmental quality and enhance the richness and diversity of rhizosphere microbial community.And co-planting three plants(M.azedarach-P.vittata-P.purpureum and B.papyrifera-P.vittata-P.purpureum)had better effect.