Remediation Characteristics Of Pennisetum Giganteum On Wasteland Of Indigenous Zinc Smelting

The development and utilization of metal mines will produce a large number of wasteland,causing serious environmental pollution.The treatment of heavy metal pollution in wasteland is a major environmental problem facing the world.Northwest Guizhou is a famous zinc smelting distribution center.Although the zinc smelting has been curbed,the large-scale wasteland left behind,such as the zinc smelting waste residue in Bijie area,covers an area of 960 hm²,of which Hezhang County covers an area of 637 hm²,and the smelting waste residue is about 20 million tons.The contents of Pb,Zn,Cu,As and Cd in smelting waste residue,soil and river sediments of wasteland are far more than the background values,and the ecological environment has been seriously damaged.After the destruction of vegetation,the soil erosion is serious,which leads to the migration and diffusion of heavy metals to form a wider range of pollution.Therefore,the treatment of heavy metal pollution and vegetation reconstruction in the wasteland of indigenous zinc smelting has become an important issue in Northwest Guizhou.Phytoremediation technology is widely used in the field of heavy metal pollution remediation because of its low cost,green environmental protection and less soil disturbance.The restoration of indigenous zinc smelting wasteland will be an important way to increase agricultural and forestry land and solve the contradiction between more people and less land.Phytoremediation has become the first choice for the restoration of indigenous zinc smelting wasteland because of its advantages.Pennisetum giganteum was introduced according to the limiting factor of plant growth in the soil zinc smelting waste land.It was found that its growth rate,tillering ability,biomass and tolerance to heavy metals were significantly stronger than other repair plants,which was suitable for phytoremediation in the soil zinc smelting waste land.In this study,Pennisetum giganteum was selected as a remediation plant,and experiments were carried out in two substrates of contaminated soil and waste residue in the waste land of zinc smelting in northwest Guizhou.The growth status,physiological response,heavy metal migration and transformation characteristics and rhizosphere microbial characteristics of Pennisetum giganteum in two substrates of waste land were studied.The tolerance mechanism and influence mechanism of Pennisetum giganteum to heavy metals in the habitat of zinc smelting waste land were discussed,which provided a theoretical basis for the utilization of Pennisetum giganteum in mine ecological environment restoration.Field experiments were carried out in the wasteland of indigenous zinc smelting in northwest Guizhou and the nursery experiments were carried out by sampling the contaminated soil and waste residue of the wasteland.The results are as follow:1.The priority control of heavy metal elements in the abandoned land of zinc smelting in Magu Town,Hezhang County,Northwest Guizhou is Pb,As and Cd.The main limiting factors of plant growth in abandoned land are as follows:（1）Extreme p H value,the contaminated soil is strongly acidic（p H=4.41）,which is easy to aggravate the dissolution of toxic substances,and the waste residue is strongly alkaline（p H=8.75）,which is easy to cause the deficiency of plant nutrients and the instability of enzymes.（2）The abandoned land is extremely barren,and the main nutrients such as organic matter,N and P are scarce.The total carbon content is 10.31 g·kg^-1,and the imbalance of nutrients is not conducive to plant growth.（3）The content of Pb,As and Cd in contaminated soil reached 1612.56 mg·kg^-1,581.78 mg·kg^-1 and 187.31 mg·kg^-1,respectively.The content of heavy metals in waste residue was higher,which would seriously affect plant metabolism and inhibit plant absorption of nutrients.（4）The physical properties of the waste residue are coarse bone,poor water and fertilizer holding capacity,easy to produce high temperature on the surface,and high salt content can easily lead to plant physiological drought.（5）The temperature of abandoned land is low in winter,which is easy to cause plant freezing damage and is not conducive to plant growth.2.The contents of Pb,As and Cd in contaminated soil and waste residue in the test area far exceed the soil background value,and there is saline-alkali stress,so it is difficult for plants to grow.Pennisetum giganteum has certain tolerance to heavy metals and low requirements for soil,showing adaptability to the habitat of zinc smelting wasteland.Giant Juncao has fast growth rate,large biomass and high coverage,which can effectively prevent soil erosion and reduce the migration and diffusion of heavy metals.The matrix nutrient in the study area is insufficient,and it is necessary to increase the nitrogen fertilizer to strengthen the repair ability of Pennisetum giganteum In addition,due to the low temperature in winter in this area,Pennisetum giganteum will face the problem of wintering.Some heat preservation measures must be taken for the preservation of the stems.The last stubble should be mowed before wintering,and the stubble height is 1～2 cm.Appropriate fertilization and covering soil or covering with hay and plastic film overwinter.3.In the two substrates of contaminated soil and waste residue in the waste land of indigenous zinc smelting,the plant stability effect of Pennisetum giganteum on heavy metals Pb,As and Cd is due to the plant stability effect of Buddleja officinalis and Artemisia argyi,and has a certain enrichment and transport capacity,which can transport some heavy metals to the ground part.Pb and As were mainly enriched in the roots of Pennisetum giganteum,and Cd was higher in the stems and leaves of Pennisetum giganteum.The contents of Pb,As and Cd in Pennisetum giganteum exceeded the“Feed Hygienic Standard”（GB13078-2017）,which was not suitable for feeding forage and could be used as biomass energy.Pennisetum giganteum has strong tolerance to heavy metals in the waste land of zinc smelting in northwest Guizhou.With the increase of planting years and cutting times,the heavy metals in the soil are gradually taken away.It can produce biomass energy while repairing the contaminated soil.It has good environmental and economic benefits and is a good in-situ remediation plant.4.Compared with the background soil,the microbial diversity of the rhizosphere soil in the two substrates of the soil zinc smelting wasteland decreased,and the relative abundance of dominant microorganisms was not significantly different.In all soil samples,the relative abundance of rhizosphere soil microorganisms was higher than that of non-rhizosphere soil.The microbial composition of rhizosphere soil and non-rhizosphere soil of Pennisetum giganteum in contaminated soil and waste residue was quite different,and the microbial composition of rhizosphere soil was more complex.Through high-throughput sequencing,it was found that P3OB＿42,MND1 and Dokdonella in the rhizosphere soil of Pennisetum giganteum had a greater contribution to the difference between groups（greater than 0.02）,and they were also the dominant bacteria in contaminated soil and waste residue,and all of them were under the Proteobacteria.Therefore,Proteobacteria is a special bacteria in the rhizosphere soil of Pennisetum giganteum,which can promote the heavy metal tolerance,enrichment and transport ability of Pennisetum giganteum.