Remote Sensing Monitoring Of Damaged Soil And Ecological Remediation In Ion-adsorption Type Rare Earths Ore In South Jiangxi Of China

Ionic rare earth ore is an important strategic resource in China.For a long time,environmental problems such as vegetation destruction,soil erosion,soil desertification and water pollution have occurred in the rare earth mining area due to the low mining technology and indiscriminate mining,which have caused wide attention from all walks of life,and the ecological restoration of the mining area is imminent.However,due to the long history of rare-earth mining and most of them are located in remote mountainous areas,the remaining rare-earth mining areas in southern Jiangxi Province have been plagued by problems such as unclear reserves and unknown distribution.At the same time,there are still few studies on the characteristics of soil damage and soil remediation obstacle factors in mining areas,only some surveys are not comprehensive enough,the results obtained are different,and the existing mining area remediation also has poor results.Based on the above reasons,this paper used Landsat as the data source and BP neural network model to analyze the spatial and temporal distribution of mining in southern Jiangxi in the past 30 years,and comprehensively analyzed the obstacle factors of mining soil remediation from the aspects of soil physical,chemical and heavy metal pollution.On these grounds,field experiments were carried out in typical rare earth mining areas to explore the action mechanism of the combined remediation of soil by clover,biological carbon and fly ash,and the response mechanism of navel orange growth and fruit quality to the soil remediation effect,so as to provide scientific basis and practical reference for the soil remediation in rare earth mining areas.The main research contents and results of this paper are as follows:(1)Taking Landsat as the data source,BP neural network classification method was used to establish classification rules based on spectral information,elevation information,vegetation index,bare soil index and other characteristics of the image,then spatial and temporal changes of ionized rare earth mining areas in southern Jiangxi in the past 30 years were extracted.The results indicate that:From 1987 to 2017,the rare earth mining area shows a trend of"small amount of mining--rapid expansion--gradual reduction".In 1997,the damaged soil area is 7308.36 ha,showing a trend of obvious expansion.In 2007,the damaged soil area reached 17100.18ha,which reached the peak.In 2017,the the damaged soil area of the land is 6471.99 ha,and the damaged area of the mining area decreased,mainly due to the further strengthening of state control and soil remediation.From the perspective of spatial distribution,the the damaged soil area of rare earth mining area in southern Jiangxi is mainly concentrated in 8 counties(districts)of Xunwu,Dingnan,Anyuan,Xinfeng,Ningdu,Ganxian and Longnan.In 1987,1997,2007 and 2017,the total area of rare earth mining in 8 counties(districts)is 16.74 ha,68.80 ha,164.67 ha and 67.37ha,respectively,accounting for 89.74%,94.13%,96.30%and 94.82%of the mining area in the whole city.(2)Six soil surface samples of typical rare earth mining areas were selected to systematically analyze the characteristics of soil damage from the aspects of soil physics,chemistry,nutrient content and heavy metal pollution.Results show that rare earth mining process,due to the repeated use of oxalic acid,ammonium sulphate,such as leaching of soil,total destruction of the mining area of soil physical structure and soil clay content is low,and the soil"desertification"is serious,per unit volume of soil solid phase components increased,Soil porosity decreases,soil water content and water holding capacity decrease,which indirectly affects soil chemical properties,reduce the soil p H,EC,CEC,soil organic matter,alkali solution nitrogen,effective phosphorus and available potassium content almost disappear that maketh all,The single index of soil in mining area is at the level of deficient-extremely deficient.Compared with the control,the soil indexes in different mining areas are all lower than the surrounding normal soil,and the soil damage degree of heap leaching mining area is greater than that of in-situ leaching mining area.Soil heavy metal analysis show that:The content of heavy metals in all soils is far lower than the second-level standard of soil environmental quality,and the total amount of rare earth is also lower than the surrounding normal soil.the soil P_Nem in mining areas with different mining processes is not different and far less than 0.7,which means the pollution level of heavy metals is clean.Therefore,soil desertification,water leakage,fertilizer leakage and nutrient deficiency are the main obstacles to soil restoration in mining areas.(3)According to the characteristic factors of soil damage,combined with the results of soil culture and pot experiment,biochar and ash were used as the modifier,and clover was used as the vegetation restoration.A field soil restoration experiment was conducted on an ionic abandoned rare earth mine in Xinfeng County,Ganzhou City.The results show that:At the same level of organic fertilizer,adding biochar and fly ash to repair damaged soil in mining areas can significantly improve soil clay content,soil porosity and soil water holding property,and soil organic matter,total nitrogen,alkali-hydrolyzed nitrogen and available phosphorus were also significantly increased.Soil microorganisms increased significantly,and in 2019-2020 the biomass carbon content increased by 114.79-295.49mg·kg^-1 and 120.27-310.21 mg·kg^-1,respectively,compared with the control group.Soil microbial biomass nitrogen content increased by 1.08-36.33mg·kg^-1 and 5.03-38.05mg·kg^-1.The above results show that the combined remediation of soil with bioash and fly ash can effectively improve the ability of soil to retain water and fertilizer,and promote the improvement of soil environment.(4)The quality of fruit trees with different levels of biochar and fly ash was tested to further explore the response mechanism of navel orange growth and fruit quality to soil remediation.The results show that the navel orange trunk height is 0-10.8cm and3.07-25.13cm higher than the control from 2019 to 2020.The thickness of navel orange trunk is 2.83-8.5mm and 4.05-18.45mm,respectively.The crown amplitude of navel orange is larger than 0.04-0.33m² and 0.19-0.80m² respectively.The leaf area index is0.05-0.34 and 0.23-1.12 higher than that of the control group.The content of chlorophyll is1.4-16.89 and 6.30-21.83 higher than that of the control group.The relative length of navel orange is 2.03-12.57cm and 2.1-22.42cm respectively.Navel orange slightly thicker than the control 0.16-0.76 mm,0.00-1.33 mm.Navel orange single fruit weight than the control4.96-83.82g;Navel orange juice weight than control weight 1.4-7.67g;The juice yield of navel orange increased by 3.83-15.87%.The longitudinal diameter of the fruit is 0.56-6.18mm longer than the control.The fruit is 0.25-5.98 mm longer than the control.The fruit shape index of each group is close to 1,and the whole fruit is round and smooth.The soluble solids increased by 0.84-24.28.Titratable acidity decreased by 0.02-0.19.The ratio of solid to acid increased by 0.33-8.79.On the whole,Applying 50 g·kg^-1 biochar and fly ash respectively has the best comprehensive effect,which is conducive to promoting navel orange growth and improving navel orange fruit quality.In summary,the ionized rare earth mining areas in southern of Jiangxi Pronvice are widely distributed and have a large distribution area,with deep damage to soil and ecology.The obstacle factors of soil remediation in mining area are high soil sand content,poor water holding and nutrient retention and poor fertility.Soil remediation is the precondition and key of ecological environment restoration in mining areas,so it is necessary to introduce artificial intervention to improve soil and promote soil reconstruction in mining areas.This paper based on the results of soil culture and pot experiment in the early stage,when under the condition of the same level of organic fertilizer,the combined restoration of clover,biochar and fly ash significantly improved the physical and chemical properties of the soil in ionized rare earth mining areas,and improved the soil microbial diversity,which promoted the growth of navel orange,and improved the quality of navel orange fruit.The key integrated technology of soil remediation in ionized rare earth mining area in southern Jiangxi Province has been formed,and good ecological and economic benefits have been obtained,which provides scientific basis and practical reference for ecological remediation in rare earth mining area.