Response And Its Mechanism Of PGPB In The Ecological Restoration Of Rare Earth Tailings In South China

The exploitation of rare earth resources has seriously damaged the local ecological environment,resulting in serious problems such as sandy soil,poor ability of soil water retention,nutrient loss,soil acidification and heavy metal pollution.Vegetation restoration and reconstruction have always been the focus and difficulty of environmental treatment of ion adsorption rare earth tailings in southern China.Soil microorganisms are essential to plant growth and development,among which plant growth promoting-bacteria(PGPB)can promote plant growth and improve plant stress resistance in stressed habitats,thus playing an important role in vegetation restoration in mining areas.However,the response and mechanism of PGPB,an important soil microorganism,in the vegetation restoration process of rare earth tailings is still unclear.This selected a plot of rare earth tailings from Southern Jiangxi,which was restored for eight years by planting Pinus elliottii-Pleioblastus amarus keng-Miscanthus floridulus,using field vegetation survey,soil physicochemical analysis,16S amplicons high-throughput sequencing,quantitative PCR absolute quantitative and metagenomic sequencing and box-sorting techniques,to explore the aboveground vegetation,soil physicochemical properties,extracellular enzyme activities,bacterial community structure as well as the abundance and composition of PGPB community after one year and eight years of vegetation restoration,respectively.The aim is to clarify the synergistic effect among plant,soil and microorganism in the process of tailings vegetation restoration,and to provide theoretical support and a scientific basis for the further use of PGPB functional flora to accelerate the vegetation restoration of ion-type rare earth tailings in southern China.The conclusions of this study are as follows:(1)Vegetation restoration effectively improved the soil physicochemical properties of rare earth tailings.Total soil organic carbon(from 2.41 g/kg to 5.31 g/kg),microbial biomass carbon(from 3.63 g/kg to 3.99 g/kg to 51.84 g/kg to 75.01 g/kg),soil water content(increased by 10%-32%)and p H(increased from 4.52 to 4.71)were significantly improved after eight years of vegetation restoration.Total nitrogen(from 1.32 g/kg-1.42 g/kg to 0.52 g/kg-0.56 g/kg)and soil bulk density(from 1.31g/cm~3-1.45 g/cm~3 to 0.99 g/cm~3-1.01 g/cm~3)were significantly decreased.The extracellular enzyme activities related to soil carbon,nitrogen and phosphorus cycling were also significantly enhanced.In total,vegetation restoration measures significantly improved soil physicochemical characteristics,enhanced soil nutrient levels,and effectively alleviated the problems of soil acidification and poor soil water retention capacity of rare earth tailings.(2)Vegetation restoration significantly increased the alpha-diversity of bacterial community only at some specific sampling time.In addition,vegetation restoration significantly changed the structure of soil bacterial community,and the dominant categories in the soil after vegetation restoration were as follows:Proteobacteria(33.4%),Actinobacteriota(20.0%),Acidobacteriota(18.3%),Chloroflexi(15.9%);The dominant bacteria in tailings soil without vegetation restoration are:Chloroflexi(36.2%),Proteobacteria(22.0%),Actinobacteriota(14.0%),Acidobacteriota(8.0%).Based on indicator species analysis,we found that Atinobacterita,Verrucomicrobiota,Proteobacteria,Acidobacteriota became the indicator species of vegetation restoration soil.Chloroflexi,WPS-2 and Nitrospirota were the indicator species of unrepaired rare earth tailings soils.(3)Vegetation restoration significantly improved the stability of the soil microbial community network.The stability of the bacterial network gradually improved from one to eight years after restoration,with negative/positive cohesion increasing from0.193 to 0.456,and the modular coefficient increasing from 0.667 to 0.895.Without vegetation restoration,the modularity coefficient of the tailings bacterial network was only 0.215,which was more similar to the random network.(4)To further explore the response law of PGPB flora abundance and species composition in the process of vegetation restoration.The PGPB database was constructed with ACC deaminase activity as an important identification basis based on the bibliometric analysis.Comparing the high throughput sequencing data of the16S r RNA gene with the self-built PGPB database,we found that the abundance of PGPB decreased gradually with the process of vegetation restoration,and the PGPB mainly belonged to Proteobacteria(40.6%)and actinomycetes(15.8%),and a few belonged to Campylobacter(2.1%).The functional gene acdS encoding ACC deaminase was analyzed by q PCR.The variation rule of functional gene acdS in the process of vegetation restoration was consistent with the results of database comparison(R~2=0.64,P≤0.001),that is,the abundance of acdS gradually decreased with the process of vegetation restoration.In addition,linear fitting analysis showed that the abundance of functional genes acdS was negatively correlated with TOC(P≤0.01).(5)Macrofactor component box technology was used to analyze medium-quality MAGs(integrity>50%,pollution<20%)and high-quality MAGs(integrity>90%,pollution<5%)respectively,to further study the mechanism of PGPB in the process of vegetation restoration of rare earth tailings.Nineteen of 142 medium-quality MAGs were found to have ACC deaminase production and several other plant growth-promoting properties(including IAA production,siderophore production,nitrogen fixation,and phosphorus solubilization).Among them,bin 91 and Bin 311 findings have not been reported and may be potential new PGPB.Among the 12 high-quality MAGs,the interpretation of the complete genome of bin 341,which can synthesize ACC deaminase,found that its genome contains many functional genes related to the resistance to heavy metal toxicity,the cycle of carbon,nitrogen and phosphorus and the synthesis of cytosine functional genes promoting plant growth.In addition,GC content mutations in DNA sections were found in the genome,suggesting that horizontal gene transfer may exist in functional genes in the genome.It is speculated that ACC deaminase producing PGPB may enable more soil microorganisms to acquire the ability to resist environmental stress and promote plant growth through gene horizontal transfer.