Study On The Bacterial Diversity In The Restoration Area Of Rare Earth Mine Tailings Of Gannan

The rare earth elements?REEs?are essential raw materials in modern technology.However,surface/mountaintop mining and accumulation of mine tailings have caused a series of negative impacts on the surrounding environment,such as heavy metal pollution,soil acidification,water and soil erosion,etc.It results in the soil nutrient deficiency,low organic carbon and pH,poor soil structure,the vegetation is difficult to recover naturally.Thus,REE mine tailings disposal and mine land restoration are especially important.Compared with physical and chemical methods of restoration,bioremediation,especially phytoremediation due to the advantages of lower cost and environmental-friendly,was widely used in the remediation of mine tailings.Nevertheless,evaluating the effect of remediation is usually limited to vegetation norms such as plant coverage and diversity.Soil microbial community is also an intrinsic factor in deciding biogeochemical cycle and organic matter transformation in soils,and therefore examining the changes of soil microbial community composition and diversity are very important in evaluating soil ecological processes during phytoremediation.In this paper,soil samples were collected from Wuxi area,Anxi County,Xunwu County and the remediation sites of rare earth tailings,these areas have similar climatic characteristics.The composition patterns and diversities of soil microbial communities in subtropical areas of Southeast China were analyzed.The effects of bioremediation on bacterial community diversity and heavy metal contaminants were analyzed.Differences of microbial community composition and diversity during different restoration processes were compared.The main results were shown below.?1?The dominant phyla were Proteobacteria,Acidobacteria,Firmicutes and Bacteroidetes in Wuxi area,accounting for 32.23%-38.45%,15.4%-28.78%,10.92%-19.03%and 5.75%-10.98%of the total effective sequences in each soil sample,respectively.The subgroups of Acidobacteria showed different distribution between evergreen tree soils and deciduous tree soils.The subgroups of Gp1,Gp2 and Gp3 were abundant in evergreen tree soils,while Gp3,Gp4,Gp5 and Gp6 were abundant in deciduous tree soils.The bacterial community composition and diversity were mainly affected by both soil pH and tree species.Soil pH was the key factor influencing bacterial diversity,with lower pH associated with less diverse communities.Meanwhile,the contents of NO₃^-were higher in evergreen tree soils than those in deciduous tree soils,while less NH₄⁺than those associated with deciduous trees,leading to a lower pH in evergreen tree soils and indirectly influencing the diversity and composition of the bacteria.Shannon indices of bacterial communities were 5.29-6.88 in different soil samples of Wuxi area.?2?The abundant phyla were Proteobacteria,Acidobacteria,and Actinobacteria in Anxi area,accounting for 25.21%-40.16%,25.88%-44.60%,6.29%-11.76%of the good-quality sequences in each group of soil samples,respectively.The relative abundance of Acidobacteria in the soils of tea garden was more than 40%,which was significantly higher than that in other soils?P<0.05?.The bacterial community diversity showed significantly positive correlations with the contents of soil total nitrogen and available phosphorus?P<0.05?,while markedly negatively correlated with soil pH?P<0.01?.Four environmental variations?silt,clay,sand and AP?showed significant effects using an interactive forward selection procedure?P<0.05?,with contributions 42.8%,42.5%,32.4%and 30.1%,respectively.The results indicated not only soil nutrients,but also soil texture could affect microbial community composition.Shannon indices of bacterial communities were 6.84-9.25 in Anxi area.?3?Soil nutrient contents?SOC,AP and TN?in the rehabilitation site by Eucalyptus for 5years,were significantly lower than that of in the navel orange orchard soils?P<0.05?.However,compared with woodland soil samples,the contents of SOC and TN,and soil pH in the restoration area showed no significant difference.The results suggested that the fertility of reclaimed land was increased.The bacterial community diversity was positively correlated with soil pH and the content of SOC?P<0.05?.The dominant phyla in Xunwu County included Proteobacteria,Acidobacteria,Actinobacteria,Firmicutes and Chloroflexi,accounting for27.34%-34.54%,28.03%-30.87%,6.23%-11.09%,6.69%-12.87%and 5.42%-9.55%of the effective sequences in each group of soil samples,respectively.Shannon indices of bacterial communities in different soil samples were 5.86-6.17.?4?Considering the microbial community structures and dominant phyla of healthy soils in different regions easily be influenced by local soil physicochemical characteristics.Shannon index is widely used to evaluate species diversity in ecology and therefore Shannon index was selected as an index to analyze soil remediation of rare earth tailings.In the above chapters,Shannon indices of bacterial communities were 5.29-9.25 in healthy soil samples.?5?After restoration of rare earth tailings by plant and microorganism,compared with the tailings,the contents of five elements?Pb,Cd,Hg,La and As?in the remediation area decreased significantly?P<0.05?.Among them,the contents of Hg and As decreased most significantly.The content of Hg decreased 65.45%and 80.00%in MQ and MC remediation areas,respectively,while the content of As decreased 82.51%and 86.65%in MQ and MC,respectively.The number of OTUs,Chao1 and Shannon indices of microbial communities increased significantly from tailings to restored areas?P<0.05?.The number of OTUs?MQ:1343;MC:1345?and Shannon indices?MQ:5.04;MC:5.13?of the remediation area are close to those of bacterial OTUs?1435?and Shannon index?5.29?of rhizosphere soil of Osmanthus fragrans in Wuxi area,which indicated that bioremediation effectively improved the ecological environment in the tailings.Soil physicochemical characteristics were the key factors of influencing the microbial communities,namely available phosphorus?AP?,total phosphorus?TP?,soil organic carbon?SOC?,total nitrogen?TN?and soil pH,which affected the soil bacterial community structures,with contributions decreasing from 83.70%to 45.50%.Shannon index showed significantly positive correlations with the contents of SOC,TN and AP?P<0.001?,however,there was a significant negative relationship between the Shannon index and soil pH?P<0.01?.The activities of?-glucosidase,phosphatase and urease in the reclaimed soils were significantly higher than those of in the tailings and the control samples?P<0.05?.Proteobacteria,Acidobacteria and Firmicutes were the dominant phyla,accounting for51.91%-83.72%,1.11%-23.73%,1.83%-12.37%of the good-quality sequences in each group of soil samples,respectively.Relative abundances of Proteobacteria and Firmicutes in the recovered sites were significantly lower than those of in the tailings?P<0.05?,while the abundances of Acidobacteria,Verrucomicrobia,Planctomycetes and Actinobacteria?except MC?in the restoration area showed significantly higher than those of in the tailings?P<0.05?.?6?Between the two restored lands after different restoration periods,the microbial diversity indices and OTU number in the restoration area by Eucalyptus after 5 years were significantly higher than those in the restored site after 2 years.In the 2-year-old restoration area,Proteobacteria was more abundant and the abundances were more than 50%in two group soil samples.While the relative abundance of Proteobacteria in the 5 year old restored land decreased,and other microbial phyla significantly proliferated.The microbial community further changed.Bacterial`community composition was significantly negatively correlated with the contents of AP and SOC,as well as soil pH?P<0.01?,the contributions of three variables were 45.4%,41.1%and 47.6%,respectively.Shannon index of the bacterial community showed markedly negative correlations with the contents of soil SOC and AP?P<0.01?.Among different stages of restoration,the more microbial groups with a longer time.The results showed that the tailings was further restored by longer time restoration.