The Influence Of Underground Coal Mining On Soil Physicochemical Properties And Bacterial Community In Lingwu City

The underground coal mining destroys soil structure,causes surface subsidence and cracks,leads nutrient leakage and loss,and soil degradation.Ningdong mining area belongs to the arid area of northwest China with fragile ecological environment and soil desertification.The soil ecosystem is stressed by coal mining activities and natural environment.Therefore,it is urgent to study the influence of coal mining subsidence on soil quality in Ningdong mining area.The driving mechanism of soil bacterial community structure in arid area of northwest China is still unclear.The response mechanism of soil bacterial functional genes to coal mining is also unclear.The temporal and spatial succession of soil bacterial community structure in coal mining subsidence area is also rarely reported.Therefore,this thesis studied the influence mechanism of coal mining subsidence on soil quality in arid area of northwest China.Taking Lingwu City of Ningdong Mining Area as an example,this study collected the soil in the unexploited areas and coal mining subsidence areas for research.This study is expected to provide reference value and theoretical basis for remediation of coal mining subsidence soil in arid area of northwest China.The specific results are as follows:（1）The environmental factors driving soil bacterial community in Lingwu city of arid desert area in northwest China were investigated.The soil texture of Lingwu city in northwest arid desert area was mainly composed of sand（60-100%）and the content of clay was very low（0-4.12%）.The soil water content（SWC）varied from 3.58 to 9.17%,electrical conductivity（EC）varied from 21.45 to 55.60μs·cm^-1,the soil is slightly alkaline（p H varied from 7.9 to 8.1）,and soil nutrient contents were low.The nutrient（soil organic carbon（SOC）,total nitrogen（TN）,total phosphorus（TP）,dissolved organic carbon（DOC）,available nitrogen（AN）,available phosphorus（AP）,available potassium（AK））decreased with soil depth.The changes of microbial biomass carbon（MBC）,microbial biomass nitrogen（MBN）and soil enzyme activity(（β-glucosidase（BG）,β-N-acetylglucosaminidase（NAG）,alkaline phosphatase（PP）and catalase（CAT））with soil depth were consistent with the trend of nutrient change.The dominant genera in soil of Lingwu city were Sphingomonas,Gemmatimonas,Pseudomonas,Gp6,Gaiella,Gp4,Gp16 and WPS-1＿genera＿incertae＿sedis.These bacteria were mainly involved in soil C,N,P,K and other nutrient cycling,and promoted the formation of soil aggregates,which could enhance the resistance of soil desertification.Some bacteria had the function of degrading polycyclic aromatic hydrocarbons（PAHs）and phenolic compounds.The results of RDA（redundant analysis）showed that soil texture,soil EC,SWC and soil depth drived soil bacterial community structure in northwest arid desert area.（2）The coal mining activity significantly reduces the soil quality in marginal subsidence area of coal working face.Compared with the unexploited area,the leakage of available nutrients（DOC,AK,AN and AP）in marginal subsidence area of coal working face（ZP）was serious and MBC and the activities of BG and PP were significantly inhibited,which affected the turnover of available nutrients,leading to the decrease of soil quality.Additionally,the activities of CAT in ZP increased,indicating that coal mining activities causes more stress on soil in ZP.Analysis of bacterial community structure showed that,compared with the control area,the relative abundances of bacteria Sphingomonas involved in C and N cycles,Pseudomonas involved in C,N,K cycles and PAHs degradation and Arthrobacter involved in K cycle in ZP decreased,indicating that the coal mining activity reduced the relative abundance of bacteria involved in nutrient cycling in the subsidence area,which led to the decrease of soil nutrient turnover rate;meanwhile,the relative abundance of Gemmatimonas,Gaiella,Nitrospira,Solirubrobacte,Thermoleophilum and Aciditerrimona increased in ZP,suggesting that bacteria were functionally redundant and more likely to cooperate with each other;However,the coal mining activities did not affect the composition of soil bacterial community,indicating that soil bacterial community had certain resilience and resistance to environmental disturbance,and provided microbial resources for the restoration of soil quality.Based on the KEGG database,the soil bacterial functional genes were predicted by PICRUSt.At third levels,the nitrogen metabolism,metabolism of xenobiotics by cytochrome P450 and drug metabolism-cytochrome P450 involved in PAHs degragation were inhibited in ZP,while peroxisomes was promoted,which was further indicated that coal mining activity had a greater stress on the marginal subsidence area.Thus,the cracks in the subsidence area should be filled manually and restored in time,especially in marginal zone.（3）The soil quality in coal mining subsidence areas showed a trend of recovery with the increase of age.The temporal and spatial succession of soil nutrients,biological properties and soil bacterial community structure in coal mining subsidence area was studied by using the method of space insteads of time.The results showed that,during the subsidence period of 4-6 years,the contents of soil nutrients（SOC,AP and AK）,enzyme activities（BG,NAG and PP）,MBC and MBN showed a fluctuating downward trend and decreased to the lowest in 4-6 years,which caused the soil quality decreaed.During the subsidence period of 8-10 years,soil silt content increased,soil nutrients,enzyme activities and microbial biomass began to show an increasing trend due to natural remediation,but NH₄⁺-N and NO₃^--N continued to decreased,indicating that the soil quality did not recover to the control level after 10 years of subsidence.Analysis of bacterial community structure showed that,the subsidence of 0-6 years had a great disturbance to the bacterial community structure and the relative abundance of Gemmatimonas,Gp6,Gaiella and Gp4 invovled in nutrient cyclings decreased due to leakage and loss of soil available nutrients.Specially,the relative abundance of Gemmatimonas,Gp4 and Gp6 reached the lowest in 6 years;Gaiella was lowest in 4years and 6 years;the relative abundance of Aciditerrimonas and Sphingomonas invovled in degradation of refractory organic pollutants increased in 6 years,and Arthrobacter and Nocardioides were the highest in the 4 years of subsidence,suggesting that there may be organic pollutants in the mining area,which were used by microorganisms as C source to survive in the poor soil.At the third level by KEGG,the relative abundance of genes involved in N metabolism and PAHs degradation varied greatly in soils of 4-6 years after subsidence.The relative abundance of genes involved in PAHs degradation,benzoate degradation,aminobenzoate degradation,toluene degradation and fatty acid metabolism increased,and the relative abundance of genes involved in PAHs degradation-cytochrome P450 and cytochrome P450 metabolites decreased,indicating that the subsidence of 4-6years have the most disturbance effect on soil.The disturbance law of coal mining subsidence to soil is as follows:the soil quality decrease in 0-6 years,the disturbance effect decreases for 8-10 years and the soil quality begins to recover.Therefore,in the process of coal mining,engineering restoration should be carried out in time to reduce the influence of subsidence on soil quality and promote the rapid ecological recovery in the coal mining area.（4）Coal mining changed the microbial community structure under different plant canopies.The soil p H under the canopies of Ammopiptanthus mongolicus,Artemisia desertorum and Caragana korshinskii were lower than that at the edge of canopies,and SWCand AP under the canopies were deficient.While other soil nutrient content,enzyme activity and microbial biomass were obviously enriched under the canopies.Compared with the unmined area,the coal mining subsidence reduced the soil fertility quality of different plants.The soil bacterial community composition under different plant canopies was similar,but the relative abundance was different.Gp6,Sphingomonas,Gemmatimonas,Gp16,Gaiella and Nocardioides were dominant in the soils under different plant canopies.Compared with the control area,the relative abundance of Gemmatimonas increased under the canopies of the 3 plants in the subsidence area,while the relative abundance of Sphingomonas increased in the surface layer and decreased in the deep layer,suggesting coal mining activities changed the microenvironment under the canopies of three plants in the subsidence area,resulting in the change of soil bacterial community structure.The RDA showed that soil p H,EC and soil depth were the main factors determining soil bacterial community structure under different plant canopies in the northwest arid desert area.