| The contamination of soil ecosystem by heavy metals is considered as global environmental issue.Huaibei Coalfield is one of the biggest coal mines in Anhui province,China,the exploitation of coal resources promoted the development of the economy and society of Huaibei City,but it resulted in a large-scale coal mining area being subsided or to be subsided,and damaged the local ecological environment.In addition,heavy metals emission may increase during coal exploitation,storage and utilization,the disposal of coal gangue and fly ash.Previous study only described arsenic,copper,and zinc contamination in soil and wheat during coal mining and assessed health risks for the inhabitants of Huaibei Coalfield.However,up until now,the heavy metal pollution and the effect of heavy metals on soil enzyme activities,microbial community structures in Linhuan subsidence of Huaibei Coalfield have not been reported.Thus,in the present study,we investigate Linhuan subsidence area of Huaibei Coalfield for the first time.Total of 42 representative soil samples were collected from the Linhuan subsidence of Huaibei Coalfield,Anhui province,China.This study aims to(1)determine the chemical forms of heavy metals(Cd,Cr,Cu,Ni,Pb and Zn)in Linhuan subsidence area of Huaibei Coalfield by applying the sequential extraction procedure;(2)assess the potential ecological risk of heavy metals contamination in the soils;(3)evaluate the impact of heavy metals contamination in agricultural soils on the enzymatic activities,soil basal respiration and microbial community structure.The main results showed as follows:(1)The mean concentrations of heavy metals in soil samples decreased as follow:Zn>Cr>Ni>Pb>Cu>Cd.The mean concentrations of these heavy metals were higher than background values of Huaibei City surface soil by a factor of 1.16 to 3.15(Cd:3.15;Cr:1.19;Cu:1.16;Ni:1.22;Zn:1.84)except Pb(0.89).In addition,the mean concentrations of these metals were higher than their background values of China soil by a factor of 1.13-3.51(Cd:3.51;Cr:1.38;Cu:1.13;Ni:1.69;Pb:1.16;Zn:2.09),and the contents of Cd(1.30)and Zn(1.42)were higher than Natural Environmental Quality Standard values for the soils in China,suggesting that heavy metal concentrations in the studied soils are of the danger levels.(2)Significant positive correlation was found among heavy metals(p<0.01)in surface soils.The strong correlation among these heavy metals indicates their common origin,especially from carbonate and sulfide minerals that are associated with coal seams.However,the concentrations of Cr and Ni showed positive relationship with each other but very weak correlations with Cd,Cu,Pb and Zn.This indicates that Cr and Ni may have different geochemical behaviours and sources from other metals.Four principal components were considered,which account for 69.61%of the total variance.The first principal component PC1 was dominated by Cd,Cu,pH,WC,OM,TN and TC accounting for 28.28%of the total variance.A high loading of pH(0.626),WC(0.692),OM(0.756),TN(0.585)and TC(0.858)on PC1 may strongly relate this component to the soil physicochemical properties.Cr(0.829)and Ni(0.892)were classified into PC3 with a variance loading of 13.25%,Cr and Ni in PC3 are strongly correlated and clearly separated from the other heavy metals,this separation may suggest that Cr and Ni mainly came from non-anthropogenic sources.(3)The most of the total Cr,Cu,Ni,Pb and Zn was mainly present in the residual(above 70%),while the predominant fraction of Cd was the exchangeable fraction accounting for 41.92%.The mobility sequence based on the non-residual fraction decreased in the order of Cd(87%)>Pb(23.55%)>Cu(20.24%)>Zn(10.3%)>Ni(8.61%)>Cr(7.71%).The exchangeable and carbonate fractions of Cd in the soils were very high.Cd needs to be payed more attention by the authority.(4)The assessment of heavy metal ecological risk results indicated that in the mining areas the ecological risk index for all factors(RI)was between low risk and moderate risk.The mean Eri of Cr,Cu,Ni,Pb and Zn values were less than 40,which is classified as low ecological risk.However,the Erv values of Cd in the studied area were 55.56-188.89,with a mean of 94.11,lead to moderate risk to high risk,suggesting that Cd would bring about significant potential ecological risk in the studied area.The ecological risk Cd was far greater than other heavy metals.The results of RAC indicated that ecological risk of Cd was highest,with all samples present high or very high risk,whereas the other heavy metals pose no or low ecological risk.The RAC results indicate that high proportions of Cd can transfer from agricultural soils to crops,which was in accordance with the assessment of heavy metal ecological risk results by Eri values.(5)In Linhuan subsidence area of Huaibei Coalfield,the correlations between soil physical and chemical properties,total amount of heavy metals,different chemical forms and soil basal respiration,microbial biomass C,N,soil metabolic quotient,ratio of microbial biomass C,N were not significant.Wheras the correlations between soil physical and chemical properties and soil enzyme activity were significant.Therein,soil enzyme activities were especially influnced by pH,which had an activation effect on dehydrogenase,urease and catalase and certain inhibitory effect on the acid phosphatase,but the inhibition effect is not significant.(6)The analysis between microbial ecological characteristics and environmental factors by CCA showed that soil microbiol properties,soil enzyme activity were influenced by exchangeable and carbonatefractions of Cd,exchangeable fraction of Cr,carbonatefraction of Ni,total Zn and pH.(7)Results from our experiment showed that the effect of elevated heavy metal concentration on soil enzyme activities was different.Compared with the control,Enzyme activity decreased with increasing concentrations of heavy metals added into the samples.Under the same heavy metal concentration,the law of soil basal respiration was Hg + Cd>Hg>Cd,which was probably due to synergistic interaction in the metal combinations.(8)Compared with the control,soil basal respiration was higher than the control.The soil basal respiration increased with the increasing of heavy metal concentrations at the first two weeks and then showed a large decrease at later two weeks,but was still higher than the controls.Under the same heavy metal concentration,the law of soil basal respiration was Hg + Cd>Hg>Cd,which was probably due to synergistic interaction in the metal combinations.The behaviors of soil basal respiration under single and combined treatments showed significant difference with increasing culture time,and the highest basal respiration were found in all treatments at the second week and the values reached 3.6,3.9,4.6 mgkg-1h-1 in Cd30,Hg30,CdHg30 respectively.(9)The NMDS ordinations of bacterial communities showed clear differences between the highest Hg concentrations(treatments of Hg30 and CdHg30)and the other treatments,but no distinct difference was observed in Cd treatments.Hg30 and CdHg30 treatments were strongly separated along PC1,whereas treatments with different levels of Cd were not separated to any obvious degree along this component.The bacterial community structures were significantly affected by Hg,combined Cd and Hg but not by single Cd.In contrast,the NMDS ordinations of fungal communities showed no obvious separation between all the treatments compared with the control.The fungal communities were less sensitive to Cd and Hg in all treatments.Illumina sequencing showed that bacterial communities were dominated by Proteobacteria,which accounted for 26.5-54.4%of total bacterial sequences.Other abundant phyla found in the different treatments included Firmicutes(4.1-25%),Actinobacteria(4.8-8.7%),Chloroflexi(3.1-8.2%),Bacteroidetes(1.9-14.1%),Gemmatimonadetes(1.9-5.2%),Candidate_division_TM7(1.0-5.6%)and Nitrospirae(1.7-4.9%).Compared to controls and low concentration of Cd and Hg treated samples,Proteobacteria,Firmicutes and Bacteroidetes became more abundant(p<0.05)upon exposure to high concentrations of Cd and Hg,while Acidobacteria,Chloroflexi,Gemmatimonadetes and Candidate_division_TM7 were negatively influenced by increasing Cd and Hg levels.Fungal communities were dominated by Ascomycota,which accounted for 43.7-99.3%of total fungal sequences.Other abundant phyla found in the different treatments included Basidiomycota,Blastocladiomycota,Chytridiomycota,Glomeromycota and Zygomycota.Basidiomycota and Blastocladiomycota became more abundant in treatments of Cd0.3 and CdHg3. |
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