| Mining and smelting activities are the primary sources of toxic metal pollution in China.The purpose of this study was to investigate the pollution risk and identify sources of metals in the arable soil of a Zn/Pb mining and smelting district located in Huize,in Southwest China.Topsoil?346?and profile?three?samples were collected and analyzed to determine the total concentrations of eight toxic elements?Cd,Hg,As,Pb,Cr,Cu,Zn and Ni?.The results showed that the mean Cd,Hg,As,Pb,Cr,Cu,Zn and Ni concentrations were 9.07,0.37,25.0,512,88.7,239,1761 and 90.3 mg·kg-1,respectively,all of which exceeded both the Huize and Yunnan soil background levels.Overall the topsoil was quite acidic,with a mean p H of 5.51.The mean geoaccumulation index?Igeo?revealed that the pollution level was in the order of Pb > Zn > Cd > Hg > As > Ni > Cu > Cr.The ecological risk index?Ei?indicated that there were serious contamination risks for Cd and Hg,high risk for Pb,moderate risk for As,and Cd and Hg were the dominant contributors to the high combined ecological risk index?Er?with a mean parameter of 699 meaning a serious ecological risk.The Nemerow pollution index?Pn?showed that 99.1% of soil samples were highly polluted or worse.Horizontally,high concentrations of Cd,Hg,As,Pb and Zn appeared in the north and middle of the study area,while Cr,Cu and Ni showed an opposite trend.Vertically,as the depth increased,Cd,Hg,As,Pb and Zn contents declined,but Cr,Cu and Ni exhibited an increasing trend.The mobilities of the metals were in the order of Zn > Cd > Hg > As > Pb.Horizontal and vertical distribution,coupled with correlation analysis,PCA and CA suggested that Cd,Hg,As,Pb and Zn mainly came from the anthropogenic sources,whereas Cr and Ni had a lithogenic origin.The source of Cu was a combination of the presence of parent materials as well as human activities.This study provides a base for the local government to control the toxic metal pollution and restore the soil environment system and an effective method to identify the sources of the studied pollutants.Soil from three regions,Smelting Zone?A?,Production Zone?B?,and Reservation Zone?C?,surrounding the typical lead-zinc mineralization area of Zhahai Town,Huize County,Yunnan Province,was selected as the research object,and the soil physicochemical properties including p H,soil electrical conductivity?EC?,total sulfur?TS?,total phosphorus?TP?,organic matter?OM?,total carbon?TC?,total nitrogen?TN?and heavy metals?Cd,Hg,As,Pb,Cr,Cu,Zn,Ni?contents in forest lands?FL?and agricultural lands?AL?were determined.Redundancy analysis?RDA?was used to study the relationship between soil physicochemical properties and heavy metals in soils under different land use patterns.The results showed that,the soil p H of the Zhehai lead-zinc smelting area influenced by the alkaline leachate after lead-zinc ore flotation generally exhibited alkaline,and the soil in this area was weakly acidic under natural conditions.The accumulation of litter in the forest land,and its stable soil aggregate structure and slow rate of microbial metabolism made the soil OM,TC,TN and C/N in the study area generally showed that their concentrations in the forest land was larger than those in the agricultural land.The artificial application of a large amount of phosphorus in agricultural land showed that the soil TP content was higher than that of forest land.The heavy metal pollution of forest land and agricultural land in Zone A,the smelting area,was the most serious.The contents of heavy metals in the main grain producing areas,Zone B,and protected reserve area,Zone C,were similar,which were significantly lower than those in the smelting areas,Zone A?P<0.05?.The decomposition of forest organic matter produces a large amount of organic acid,which makes the soil p H of the forest soil negatively correlated with OM,TC,TN and C/N ratio.The application of phosphorus and potassium fertilizer would cause the soil p H and TP,TS positively correlated with each other.Among the soil physical and chemical factors involved in this paper,the effect of p H on heavy metal content were the stronger factors in forest land and agricultural land,followed by OM,TC,TN and C/N ratio.The smelting waste gas,waste water and waste residue which contained much heavy metals all exhibited acidity and made the soil p H positively correlated with Cd,Hg,As,Pb,Cr,Cu and Zn in the study area.But EC was positively correlated with heavy metals in agricultural land and negatively correlated with heavy metals in forest soil except for Ni.And the Cd,Hg,As,Pb,Cr,Cu and Zn made the OM,TC,TN contents and C/N ratio to decline due to the toxicity and alkaline.As,Cr,Cu and Ni had a large percentage of the residual fraction?50.7-76.6%?and relatively small proportion of the mobile fraction?5.79–9.41%?,implying that these metals had limited bioavailability and biotoxicity,and great contribution from the lithogenic source;while Cd,Hg,Pb and Zn had a relatively low percentage of residual fraction?12.8–52.8%?and large proportion of mobile fraction?31.2–80.1%?,indicating the these metals had a strong contribution from anthropogenic source in soils and a high probability transferring from soil to crops and underground water,implying that they may have great potential bioavailability and biotoxicity,and may pose more potential risk of soil contamination than other metals.Therefore,anthropogenic Cd,Hg,Pb,and Zn in the studied soil were mostly bound in the non-residual fraction,and lithogenic and pedogenic Cr,Cu and Ni were mostly bound in the residual fraction.In addition,the higher proportions of Cd and Zn in the acid extractable fraction might lead to deeper downward migration than that of Hg and Pb.Proteobacteria,Actinobacteria,Chloroflexi and Acidobacteria were major dominant phyla?relative abundance was greater than 1%?.Gemmatimonadetes,Firmicutes,Bacteroidetes,Saccharibacteria and Nitrospirac also accounted for very large proportions of OTUs.Among them,the first five phylum dominate the soil bacterial communities,accounting for 88.3%-92.7% of all bacteria.PCo A results showed that the samples of the same land use types were clustered and FL soil was totally different from AL soil in bacterial structure.The predominant phyla of FL soil were Chloroflexi,Bacteroidetes,Acidobacteria and Proteobacteria.While Actinobacteria,Gemmatimonadetes,Firmicutes,Saccharibacteria and Nitrospirae dominated the microbial community of AL soil.LEf Se analysis showed that,at the phylum level,Acidobacteria and Gemmatimonadetes were the differential indicator species enriched in AL soil samples.While the differential indicator species enriched in FL soil samples with LDA value higher than 3 was Chloroflexi.High TP,OM,p H and C/N ratio were identified to have important influences in FL soil on bacterial communities,which could increase the abundance of Chloroflexi,Acidobacteria and Saccharibacteria.While in AL soil,OM,TN,p H and C/N ratio had a greater impact on soil bacteria,and they were positively correlated with Proteobacteria,Gemmatimonadetes,Firmicutes,Chloroflexi,Bacteroidetes and Nitrospirac.In FL soil samples,heavy metals including Ni,Cu,As,Cd,and Cr were identified to have important influences on bacterial communities,which could decrease the abundance of Chloroflexi,Saccharibacteria,Firmicutes,Gemmatimonadetes and Nitrospirac.The heavy metals of AL including Ni,Hg,Zn and Pb were identified to have important influences on bacterial communities which could decrease the abundance of Proteobacteria,Bacteroidetes,Chloroflexi,Firmicutes,Gemmatimonadetes and Nitrospirac. |
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