| As a typical karst region,the soil has high-geological background levels of many heavy metals derived from carbonate rock weathering in southwestern China.In addition to natural sources,human activities such as mining and smelting also contribute to soil heavy metal accumulation in the karst region.Previous studies have mainly focused on the sources and ecological risks of soil heavy metals at regional scales,but few have examined how landscape changes induced by mining activities superimposed on high-geological backgrounds affect soil heavy metal accumulation.Maochang ore district is a significant concentration of buried bauxite deposits in Guizhou province,characterized by frequent mining activities and diverse extraction techniques.In addition to bauxite,the area also boasts coal,iron ore,basalt and other mineral resources.The aluminiferous rock series found in Maochang’s deposit concentration zone often contains abundant trace rare earth elements,and concomitant hematite,pyrite and other minerals.This study aims to fill this gap by taking the Maochang ore district as a case study.We investigated the soil heavy metal contents and applied various methods such as correlation analysis,redundancy analysis,geoaccumulation index,variance decomposition,and geographical detector to explore the influences of geological backgrounds and landscape changes on soil heavy metal accumulation,and also revealed their interaction mechanisms on soil heavy metal accumulation at different spatial scales.The main conclusions are as follows:(1)The study area is dominated by carbonate rocks interbedded with clastic rocks,followed by pure carbonate rocks,clastic rocks,and basalt.The soil As,Cd,Cr,and Hg accumulation varies according to the underlying rock type.Basalt-derived soils have an average Cr content of 173 mg/kg,which is markedly higher than that of other rock types.Similarly,soils derived from carbonate rocks interbedded with clastic rocks have higher average contents of As(37.93 mg/kg),Cd(1.06 mg/kg)and Hg(0.71 mg/kg)than those derived from other rock types.The average contents of As,Cd,Cr and Hg in soils of the study area exceed the background values of soil As(13.8 mg/kg),Cd(0.4mg/kg),Cr(98.98 mg/kg)and Hg(0.13 mg/kg)in Guizhou Province by 2.1 times,2.3times,1.6 times and 3.5 times respectively,indicating high geochemical backgrounds.(2)Heavy metal accumulation in soils varies significantly with soil type in the study area.Yellow soils,which are the most widespread,also have the highest As,Cd,Cr,and Hg contents(mean values of 33.39 mg/kg,1.12 mg/kg,165.90 mg/kg,and 0.62mg/kg respectively).Acidic soils(p H<6.5)tend to accumulate more heavy metals than alkaline and neutral soils.The contents in forest land are significantly higher than those in cultivated land and grassland,with values of 38.63mg/kg、1.29mg/kg、175.45mg/kg、0.84mg/kg respectively.Mining area soils have significantly higher As(39.38 mg/kg),Cd(0.73 mg/kg),Cr(162.85 mg/kg),and Hg(0.84 mg/kg)contents than non-mining area soils.Soil As,Cd,and Hg are at a moderate accumulation level.They exhibit a spatial clustering pattern that is highly correlated with mining activities(3)Large-scale landscape changes occurred in the study area under the dominance of mining activities over 15 years,resulting in reduced natural forest area,increased landscape fragmentation,diversity and heterogeneity,accelerated soil erosion,and weakened vegetation absorption and purification of heavy metals.Landscape changes also exposed a large amount of ore and waste to air,enhancing weathering and erosion effects,and directly and indirectly aggravating soil heavy metal accumulation risk.Soil heavy metal accumulation in newly added grassland and forest areas within the mining area was higher than that in unchanged areas inside and outside the mining area.Within1.5 km from the mining area,soil heavy metal accumulation was significantly negatively correlated with distance from the mining area.(4)Factor detection analysis using geographic detector1 showed that geological background was a key factor influencing soil heavy metal accumulation without mining activities.Rock type and stratum were the main geological factors affecting soil As and Hg accumulation and soil Cd and Cr accumulation respectively,accounting for 9.23%-12.31% of the variation.Interaction detection indicated that p H was the leading factor among natural factors interacting with elevation,slope,and organic matter for heavy metal accumulation,explaining over 30% of the variation.After adding mining activities,stratum still controlled soil Cr accumulation,while soil As,Cd and Hg accumulation were mainly determined by mining activities and landscape changes;mining scale had the largest impact on soil Cd and Hg accumulation,accounting for9.21% and 25.1% of the variation respectively;land use/cover change(LUCC)had the largest impact on soil As accumulation,accounting for 28.98% of the variation;interaction between mining activities and natural geographical factors or LUCC increased soil heavy metal accumulation in a two-factor or nonlinear manner,accounting for 31.78%-47.93% of the variation.(5)Integrating geographic detector and redundancy analysis results showed that at different spatial scales,stratum generally had a dominant role on soil heavy metal accumulation despite heavy metal input from mining activities;geological background,natural factors and landscape changes had different scale effects on soil As,Cd,Cr and Hg element accumulation;overall,landscape changes dominated by mining activities enhanced the influence of natural factors on soil As,Cd,Cr and Hg element accumulation in karst areas in a two-factor or nonlinear way,and this effect increased with increasing spatial scale.Karst soil have high geological background attributes of various heavy metals,while mining activities alter soil physicochemical and geomorphological characteristics and cause heavy metal dispersion,which increase soil As,Cd,Cr and Hg accumulation.Moreover,the interaction between landscape changes and natural geographical factors amplifies the effect of mining on soil heavy metal accumulation and creates different spatial patterns. |
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