Ecotoxicological Effects Of Rare Earth Accumulation On Soil Animals In Desert Grassland Around Rare Earth Mine

Rare earth elements(REEs)are a group of metallic elements with similar chemical properties found in nature,which have very wide applications in the field of new energy,high technology,electronics industry,agriculture and national defense.With the increasing scale of production and use,REEs have been entering the soil in large quantities through mining,electronic waste piles and the use of rare earth compound fertilizers.Soil animals directly participate in the decomposition of soil organic matter and the cycling of ecosystem materials and energy flows,which affect soil biochemical processes by regulating the activities of soil enzymes and microorganisms,exerting an important function in the soil ecosystem.The animals live in the soil during part or all of their life cycle.Thus,they are at risk of exposure to REEs accumulated in the soil.To assess ecological and toxic effects of REEs on soil fauna in the desert grassland around Bayan Obo mining area,we examined the content and spatial distribution of soil REEs through REE content detection,field soil fauna community survey,and indoor soil fauna toxicity assays.The main findings were as follows:The soil REE content in the desert grasslands around Bayan Obo mining area was significantly higher than the background value of the area.The average content of mixed REEs in 0～10 cm soil of the area was 431.24 mg/kg,with a maximum value of 1064.97 mg/kg.According to the average content,REEs were ranked as follows:Ce>La>Nd>Pr>Sm>Gd>Dy>Eu>Er>Yb>Tb>Ho>Tm.More REEs were accumulated in the southeast and southwest regions of the mining area compared with the northwest region.The southeast and southwest regions of the mining area had a higher content of REEs than the northwest regions,especially those located 10 km away from the mining area.The REE distribution in the soil exhibited a pattern with a right-sloping light rare earth enrichment.Wind direction and distance were the main factors impacting the accumulation of REEs in the soil of Bayan Obo desert grassland.The accumulation of soil REEs had no significant impact on soil physicochemical properties.In the southwest and southeast directions,the total number of terrestrial arthropods in the region within 10 km from the mining area,especially the number of individuals of Coleoptera beetles and ants,was significantly decreased,and the community structure tended to be unstable compared to the region 10-20 km away from the mining area.The evolution of the community was driven mainly by changes in environmental factors,such as REEs(2.39%),soil moisture content(6.05%),herbaceous biomass(4.09%),and vegetation cover(1.95%).Meanwhile,the total number of macrofauna and taxa decreased significantly in the sampling region within 10 km from the mining area,with the extinction of some sensitive and rare taxa,including Cicindelidae larvae,Meloidae larvae,and Staphylinidae larvae.Besides,a 32%decrease in the total number of Coleoptera larvae(the most dominant taxa),a 45%decrease in Curculionidaelarvae,and a 61%decrease in Scarabaeidae larvae were observed.The above observations indicated that the accumulation of soil REEs causes serious disturbance to the soil fauna of the desert grassland.Indoor toxicity assays revealed that REEs cerium,lanthanum,and neodymium had significant toxic effects on soil animals such as Eisenia fetida,Mongoloniscus sinensis,Harpalus sinicus,and Folsomia candida.In this case,we observed:(1)A moderate concentration of lanthanum and cerium(1200 mg/kg)displayed lethal effects on earthworms and tide worms,respectively,while a low concentration of cerium,lanthanum or neodymium(80 or 240 mg/kg)inhibited the growth of tide worms.Moreover,earthworms and tide worms showed significant avoidance behavior(avoidance rate>80%)to a high concentration of lanthanum or neodymium(3360 mg/kg)and neodymium(1680 mg/kg),respectively;(2)Exposure to 720 mg/kg cerium led to a marked decrease in the reproduction rate of earthworms,a slight damage of the earthworm epidermis,and fibrosis of the muscle layer.By contrast,exposure to 1680 mg/kg cerium fully inhibited earthworm reproduction and caused severe pathological changes and inflammatory reactions in earthworm tissues;(3)The levels of MDA(malondialdehyde)and 8-OHdG(8-hydroxydeoxyguanosine)in earthworms significantly increased with exposure to the increasing concentration of cerium,indicating that cerium induced remarkable oxidative stress and DNA damage.While the activity of SOD,GST,and GPx was inhibited at the low(80 or 240 mg/kg)or medium(720 or 1200 mg/kg)concentration of cerium,a high concentration of cerium(3360 mg/kg)downregulated the activity of SOD and GPx.These results suggested that a low concentration of cerium exerts certain antioxidant effects,whereas a medium or high concentration of cerium interferes with the antioxidant defense system of earthworms;and(4)Exposure to cerium caused changes in transcriptional levels,which resulted in dysregulation of nutrient uptake and metabolism,disrupting protein and calcium ion homeostasis in earthworms.Particularly,this study identified the rough endoplasmic reticulum as the most affected subcellular organelle and SERCA(Sarco/Endoplasmic Reticulum Calcium ATPase)as one of the key genes involved in the effects of cerium.Bayan Obo rare earth mine,the world’s largest rare earth production base,is a long-term and huge source of pollution.The present study found that the massive accumulation of various soil REEs in the desert grassland around the mining area led to toxic effects on the survival,growth,reproduction,antioxidant system,and even gene expression of certain soil fauna taxa,thus altering the community composition and structure of the soil fauna to some extent.Soil animals are important decomposers and consumers in the ecosystem,directly participating in the geochemical cycling of elements and regulating ecological processes such as microbial decomposition and nutrient mineralization.Ecological and toxic effects of REEs on soil animals reflect the serious disturbance of desert grassland ecosystems caused by rare earth mining activities.This study provides not only scientific data for the evaluation of ecological risk of rare earth pollution,but also theoretical support for ecological protection and restoration in and around Bayan Obo mining area,thereby possessing scientific significance and application value.