Migration characteristics and human health risk assessment of selenium and heavy metals in rhizosphere soil-crop system in high geological background area of southern Qinling Mountains: A case study of Shiquan County, Shaanxi, China

Elevated heavy metal concentrations in selenium-enriched soils from high geological background areas raise concerns about crop safety and human health. This study investigates the migration characteristics and influencing factors of selenium and heavy metals in the rhizosphere soil-crop systems of the southern Qinling Mountains, with a focus on potential human health risks. Results revealed that selenium and heavy metal concentrations in rocks and rhizosphere soils exceeded local background values, with the highest exceedance ratios observed for chromium in rocks (48.01 %) and cadmium in rhizosphere soil (49.17 %). The selenium enrichment rates in rice, maize, pepper, and konjac reached or exceeded 80 %, whereas the selenium content in radish and rapeseed also showed an exceedance rate of over 80 %. These findings suggest that the cultivation of selenium-rich radish and rapeseed should be minimized in the study area to mitigate potential risks. Additionally, the heavy metal exceedance rates in vegetable crops (pepper, rapeseed, konjac, and radish) were significantly higher than those in cereal crops (rice and maize). Heavy metal migration varied across crop types, with accumulation levels following the order: radish (root) > konjac (stem) > pepper (fruit) > rapeseed (leafy) > rice/maize (cereal). Soil selenium promoted selenium absorption in konjac and cadmium uptake in rapeseed, konjac, and radish, while inhibiting selenium uptake in rapeseed. Iron and manganese oxides reduced copper migration in the soil-pepper system, while acidic soil environments enhanced cadmium uptake in certain crops. Multiple linear regression models successfully predicted bioconcentration factors (BCFs) for selenium in konjac and cadmium, selenium, and zinc in radish. These findings highlight the critical role of soil properties and altitude in regulating metal migration, providing a scientific basis for mitigating health risks through soil property adjustments and optimized crop cultivation practices.