Specification and Spatial Heterogeneity of Bioavailability-Influencing Factors for Soil Se, Zn, Cu, and Cd in the Yanqi Basin, Xinjiang

This study investigates the specification and the spatial heterogeneity of factors influencing the bioavailability of selenium (Se), zinc (Zn), copper (Cu), and cadmium (Cd) in the soils of the Yanqi Basin, Xinjiang. Based on regional geochemical data, sixty-four representative soil samples were collected and analyzed for total concentrations and chemical speciation via a 7-step sequential extraction procedure. The mobility factor (MF) was calculated to assess bioavailability, followed by correlation analysis and Geographically Weighted Regression (GWR) modeling. The results indicate that: The mean soil Se content (0.37×10-6) is higher than the national background, with 64.20% of samples meeting the Se-rich standard. Zn and Cu levels are comparable to the regional background, with portions reaching Zn-rich and Cu-rich standards, while Cd content is below the agricultural soil pollution risk threshold. The specification of Se and Cd are similar, conferring higher bioavailability (mean MF 7.03% and 6.26%, respectively) compared to the lower bioavailability of Zn and Cu. A mirrored correlation was observed between soil properties and the total content versus MF of elements, indicating that adsorption onto clay and Fe-Mn oxides reduces bioavailability. GWR analysis revealed that soil organic matter, iron oxides, and silicates are the primary factors influencing bioavailability, with their effects exhibiting significant spatial heterogeneity. In conclusion, tailored soil management is recommended to enhance the bioavailability of Se and Zn in respective rich areas while ensuring agricultural product safety.