Abstract:Abnormal heavy metal accumulation has been observed in secondary weathered farmlands in the karst areas of Southwest China, but there is no significant correlation between crop heavy metal accumulation and soil background values. This study focused on a typical karst region in the Eastern Sichuan Plain, collecting 176 soil and rice grain samples to systematically analyze their physicochemical properties, Cd and As concentrations, migration and enrichment characteristics, and to evaluate the applicability of national standards (GB 15618-2018) in the region. The results showed that the soil pH in the study area ranged from 4.49 to 8.94, with an overall trend of neutral to slightly acidic conditions, and localized significant acidification. The average Cd concentration was 0.26 mg/kg, higher than the national (0.15 mg/kg) and Chengdu Plain (0.207 mg/kg) background values, with a high bioavailability of exchangeable Cd (average 0.12 mg/kg). The average As concentration was 4.39 mg/kg, lower than the national and Chengdu Plain background values, but there was a risk of activation in acidic soils. The Cd exceedance rate in rice grains was 8%, significantly lower than the potential soil risk proportion (21%), while As exceedance rates were low but showed a significant negative correlation with soil pH, with acidification increasing accumulation risks. Further correlation analysis revealed that soil pH, soil organic matter (SOM), and Fe-Mn oxides were key factors influencing the migration and enrichment of Cd and As. Acidification enhanced the release and enrichment of Cd and As, whereas higher SOM and Fe-Mn oxide contents significantly reduced their bioavailability. Based on these findings, this study proposed targeted risk management measures for the safe use of Cd-contaminated soils in karst areas, providing a scientific basis for regional soil pollution risk assessment and management standards in Southwest China, and offering data support for developing a more reasonable soil safety evaluation system.