Exploration of the Soil Cadmium Thresholds for Safe Pepper Production in the High Geochemical Background Area of Shilin County

<p indent="0mm"> This study aims to investigate the characteristics of Cd content in soil-pepper systems in geological high-background areas and to explore the soil threshold for safe pepper cultivation in such regions. The findings will provide references for differentiated soil management and safe utilization of agricultural land in high-background areas. A total of 89 soil-pepper samples were collected from Shilin County, Yunnan Province. The available Cd content extracted using three extractants <sc>(0.1 mol/L</sc> hydrochloric acid, DTPA, and <sc>0.01 mol/L</sc> calcium chloride solution) was analyzed. The correlation between Cd content in peppers and soil physicochemical properties was discussed, and the soil Cd thresholds for ensuring safe pepper production under different pH conditions were derived using the ordinal distribution method. The soil in the geological high-background area is generally acidic (pH range: 4.25-8.33, mean: 6.16). The exceedance rate of total soil Cd is 87.6%, while the exceedance rate of pepper Cd is 68.5%. The extraction rates of total soil Cd by hydrochloric acid, DTPA, and CaCl₂ were 49%, 24%, and 6%, respectively. Significant correlations were found between pepper Cd content and total soil Cd (<italic>r</italic>=0.571; <italic>P</italic>&lt;0.01), HCl-Cd (<italic>r</italic>=0.654; <italic>P</italic>&lt;0.01), DTPA-Cd (<italic>r</italic>=0.604; <italic>P</italic>&lt;0.01), and CaCl₂-Cd (<italic>r</italic>=0.735; <italic>P</italic>&lt;0.01). The soil total Cd thresholds for different pH ranges in high-background areas were determined to be <sc>0.08 mg/kg</sc> (pH≤5.5), <sc>0.17 mg/kg</sc> (5.5&lt;pH≤6.5), and <sc>0.47 mg/kg</sc> (6.5&lt;pH≤7.5), with the soil CaCl₂-Cd threshold being <sc>0.006 mg/kg.</sc> The Cd content in soils of geological high-background areas in Shilin County exceeds the standard significantly. Although the proportion of available Cd is relatively low, the high accumulation capacity of peppers still poses a considerable risk of exceeding the standard.