Land-use and depth-dependent distribution of potentially toxic elements and ecological risk in soils of a phosphate-mining watershed upstream of Dianchi Lake, China.

Abstract

Phosphate mining can alter soil geochemistry and increase potentially toxic element (PTE) concentrations, but the combined effects of land use and soil depth on PTE distributions in mining areas remain insufficiently understood. This study investigated soil physicochemical properties, nutrient status, PTE concentrations, and ecological risk across tailings land, farmland, bare land, grassland, and natural forest land in the Jinning phosphate-mining area upstream of Dianchi Lake, southwestern China. Sixty composite soil samples were collected from four depth intervals (0-10, 10-30, 30-60, and 60-90 cm). Concentrations of Cd, Cr, Cu, Ni, Pb, Zn, and Hg were determined, and contamination was assessed using the single-factor pollution index, Nemerow comprehensive pollution index, geo-accumulation index, enrichment ratio, and potential ecological risk index (RI). Soil properties and PTE concentrations varied among land-use types and soil depths. Soil organic matter, total nitrogen, available phosphorus, and available potassium generally declined with depth, whereas pH increased. Tailings land was the principal contamination hotspot, with surface Cd and Pb concentrations of 0.70 ± 0.35 and 176.8 ± 130.0 mg kg^-1, respectively, and a Nemerow index of 2.085. The RI of tailings land decreased from 184.52 ± 57.18 in the 0-10 cm layer to 110.34 ± 20.36 in the 60-90 cm layer. Mean RI values exceeded 150 only in the upper 30 cm, indicating moderate ecological risk. Cd and Hg made the largest contributions to RI. These findings identify tailings-affected soils as the priority management unit and show that depth-resolved assessment is necessary for characterizing PTE risks in the Jinning phosphate-mining area.