Soil cation exchange property (SCEP) is important in soil development and environmental buffering. However, the variations in SCEP and its correlation with soil-forming factors in subsurface soil are not fully understood. In this study, we quantified the surface and subsurface SCEP variations as a function of parent material, vegetation, hillslope position, and soil type. Fifty upland soil profiles from mid-subtropical China were selected. The cation exchange capacity (CEC) and effective CEC (ECEC) of subsurface soil were significantly higher in soils derived from slate, Quaternary red clay (QRC), and limestone than in soils derived from granite and sandstone. The subsurface soils derived from limestone had the highest base saturation (BS) and the sum of base cations, and the lowest aluminum (Al) saturation. The SCEP in surface soil significantly varied with vegetation and hillslope position. The surface soil CEC was the highest in mixed-forest vegetation, whereas the ECEC and exchangeable acidity (EA) were the highest in arable vegetation. Exchangeable potassium (K⁺) was lowest and the EA was highest in soil orders at the strong development phase. Exchangeable calcium (Ca²⁺), magnesium (Mg²⁺), CEC, and BS were the highest in soil orders at the intermediate development phase. The prediction accuracy of soil CEC using the random forest model was higher than that obtained using multiple stepwise regression, with the best results (R² = 0.92) obtained in the surface soil. Our study indicated that the SCEP in surface and subsurface soils was controlled by different soil-forming factors and could be effectively predicted by soil properties in subtropical China.

Heavy metal(loid)s (HMs) pollution in farmland soil and its induced health risk have attracted great attention worldwide. Hunan Province is recognized as one of the principal rice-producing regions in China. Its farmland is seriously threatened by HM pollution, which poses a great risk to food safety and residents’ health. However, the spatial and vertical distribution, sources of HMs, the main factor affecting HMs accumulation, and related pollution and health risks across Hunan Province’s farmland largely remain unclear. To fill this gap, 62 profile soil samples were collected across the farmland of Hunan Province with depths up to 140 cm, and their Cd, Cr, Hg, Pb, and As content were measured. After that, the source of HMs was appointed using the positive matrix factorization model. Then, the Shapley additive explanations methodology was adopted to quantify the relative contributions of different natural and anthropogenic variables to HM accumulation. Finally, human health risks and potential ecological risks were also assessed. Our results revealed a significant concentration of Cr, Hg, Pb, and As in the farmland of Hunan Province. Only the mean value of Cd (0.42 mg kg-1) in the surface soil surpassed the screening value for assessing the soil contamination risk regulated by the national standard (0.3 mg kg-1, GB 15618-2018), indicating that Cd contaminates the farmland soil in Hunan Province. The HMs were mainly enriched in the surface layer, except for As. The mean contents of Cd, Pb, Hg, and Cr exhibited a decline as soil depth increased. The Pb and Cd in the surface soil of farmland in Hunan Province was largely sourced from industrial and traffic activities, As and Cr are mainly derived from soil parental material, and Hg chiefly stems from atmospheric deposition. The mean annual temperature has the largest effect on Cd, Hg, and Pb accumulation, while Fe2O3 has the largest impact on the accumulation of As and Cr. Children and adults were treated with high carcinogenic risk due to long-term exposure to HMs. Cr was the main contaminant for carcinogenic risks, and dermal contact was the main route for carcinogenic risks. Our results offer empirical evidence to support the prevention and management of HM pollution in the farmland of Hunan Province, as well as to ensure food security.