Cadmium Distribution and Bioavailability in Paddy Soil Profiles from Regions Impacted by High Geochemical Background and Mining Activities, Southwestern China

<p indent="0mm">Paddy soils in the high geochemical background and mining activity areas of Southwest China generally exhibit cadmium (Cd) enrichment, with significant differences in the activity and risk of Cd from different sources. However, comparative studies on paddy soil profiles with high Cd content from different origins are scarce. This study compared the vertical distribution and main influencing factors of Cd content, chemical forms, and bioavailability in paddy soil profiles from high geochemical background and mining activity areas in southern Guizhou. The results show that Cd content in high geochemical background soils changed little with depth, while in mining activity areas, Cd content decreased with increasing depth. In the study area, Cd primarily occurred in the forms bound to iron and manganese oxides and in exchangeable forms. The proportion of exchangeable Cd in mining activity area soils was about twice that in high geochemical background soils, and the proportion of bioavailable Cd (13.6%) is higher than in high geochemical background soils (7.76%). The bioavailability of Cd in high geochemical background soils is influenced by factors such as soil organic matter and free iron oxides, whereas in mining activity areas, the source of Cd mainly affects its bioavailability. The downward migration of active Cd increased the bioavailability of Cd in deeper soils. These findings provide a basis for pollution control of high-Cd paddy soils of different origins in karst areas.