High geogenic cadmium with paradoxically low bioavailability in karstic Cambisols: variability across land uses of subtropical Guangxi

Soils derived from carbonate rocks are characterized by the accumulation of high levels of geogenic cadmium (Cd). Nevertheless, varying reports indicate that the bioavailability of this geogenic Cd is generally limited, possibly due to inherent soil properties, including high CaO concentration, strong alkalinity, and abundant Fe oxides that enhance Cd immobilization. However, the available evidence on the extent and consistency of this limited bioavailability remains inadequate and sometimes inconsistent across different karst systems. This study examined Cd in soils of Mashan County, Guangxi Zhuang Autonomous Region, a typical subtropical karst area in China. By conducting a comprehensive analysis of soil physicochemical properties across different land use types, we investigated the spatial distribution characteristics of soil Cd and its bioavailability, aiming to better understand the controlling factors governing Cd behavior in Cambisols. This study employed partial least squares path modeling (PLSPM) to assess the effects of lithology and land use on Cd bioavailability. Results showed that (1) Cd in these carbonate-derived Cambisols had an average concentration of 9.05 mg kg −1 , 12 times higher than that of Acrisols in non-karst areas, but with low bioavailability (0.52%) compared to Acrisols (5.45%), partly due to substantially higher Fe 2 O 3 concentrations (10.82% vs 5.52%), which may enhance Cd sorption. (2) The PLSPM revealed that land use had a strong influence on Cd bioavailability, exceeding the influence of lithology. Cd bioavailability in karst areas followed the order of drylands > abandoned farmlands > paddy fields > shrublands. (3) Soil organic matter (SOM) and pH, which are influenced by land use, were the key factors controlling Cd bioavailability in this karst region. Both Paddy fields and drylands exhibited lower SOM concentrations and lower pH levels than shrubland soils (i.e. undisturbed forest soils with minimal human disturbance). Lower pH levels increased Cd solubility while lower SOM concentrations reduced the adsorption and binding capacity for Cd mobility. These findings highlight land use and soil properties as critical controls of Cd bioavailability. Strategies, such as optimizing land use, pH buffering using different amendments, and increasing SOM, can mitigate Cd contamination, reduce crop uptake, and promote safer agriculture.