Multiscale risks-bioaccessibility coupling framework for improving derivation of soil cadmium threshold

It is imperative to derive the precise threshold of cadmium (Cd) in soil for rice safety production and human health protection. The traditional soil Cd threshold derivation is mainly based on the total amount of Cd and a single risk of Cd pollution, without considering Cd bioaccessibility, multiscale risks and spatial heterogeneity. This fails to synergistically protect safe rice production and human health, and often lead to over- and non-protection issues. Here, we conducted a systematic peri-urban soil-rice grain paired field sampling in the largest megacity of China, where a new multiscale risks-bioaccessibility coupling (MRBC) framework was established to improve the derivation of regional-specific soil Cd threshold. The framework is valid and reliable when applied to regional field-collected datasets. Soil Cd threshold is tailored based on soil properties, multiple risks protection objectives and bioaccessibility of Cd. Results indicated that pH, EC, and SOM concentration were major soil factors influencing the Cd bioaccumulation of rice and soil Cd threshold. Derived soil Cd threshold concentration considering HNO₃ and CaCl₂-extractable Cd decreased. However, when further considering the health risks of Cd intake through soil-rice-human transfer chain, integrating the gastrointestinal bioaccessibility significantly increased soil Cd threshold. Overall, the derived soil Cd threshold based on MRBC framework indicated that the current soil Cd environmental criteria in China were too conservative and may over-protect the regional-specific rice production and human health. Our findings improved existing soil threshold derivation techniques and enabled more precise and scientific soil environmental quality management.