Dynamic Changes of Colloidal Cd in Wheat Rhizosphere: Implications for Cd Bioavailability in Alkaline Agricultural Soils

Cadmium (Cd) contamination poses a serious problem in wheat fields. Although wheat is predominantly cultivated in neutral-alkaline soils, where Cd bioavailability is conventionally considered low, soil colloids constitute a dynamic Cd reservoir capable of releasing Cd²⁺ for wheat adsorption. Argosol colloids (North China Plain) caused 1.6-fold higher Cd bioaccumulation in wheat than Aridosol colloids (Loess Plateau), despite having 2.1-fold less colloidal Cd and similar pH (∼9.0). Analysis revealed mineral-organic colloids >50 nm enhance Cd bioavailability via two pathways: disaggregation-released <1 kDa soluble Cd and 1 kDa to 50 nm colloidal Cd. Crucially, pedogenic calcite prevalent in Aridosol colloids suppresses Cd²⁺ release, explaining their lower bioavailability despite the higher colloidal Cd content. This demonstrates that colloidal Cd significantly contributes to Cd uptake in alkaline wheat soils, and its bioavailability is fundamentally constrained by region-specific, pedogenically derived mineral compositions like calcite.