Effect of a novel Ca-Si composite mineral on Cd bioavailability, transport and accumulation in paddy soil-rice system

Ubiquitous cadmium (Cd) contamination in mine impacted paddy soil has been jeopardizing regional rice quality, which represents a dominant pathway of Cd exposure in populations depending on a rice diet. Two major aspects of mitigation, soil liming and Si fertilization, were integrated and investigated with a Ca-Si-rich composite mineral (CS) derived from feldspar and carbonate. With the CS amendment, bioavailable Cd in rice rhizosphere was reduced by 92-100% from tillering to maturation stage, paralleled by a marked increase in Cd bound to Fe/Mn oxides and carbonate. As indicated by XRD analysis, the much reduced labile pool of Cd in the CS-amended soil could be mainly attributed to Cd (co)precipitation (Cd(OH)₂, Cd₂(OH)₃Cl, CH₆Br₃CdN) and surface complexation on more negatively charged oxides at elevated soil pH with CS addition. EDX line scan illustrated much more intensified Si deposition along root cross-section in the CS treatment, which resulted in 1.5-2.1-fold higher Cd sequestration in the CS-amended root than control. As a direct result, the root-to-shoots Cd translocation was reduced significantly by 42-51%, while a slightly less significant decrease in brown rice Cd was obtained with the CS treatment relative to control. The CS amendment showed differing effects on brown rice mineral accumulation, with 1.2-1.5-fold increase in brown rice Zn and simultaneously reduced Fe, Mn, Mg and Cu in brown rice. Our results call the readers' attention to the potential impact of soil ameliorator on grain mineral uptake, and we suggest that proper fortification with mineral fertilizers should be supplemented to assist sustainable rice production with improved mineral nutrition.