Roles of Chloride and Sulfate Ions in Controlling Cadmium Transport in a Soil-Rice System as Evidenced by the Cd Isotope Fingerprint

Anions accompanying inorganic fertilizers, such as chloride and sulfate ions, potentially affect the solubility, uptake, and transport of Cd to rice grains. However, the role of anions in controlling Cd transport in the soil–soil solution–Fe plaque–rice plant continuum remains poorly understood. Cd isotope ratios were applied to Cd-contaminated soil pots, hydroponic rice, and adsorption experiments with or without KCl and K2SO4 treatments to decipher transport processes in the complex soil–rice system. The chloride and sulfate ions increased the Cd concentrations in the soil solution, Fe plaque, and rice plants. Accordingly, the magnitude of positive fractionation from soil to the soil solution was less pronounced, but that between soil and Fe plaque or rice plant is barely varied. The similar isotope composition of Fe plaque and soil, and the similar fractionation magnitude between Fe plaque and the solution and between goethite and the solution, suggested that desorption-sorption between iron oxides and the solution could be important at the soil–soil solution–Fe plaque continuum. This study reveals the roles of chloride and sulfate ions: (i) induce the mobility of light Cd isotopes from soil to the soil solution, (ii) chloro-Cd and sulfato-Cd complexes contribute to Cd immobilization in the Fe plaque and uptake into roots, and (iii) facilitate second leaves/node II-to-grain Cd transport within shoots. These results provide insights into the anion-induced Cd isotope effect in the soil–rice system and the roles of anions in facilitating Cd migration and transformation.