Calcium restricts cadmium accumulation in rice by competing for iron plaque adsorption sites

Calcium (Ca) amendments are widely used to restrict cadmium (Cd) accumulation in rice; however, the mechanisms governing competitive adsorption between Ca and Cd in iron plaque (IP) of rice roots remain poorly understood. Here, hydroponic and adsorption experiments using detached roots and goethite (Gt), a simulant for IP, were conducted to investigate these interactions. Results showed that exogenous Ca did not affect IP formation but decreased Cd concentrations in IP, roots, and shoots by 21%-28%, 11%-25%, and 17%-27%, respectively. Detached-root adsorption experiments showed that Ca reduced Cd adsorption in IP by 70% when the Ca: Cd ratio reached 20, with root Cd uptake declining by 50% compared with the Ca-free treatment. Gt experiments introduced competitive mechanisms through a dual-pathway model. First, Ca adsorption shifted the point of zero charge of Gt towards higher pH, weakening electrostatic attraction for Cd. Second, spectroscopic evidence supported that Ca bound to surface hydroxyl groups (Fe-OH) through ligand exchange to form inner-sphere complexes (Fe-O-Ca), competitively occupying surface binding sites. This study not only revealed molecular-scale mechanisms of Ca/Cd competition in roots IP, but also provided new insight into quantitatively estimating Ca detoxification when optimizing Ca compound strategies in Cd-contaminated paddy soils beyond pH effects.