Foliar Selenium and Silicon Decrease Root Uptake of Cadmium from Soil but Increase Its Foliar Absorption from the Atmosphere: Evidence from Stable Isotopes

Foliar application of selenium (Se) and silicon (Si) is widely used to reduce cadmium (Cd) accumulation in crops, but its effectiveness under atmospheric deposition remains unclear. Here, we combined field factorial soil-atmosphere exposure experiments, greenhouse validation, and Cd stable-isotope tracing to investigate the effects of foliar Se/Si applications on Cd uptake by rice. At the background site, foliar Se and Si applications significantly reduced Cd concentration in all rice tissues, while they increased Cd in grains, leaves, and stems by 5–33, 6–25, and 7–14% at the high-deposition site, respectively. Furthermore, foliar Se and Si applications increased stomatal conductance and transpiration rate, thereby enhancing foliar absorption of atmospheric Cd and prolonging particle retention on leaves. An optimized isotope mixing model revealed that foliar Se increased the contribution of leaf-absorbed atmospheric Cd to rice grains from 47 to 78%, while reducing the contributions of root-absorbed atmospheric Cd and soil legacy Cd from 19 to 9% and from 34 to 13%, respectively. Our results demonstrate that foliar Se and Si applications are unsuitable for reducing Cd accumulation in crops under high atmospheric deposition, due to the enhanced foliar uptake of airborne Cd. Thus, such agronomic measures should be used with caution in high-deposition areas.