Cadmium Isotopes Reveal Dominant Atmospheric Deposition Pathways in Agroecosystems

Atmospheric cadmium (Cd) deposition poses significant risks to agricultural ecosystems and food safety. However, quantitatively tracing the contribution and transport mechanisms of this Cd in soil-crop systems remains challenging. By integrating a two-year continuous field open-top chamber (OTC) and greenhouse experiments with stable isotope analysis, we elucidated the biogeochemistry of dry and wet atmospheric Cd deposition in agroecosystems. Newly deposited Cd undergoes rapid aging in soils, with its bioavailability being dependent on deposition type. The isotopic signature of soil bioavailable Cd is primarily governed by dry deposition, a pattern mirrored in crop edible parts regardless of deposition type for both root and foliar exposure pathways, indicating that the isotopic characteristics of bioavailable Cd in wet and dry deposition are congruent with its mitigation in the environment. Applying an isotope mixing model optimized with greenhouse-derived fractionation factors, we quantified that atmospheric deposition contributed 29-65% to the soil bioavailable Cd pool. Foliar uptake of atmospheric Cd contributed over 50% to rice grains and pak choi shoots, exceeding the root uptake of soil legacy Cd. This work establishes Cd isotopes as a robust tool for distinguishing Cd pathways and sources, offering a refined approach for quantitative source apportionment in agroecosystems.