Unveiling the barriers of Cd translocation from soil to rice: Insights from continuous flooding

Understanding the spatiotemporal processes governing Cd behavior at the soil-solution-root interface is crucial for developing effective remediation strategies. This study examined the processes of chemical remediation in Cd-contaminated paddy soil using rhizotrons over the entire rice growth period. One-dimensional profile sampling with a 10 cm resolution revealed that during the initial flooding, paddy soil was strongly stimulated, followed by stabilization of porewater properties. X-ray diffraction of freeze-dried porewater confirmed the generation of submicron-precipitates such as CdS under continuous flooding, resulting in low ion levels of water-soluble Cd (<1 μg/L) and sulfate (<10 mg/L) in porewater. Two-dimensional imaging technologies indicated the maximum iron‑manganese plaque (IP) within 20-110 μm of the root surface. Subsequently, monitoring O₂ in the rhizosphere with a planar optode by two 100 cm² membranes for a consecutive month revealed significant circadian O₂ variations between the root base and tip. Destructive sampling results showed that acid-soluble Cd in soils, as available Cd, is crucial for Cd uptake by rice roots under continuous flooding. The IP deposited on the root surface, as the barriers of Cd translocation, increased with rice growth and blocked Cd translocation from soil to rice by about 18.11 %-25.43 % at maturity. A Si-Ca-Mg compound amendment reduced available Cd by about 10 % and improved Cd blocking efficiency by about 7.32 % through increasing IP concentration, resulting in the absorption ratio of Cd in the amendment group being half that of the control group. By unveiling the complex Cd interactions at the soil-rice interface, this study lays the groundwork for developing effective agricultural practices to mitigate Cd-contaminated paddy and ensure food safety.