Foliar carbon dots reprogram leaf redox metabolism and rhizosphere iron cycling to mitigate cadmium accumulation in field-grown rice

Engineered carbon dots (CDs) are promising biointeractive nanomaterials; however, their field-scale efficacy and cross-scale mechanisms in crop Cd mitigation remain insufficiently resolved. In this study, a full-season field experiment was conducted to evaluate foliar carbon dots (CDs; 50 and 100 mg/L) in moderately Cd-contaminated paddy soil. The 100 mg/L treatment performed best, increasing grain yield by 18% and reducing Cd accumulation in roots and grains by 40% and 46%, respectively, without compromising grain nutritional quality. Integrated physiological, metabolomic and amplicon-sequencing analyses showed that CDs strengthened leaf antioxidant defenses (ABA, GABA, SOD, POD) and reprogrammed central metabolism, leading to the enrichment of metabolites in the ascorbate–glutathione, zeatin, and triterpenoid saponin pathways that underpin a multilayered reactive oxygen species–scavenging network. At the rhizosphere scale, CDs reshaped bacterial community composition (e.g., Sideroxydans) and promoted Fe cycling, accompanied by 1.8-fold higher Fe and 2.4-fold higher Cd sequestration in root-surface iron plaques under the 100 mg/L treatment. These findings indicate that foliar CDs can effectively couple leaf metabolic reprogramming with rhizosphere Fe-barrier formation to restrict Cd transfer to grains, thereby providing a practical nano-enabled strategy for safer rice production in Cd-contaminated paddy systems.