Biochar Promotes Cd Immobilization via Amorphous Iron Oxides in Iron Plaque by Enhancing Iron Reduction During Soil Reflooding After Drainage in a Pot Experiment

Question: Continuous flooding (FL) promotes Cd immobilization in paddy soils, whereas the effects of alternate wetting and drying (AWD) on Cd immobilization in soil and on iron plaque (IP) remain unclear. This study investigated the effects of water management and biochar application on Cd immobilization and Cd accumulation in rice across soils with different iron oxide crystallinities. Methodology: A pot experiment was conducted with three soils differing in iron oxide crystallinity (QY, TL and QX) under FL and AWD regimes, with and without biochar addition. Soil Cd fractions, iron transformation, IP morphology, and Cd uptake by rice were investigated. Results: Compared with FL, AWD increased rice grain Cd by 57.1–93.1%, suggesting enhanced Cd mobilization in soil. Nevertheless, AWD also increased IP-bound Cd (IP-Cd) and IP-associated amorphous iron oxides (Feo)-bound Cd (IP-Feo-Cd). The increases in IP-Feo-Cd was likely driven by elevated Fe(II) concentrations during reflooding, which promoted the formation of Feo on the plaque surface. Biochar significantly enhanced Fe(II) concentration, soil Feo concentration, Fe/Mn oxide-bound Cd, and Feo accumulation on IP, with stronger effects under AWD than under FL. Biochar also increased soil electron transfer capacity, suggesting its role as an electron shuttle that accelerates iron cycling and Cd immobilization. Consequently, biochar application reduced grain Cd by 39.7% under AWD, bringing it to a level comparable to that under FL without biochar. The effects of biochar on IP development and Cd immobilization became increasingly pronounced with increasing iron oxide crystallinity. Conclusions: These findings reveal a crystallinity-dependent Fe-Cd coupling mechanism and demonstrate that biochar promotes Fe(II)-mediated iron oxide formation and Cd immobilization in soil and on IP, thereby offsetting the increased risk of Cd accumulation associated with AWD.