Iron-Enriched Carbonized Straw Reduces Rice Antimony Accumulation by Shifting the Dominant Antimony Sink from Soil Adsorption to Root-Surface Iron Plaque Sequestration

Abstract Straw return is widely practiced in paddies, yet its role in antimony (Sb) transfer from soil-to-rice remains unclear. This study compared raw straw, carbonized straw (CR), and iron–manganese-enriched carbonized straw (FMCR) to resolve dynamic coupling among rhizosphere biogeochemistry, soil Sb bioavailability, iron plaque, and rice Sb accumulation. Straw return increased pH, SOC, reactive Fe, available K, and porewater sulfate, but decreased Eh and available P, driving stage-dependent Sb immobilization, reductive release, and reactivation. Dominant Sb sink shifted from soil/carbon matrix–iron oxide adsorption at tillering to iron plaque sequestration during filling/maturation, with 72.97–93.35% of iron plaque-bound Sb associated with amorphous Fe. CR and FMCR enhanced iron plaque formation by 25.35–64.15% and 25.64–119.19%, respectively, strengthening root-surface interception and reducing net rice Sb accumulation by 17.84–72.14% and 44.81–57.94%. FMCR reduced brown rice Sb by 63.09%, highlighting its potential for Sb-safe rice production under practical straw-return management.