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Sulfur autotrophic denitrification (SAD) offers a sustainable solution for nitrogen removal in low carbon to nitrogen ratio (C/N) municipal wastewater, yet its efficiency and startup time pose significant challenges. To overcome these, we developed iron-sulfur-modified carriers (FeS@MC) and integrated them into an in-situ sequencing batch reactor (S-Fe-SBR), which successfully achieved rapid startup (16 days) of SAD under low C/N conditions. The mechanisms revealed that FeS@MC's hierarchical porous structure promoted biofilm colonization and selective enrichment of sulfur-oxidizing bacteria (e.g.,Thiobacillus). FeS@MC stimulated extracellular polymeric substance (EPS) secretion to amplify sulfur oxidation gene expression (soxA: 126 % enrichment). Moreover, FeS@MC enhanced microbial electron transfer capacity, nitrate reductase activity and synergistically boosted denitrification kinetics, establishing a robust mixotrophic denitrification pathway for high total nitrogen removal efficiency. Our findings propose a novel carrier design paradigm by leveraging iron-sulfur carriers' dual role to optimize biofilm functionality and redox balance, promoting sustainable SAD application in carbon-constrained wastewater treatment.