A 4000 m2 steel slag (SS) bed was constructed in the brackish water area for water quality remediation in 2017. This study demonstrates the ability of SS to stimulate carbon sequestration based on one-year pilot and in situ SS beds. The pilot SS bed increased the soil organic matter (SOM) content by 5.1 wt %, sequestering sewage-derived organic carbon (Corg) as SOM. Notably, recalcitrant SOM increased by over 40 %, indicating long-term retention of Corg in the sediment. SS increased the Corg density in the sediment by approximately two times, exceeding that of the dense carbon ecosystems found in global seagrass meadows and peatlands. The complexation of the low-molecular-weight Corg with iron in SS was the main pathway for Corg fixation. Benthic animal investigations showed that SS capping fostered an anaerobic fauna ecosystem, aiding biotic Corg sequestration. Algae proliferation on the SS bed suggested complex benthic ecosystems with photosynthetic carbon fixation. Overall, SS beds offer a new abiotic carbon sequestration pathway with environmental restoration benefits; in situ capping with metal-containing material is potentially more economical and sustainable compared to traditional landfilling.