Mineralization-based biochar unlocks sustainable restoration of soda saline-alkaline farmlands

Soda saline–alkaline soils are expanding worldwide and pose a growing threat to soil fertility, carbon stability, and food production. Conventional amendments can alleviate salinity–alkalinity, yet they often fail to immobilize reactive carbonate, increasing the risk of secondary salinization and limiting soil organic carbon (SOC) stabilization. Here, we introduce a magnesium–iron engineered biochar (MgFeBC) that harnesses soil salinity–alkalinity to drive in-situ mineral formation. MgFeBC reduced extractable carbonate by 19.8% and enhanced Na⁺ displacement by 55.5% relative to unamended controls. MgFeBC drives the self-assembly of Mg–Fe layered double hydroxides, enabling carbonate mineralization. These mineral transformations strengthened organo–mineral associations, reorganized soil aggregates, and increased particulate and mineral-associated organic carbon. Concomitantly, microbial communities shifted toward copiotrophic taxa, and maize biomass clearly increased. These results demonstrate a mineralization-driven remediation strategy that links carbonate capture, sodicity alleviation, and SOC stabilization, offering a mechanistic pathway for restoring soda saline–alkaline soils. This study shows that Mg–Fe engineered biochar drives the formation of Mg–Fe layered double hydroxides, immobilizes carbonate, reduces sodicity, and stabilizes soil organic carbon via organo–mineral associations in soda saline–alkaline soils.