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.