The calcium-biological bridge: an integrated framework for soil organic carbon stabilization in calcareous ecosystems

Soil organic carbon (SOC) persistence is commonly understood as a function of microbial transformation and mineral protection, yet current frameworks treat minerals broadly and thus overlook calcium-driven stabilization in calcareous soils, which cover approximately 30–35% of global land. We propose the Calcium-Biological Bridge (CBB), a conceptual framework linking microbial processes with calcium-mediated stabilization across multiple timescales. High calcium availability may shape microbial communities and carbon transformation, driving production of microbial necromass. These microbial products may then interact with Ca 2+ through sequential stabilization pathways: cation bridging, aggregation, and organo‑carbonate precipitation. These processes may contribute to multi-timescale SOC persistence, though the quantitative significance of each pathway remains to be determined. Ca-driven improvements in soil structure may enhance microbial habitat, potentially creating a structural feedback loop. By integrating microbial filtering with calcium geochemistry, the CBB framework advances mineral-associated organic matter concepts and highlights mechanisms overlooked in Earth system models. Incorporating calcium-mediated processes may therefore improve SOC predictions in carbonate-rich ecosystems under changing climate.