Dual Saturation in Soil Carbon Sequestration: Biophysical Limits and the Operational Capacity of Farmers.

Global soils can store approximately 1500 Pg C in the upper meter and about 2344 Pg C within the upper 3 m, making soil organic carbon (SOC) the largest actively managed terrestrial carbon reservoir. Beyond its climate mitigation role, SOC underpins essential ecosystem services-including soil fertility, water regulation, and ecosystem stability-that support resilient agricultural landscapes and the long-term reliability of food production systems. However, discussions of soil carbon sequestration often focus on biophysical potential while overlooking the human and institutional systems that determine whether this potential can be realized. This Perspective therefore introduces the concept of "dual saturation", arguing that the global potential for soil carbon sequestration is constrained not only by biophysical limits of soil carbon stabilization but also by the operational capacity of farmers to implement complex ecological management practices. Biophysical saturation represents the desired restoration endpoint; operational saturation represents a systemic failure condition. The two are mutually exclusive in practice-and together constitute a self-defeating state, since operational saturation prevents soils from ever approaching their biophysical ceiling. Drawing on evidence from agri-environmental policy research and carbon-farming initiatives, the article argues that administrative complexity, regulatory instability, and documentation requirements can reduce participation in soil carbon-enhancing programs despite available incentives. Supporting farmers' operational capacity through stable and low-friction policy environments thus emerges as an essential enabling condition for large-scale soil carbon restoration and for safeguarding the planetary health functions of agricultural landscapes.