Coupled Dynamics of Mineral Transformation and Mineral-Associated Organic Matter (MAOM) Degradation

The stability of soil organic carbon (SOC) relies on its association with reactive minerals. However, the co-evolutionary feedback between mineral transformation and SOC persistence remains a key uncertainty. A typical example is that while iron oxides stabilize SOC, their inevitable transformation into more crystalline phases is often assumed to weaken SOC protection. Here, we examine how polygalacturonic acid (PGA), a typical SOC, actively modulates ferrihydrite transformation, and its feedback on carbon persistence, under varying PGA loading (C/Fe ratios) and Fe(II)-induced redox conditions. Our goals are to test whether SOC stability decreases as typically assumed and to reveal the underlying mechanisms of this mineral-organic interplay.

We find that goethite, the transformation product of ferrihydrite, does exhibit lower carbon protection capability due to its reduced surface area. Surprisingly, in ferrihydrite-PGA complexes, carbon protection is maintained across carbon loadings through two distinct pathways: at high carbon loading, PGA suppresses ferrihydrite transformation, forming a stable organo-mineral association; at low carbon loading, transformation proceeds but protection is sustained by the residual surface area of the evolving mineral. Moreover, we identify an important negative feedback: the degradation product, galacturonic acid (GA), more strongly inhibits mineral transformation than PGA itself, suggesting that partial degradation actively reinforces the stability of the remaining carbon.

Our results demonstrate that carbon saturation governs two functional pathways: a direct, static organo-mineral stabilization pathway under high saturation, and a resilient, dynamic stabilization pathway under low saturation. Critically, this study reveals that SOC and its degradation products can actively regulate mineral transformation, thereby influencing their own long-term persistence. This microscopic feedback illustrates a self-regulating capacity in soil systems, suggesting that such intrinsic negative feedbacks may enhance soil carbon resilience under a dynamic environment.