Kinetic and Transport Controls on Soil Organic Matter Persistence: A Coupled Framework for an Emergent Ecosystem Property

Soil organic matter persistence has been recognized as an ecosystem property emerging from environmental and biological controls rather than intrinsic molecular recalcitrance. Yet a theoretical framework operationalizing this insight into predictive equations remains elusive. Here we present a coupled transport-reaction model where persistence emerges from the dynamic interplay of water, heat, and oxygen transport with kinetically-controlled mineral associations. The framework explicitly couples: (1) environmental state variables (θ, T, O₂) that modulate all reaction rates, (2) transformation kinetics from particulate to dissolved to reactive intermediates, (3) two-stage mineral association with direction-dependent sorption and desorption rates (αs > αd) following Langmuir-Freundlich kinetics, and (4) diffusive and advective transport controlling substrate accessibility. Analytical steady-state solutions in dimensionless form reveal fundamental parameter groupings—including a combined affinity parameter β that governs saturation behavior.

The model predicts distinct persistence regimes: environmental (decomposition suppressed by moisture, temperature, or oxygen limitation), kinetic (asymmetric mineral association creates hysteresis and path-dependence), and transport-limited (micro-site isolation restricts accessibility). These regimes explain why particulate organic matter can persist for centuries under certain conditions while mineral-associated carbon exhibits dynamic exchange in others. The framework also resolves apparent contradictions between saturation theory and field observations—total soil carbon can increase linearly while mineral-associated efficiency declines. Validation against long-term field experiments demonstrates predictive capability across contrasting sites and input regimes. We show that persistence is not a property to be measured but an outcome to be predicted from coupled dynamics—providing a quantitative foundation for the paradigm shift from intrinsic to emergent controls on soil carbon.