Representation of diffusion controlled carbon stabilization in reactive transport models

Soil carbon (C) stabilization occurs through two general groups of mechanisms: (1) physical separation of the organic carbon from the reactants or enzymes required for mineralization via occlusion, movement of C to depth, isolation of C in zones without suitable electron acceptors; and (2) movement of organic C into an unavailable form, for instance through association with mineral surfaces or through chemical recalcitrance. The later group of mechanisms can be numerically represented through various sorption/desorption and co-precipitation formulations that—although still evolving—have been used in numerical models for several decades or longer. Protection of carbon via physical isolation is more challenging and currently under-explored in numerical modeling codes. We propose four simplified conceptual models of increasing complexity for protection of carbon through physical-isolation along with early-stage numerical simulation of testable `scenarios' within a reactive transport model. We then discuss the relative time-scales for carbon residence time based on numerical output for each conceptual model. The testable scenarios include: (1) Isolation of soluble carbon within low-flow domains within the soil; (2) Isolation of particulate carbon within low-flow domains; (3) Imposing oxygen diffusion constraints on decomposition of carbon isolated within low-flow domains; (4) imposing mineral sorption of soluble carbon on scenario three.