Soil Structure and Mixing Controls on Water‐Rock Contact: Implications for Enhanced Weathering

Enhanced weathering (EW), the addition of finely ground silicate rock powder (RP) to soil, has emerged as a promising carbon removal strategy. However, quantifying weathering rates in soils remains challenging, as most continuum‐scale EW models do not adequately account for the fraction of RP surface area (SA) that is wet at a given soil moisture and thus actively weathering. Here, we study how soil pore structure, RP particle size distribution, and RP mixing degree within the soil control water‐rock contact. Using a soil‐physics‐ based framework, we derive a scaling factor that quantifies the wet fraction of RP SA as a function of soil moisture and mixing degree within soil pores. This scaling factor varies nonlinearly with soil moisture for typical soil pore structures and RP particle size distributions, countering previous zero‐order (independent of soil moisture) or linear assumptions. The scaling factor evolves dynamically with hydrological fluctuations and, for a given pore structure and RP mixing degree, it can span nearly two orders of magnitude with changes in median particle size. To illustrate its application, we integrate the derived scaling factor into the Soil Model for Enhanced Weathering and examine the sensitivity of simulated weathering fluxes to mixing degree under otherwise identical conditions. Under low mixing, results show that average weathering rates are roughly two orders of magnitude lower than under perfect mixing over 1 year of application. Our work provides a mechanistic, computationally efficient framework for representing water‐rock contact in soil, offering a pathway to improve continuum‐scale EW models.