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

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2025WR04147

Authors: Shashank Kumar Anand, Matteo Bertagni, Felipe Aburto, Salvatore Calabrese

First published: 02 February 2026

https://doi.org/10.1029/2025WR041479

Abstract

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.

Plain Language Summary

Enhanced weathering (EW) is a strategy that spreads finely ground silicate rock powder (RP) over soils to remove atmospheric carbon dioxide (CO2). However, predicting how fast the RP weathers and therefore how much CO2 it can remove remains highly uncertain in soils. This is partly because most continuum-scale EW models assume that the entire surface area (SA) of the added RP is always in contact with water. Here, we show that the degree of RP mixing in the soil, the soil pore structure, and the level of soil moisture all strongly influence the portion of the rock surface that is actually in contact with water and can react. We develop a new, soil-physics-based framework to calculate the “wet” fraction of the RP SA under different soil and mixing conditions. We find that this wet fraction varies nonlinearly with soil moisture levels and the degree of mixing, which in turn affects the overall weathering rate of the added rock particles. Our framework helps explain part of why observed weathering rates in the field are often lower than predicted. It also provides a practical way to improve continuum-scale EW models and better estimate the carbon removal potential of EW.

Key Points

We developed a soil-physics-based framework to quantify the wet surface area (SA) of rock powder (RP) particles in heterogeneous, multiphase soils

Wet SA varies by orders of magnitude with changes in soil structure, moisture, and the degree of RP mixing

The framework can explain part of the theory-observation mismatch in enhanced weathering applications

Source: AGU