Assessing carbon dioxide removal across wollastonite application gradients in mesocosm enhanced rock weathering experiments

Enhanced rock weathering (ERW) is a carbon dioxide removal (CDR) technology that accelerates natural silicate weathering through the application of crushed silicate rocks to soils, commonly in agricultural settings. Identifying optimal feedstock application rates is essential for balancing operational feasibility with the rock densities necessary for robust monitoring, reporting, and verification (MRV). We investigated these dynamics in a mesocosm-scale trial, quantifying the weathering efficiency of crushed wollastonite skarn applied to a circumneutral (pH 7.2) sandy UK agricultural soil across a doubling application gradient: 0 (control), 5, 10, 20, 40, and 80 t/ha. The soil was selected for its sandy texture and relatively low cation exchange capacity (CEC) to maximize the potential for cation leaching. To isolate the vertical reactive transport of weathering products and characterize the progression of the alkalinity front, feedstock was incorporated solely into the uppermost (0–5 cm) soil horizon.

Mesocosms (30 cm soil depth) were sown with perennial ryegrass (Lolium perenne), maintained under a diurnal climatic cycle (25/17 °C, 60/80% RH day/night), and irrigated twice daily for three months. Our results reveal a clear, non-linear dose-response relationship; while soil pH and exchangeable Ca²⁺ increased significantly with application rate, we observed diminishing returns with successive doublings, particularly above 20 t/ha. This suggests that at higher application densities, weathering efficiency may be constrained by self-inhibiting geochemical feedback, including the inhibitory effect of increasing pH on proton-promoted mineral dissolution or localized pore-water saturation during the trial.

High-resolution depth profiling demonstrated considerable vertical translocation of weathering products. Although the feedstock was applied only to the top 5 cm, pH and exchangeable Ca²⁺ peaked in the 5–10 cm layer (immediately below the mixed zone) suggesting a strong downward alkalinity flux. However, despite the soil’s low CEC, the soil matrix acted as an effective sink, retaining the majority of the Ca²⁺ weathering signal within the upper 15 cm and preventing a  breakthrough in leachate chemistry for any treatment. The absence of an aqueous signal was likely obscured by high leachate variability driven by heterogeneous plant uptake of water and nutrients.

We propose that cation exchange dynamics were largely governed by competitive adsorption and mineralogical signatures. The large influx of Ca²⁺ from feedstock mineral dissolution (e.g. calcite, wollastonite) displaced more mobile native cations, driving a dose-dependent depletion of exchangeable Mg²⁺ in the 0–15 cm root zone. Conversely, exchangeable Na⁺ increased strongly (across all depths) with application rate, suggesting that the dissolution of Na-bearing minerals (e.g. readily-soluble salts) from the skarn outweighed competitive displacement effects for this highly-mobile low-background cation. Exchangeable K⁺ exhibited localized depletion in the 5–15 cm horizons, possibly due to Ca²⁺ competition for exchange sites and increased biomass-driven nutrient demand.

These findings demonstrate that wollastonite applications can trigger a complex, non-linear reconfiguration of soil geochemistry and nutrient pools. The observed weathering signals provide essential empirical constraints for calibrating reactive transport models and refining CDR accounting frameworks for scalable ERW deployment.