Soil Calcimetry Dynamics to Monitor Weathering Flux: Method Calibration in Wollastonite-Amended Croplands

Enhanced Rock Weathering (ERW) is a promising carbon dioxide removal (CDR) strategy that accelerates mineral dissolution, sequestering atmospheric CO₂ while improving soil health. This study builds on prior applications of soil calcimetry by investigating its ability to resolve short-term carbonate fluxes and rainfall-modulated weathering dynamics in wollastonite-amended croplands. Conducted over a single growing season (May–October 2024) in temperate row-crop fields near Port Colborne, Ontario—characterized by lacustrine clay soils and variable rainfall—the study tests whether calcimetry can differentiate between dissolution and precipitation phases and serve as a proxy for total weathering flux within the sentinel layer. Monthly measurements of soil pH (Milli‐Q and CaCl₂ extractions) and calcium carbonate equivalent (CCE) were collected from 10 plots. Results show significant alkalinization (p < 0.001) in both pH measures, whereas CCE exhibited high spatial and temporal variability with no consistent seasonal trend. The calcimetry‐derived weathering proxy, log (Σ ΔCCE/Δt), correlated positively with pH (r = 0.652), capturing net carbonate accumulation, while the kinetic dissolution rate model correlated strongly and negatively with pH (r ≈ −1), reflecting acid‐promoted dissolution. This divergence confirms that the two metrics capture complementary stages of the weathering–precipitation system. Rainfall exerted a strong short‐term influence on carbonate formation, with cumulative precipitation over the preceding 7–10 days showing a saturating positive effect, while dissolution fluxes were unaffected by rainfall. These findings expand calcimetry’s potential for ERW MRV, providing a direct, scalable, and dynamic measure of CO₂ sequestration in suitable climates and soils.