Tracking pedogenic carbonate formation and alkalinity migration in agricultural soils amended with crushed wollastonite ore – Evidence from field trials in Southwestern Ontario

Abstract

Considered a well-known carbon sequestration method, terrestrial enhanced rock weathering (ERW) involves the application of crushed silicate-bearing minerals to urban and agricultural soils. Once dissolved in a soil–water system, alkaline minerals adjust the pH in a range favorable for pedogenic carbonate formation and, hence, atmospheric carbon drawdown. As a fast-weathering Ca-rich mineral, wollastonite is recognized as a primary candidate for this process. Although previous studies have demonstrated the potential of wollastonite to sequester carbon in croplands, no study has investigated the fate of wollastonite over the vertical profile of soil. Furthermore, no studies have investigated changes in the elemental composition of soils due to wollastonite amendment at the field scale. The present study presents the results of multiyear sample collection from different layers (0–15, 15–30, and 30–60 cm) of agricultural soil amended with wollastonite in Woodstock, Ontario, Canada. The impact of initial soil pH on pedogenic carbonate formation was also investigated through the inclusion of two more field trials (Thorndale and Dawn-Euphemia, Ontario). The results indicated that wollastonite addition increased the inorganic carbon pool of the soil at a rate as fast as 0.55 t CO₂/(ha·month) at higher (20 t/ha) wollastonite dosages, and with efficiencies reaching up to 0.42 t CO₂/t wollastonite (as CO₃^2-). Elemental composition analyses (WDXRF) revealed increases in the Ca (0.05–0.32 %) and Mg (0.01–0.02 %) contents in the amended soils that either were inferior to the theoretical amendment change, suggesting migration of weathering products to deeper layers, or in some cases similar and thus correlating with pedogenic carbonate retention in surficial layers. The implications of composite sampling and year-over-year comparisons on the estimated uncertainty from statistical analysis (hierarchical permutation test of the Wilcoxon signed-rank test) is discussed. This study concludes that carbonate formation is not limited to surficial layers and that deeper layers also need to be taken into account when estimating carbon capture due to ERW practices.