CO2 sequestration by wollastonite-amended agricultural soils - An Ontario field study

Terrestrial enhanced weathering is the spreading of silicate powder on land, and can sequester atmospheric CO2 through carbonation of calcium- and magnesium-rich minerals. When applied to soils, at suitable geochemical conditions, alkaline minerals lead to accumulation of soil inorganic carbon as pedogenic carbonates. Agricultural land covers 37 % of the Earth's land surface, thus offering a natural sink for atmospheric CO2. Wollastonite (CaSiO3) has been commercialized in Ontario as an agricultural soil amendment for several years, but it is not known if or how much CO2 is sequestered annually. In the present study, wollastonite-amended soils were collected from three commercial-scale fields located in Ontario: a leafy vegetables farm in the town of Paris (Field 1); a potato farm in the settlement of Alliston (Field 2); and a soybean farm in the city of Woodstock (Field 3). Chemical and mineralogical assessments of these soils were conducted to determine their carbonate content and the fate of the wollastonite. The soil inorganic carbon (SIC) content was higher in all soils that had been amended with wollastonite, compared to controls, and in Field 1 the SIC content was 2.6 times higher in soil amended thrice with wollastonite compared to a single application. Mineralogical analysis showed the presence of additional polymorphs of SiO2, which may originate from the wollastonite after the dissolution of calcium. This work provides further evidence, here for the first time at a regional scale, that soil amendment with alkaline silicates can be an effective geoengineering tool for climate change mitigation.