Evidence of microbially accelerated weathering from a laboratory mesocosm experiment with sequential selective dissolution

Abstract. Microbially accelerated weathering (MAW) is a promising soil-based carbon dioxide removal (CDR) strategy that leverages beneficial soil microbes to increase weathering of preexisting silicate minerals. This approach does not require addition of a mineral feedstock, greatly reducing the carbon footprint from mining, grinding, transporting, and applying the mineral to land compared with enhanced weathering. A key obstacle to measurement, reporting, and verification for MAW is ensuring that increases in weathering products, such as base cations, are sourced from silicate dissolution rather than redistribution of pre-existing cations from the exchangeable, oxidizable, or reducible soil pools. To address this, we conducted a 63-day mesocosm study with soybean, utilizing soil sequential extractions to track the buildup and distribution of weathering products in soil columns inoculated with Bacillus subtilis strain MP1. Our results indicated that MP1-treated soils yield a net increase in available base cations, corresponding to a 4.5 % increase in base cation charge relative to control soils. The observed increase in base cations is partitioned between the carbonate and exchangeable soil pools. We estimate that 37 % to 67 % of the weathering derived base cations formed carbonates, corresponding to a CDR of 0.20 to 0.36 g CO₂ kg^-1 soil. These findings suggest that Bacillus subtilis MP1 couples silicate mineral dissolution with carbonate precipitation, providing support for MAW as a viable and scalable CDR strategy.