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

Microbial carbon dioxide mineralization (MCM) is a promising soil-based carbon dioxide removal (CDR) strategy that leverages beneficial soil microbes to accelerate weathering of native 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 MCM is ensuring that weathering products 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. Increases in base cations were primarily partitioned between the carbonate and exchangeable soil pools, with significant increases of carbonate in the MP1-treated soils. We also observed a small but significant accumulation of silicon and magnesium in the reducible fraction, suggesting secondary clay mineral formation. We estimate that 37% to 67% of the weathering-derived cations formed carbonates, resulting in a CDR of 0.20 to 0.36 g CO2 kg-1 soil. These findings demonstrate that Bacillus subtilis MP1 couples silicate mineral dissolution with carbonate precipitation, confirming MCM as a viable CDR strategy.