Overlooked Soil Acidity Leads to Significant Overestimation of Carbon Sequestration in Enhanced Rock Weathering

Enhanced rock weathering (ERW) is a promising carbon dioxide removal (CDR) strategy, yet current estimates commonly assume that cation release directly reflects CO2 sequestration. We investigated wollastonite weathering and carbon sequestration in agricultural soils spanning a wide pH range using column experiments. Weathering decreased with increasing soil pH, with 63%, 48%, and 30% of wollastonite dissolved at pH 4, 6, and 8, respectively. However, CDR estimated from cation release exceeded measured inorganic carbon formation by 1.5-3.1 times, with the largest discrepancy in acidic soil. Charge balance analysis showed that, under acidic conditions, released cations were accompanied by strong acid anions and decreases in exchangeable H+/Al3+, rather than by bicarbonate formation, resulting in only 0.63-1.30 mol CO2 fixed per mol divalent cation released, well below the theoretical value of 2.0. Wollastonite also enhanced CO2 emissions in acidic soil through native carbonate dissolution and soil organic carbon (SOC) mineralization. After accounting for these emissions, net CDR ranged from 0.06 to 0.46 g C kg-1 soil and increased with soil pH. These results demonstrate that soil pH, through charge balance, rock carbonate dissolution, and SOC mineralization, regulates ERW efficiency and should be incorporated into CDR accounting to avoid substantial overestimation of carbon sequestration potential.