Enhanced Rock Weathering Affects Formation of Mineral-Associated Organic Carbon in Soil

Abstract

Enhanced silicate rock weathering (ERW) is increasingly considered for climate mitigation through inorganic carbon (IC) formation, yet its effects on soil organic carbon (OC) turnover remain poorly constrained and mechanistically unresolved. We investigated how the basalt weathering state influences mineral-associated organic matter (MAOM) in a 6 month microcosm incubation of a slightly acidic Cambisol. Fresh basalt (olivine-rich) and naturally weathered basalt (olivine-depleted and phyllosilicate-enriched) were applied, and ¹³C-labeled straw was used to trace the incorporation of plant-derived C into MAOM. Fresh basalt caused more native MAOM-C loss (∼13%) by 0.86 ± 0.29 mg of C (g of soil)⁻¹, likely driven by pH, whereas weathered basalt caused minimal pH change and marginal native MAOM-C loss (∼0%) compared to the control. In ¹³C-labeled straw-amended soils, weathered basalt, characterized by a larger specific surface area and limited pH alteration, increased straw-derived new MAOM-C formation by 20% and 8.5% compared to fresh basalt and the control, respectively. These microcosm incubation results suggest that basalt weathering within 6 months of application may strongly modulate native MAOM-C loss and new MAOM-C formation. Our findings indicate that ERW can influence soil MAOM-C beyond IC sequestration, which implies the need to include MAOM-C responses in ERW carbon accounting to improve its climate mitigation potential assessments.