Balancing Organic and Inorganic Carbon Dynamics in Enhanced Rock Weathering: Implications for Carbon Sequestration

Enhanced rock weathering (ERW) is a promising strategy for CO₂ removal via promoting inorganic carbon (IC) sequestration. However, knowledge gaps persist regarding its influence on the largest terrestrial carbon pool, soil organic carbon (SOC) and how these effects evolve as weathering progresses. This study investigated how basalt weathering influences soil carbon fluxes and organic matter (OM) turnover. Over a 6th-month incubation, we applied fresh basalt (fine-sized, olivine-rich) and weathered basalt (coarse- and fine-sized, olivine-depleted) to temperate cropland topsoil, incorporating with ¹³C-labelled straw. Fresh basalt increases soil pH via rapid H⁺ neutralization during olivine dissolution, releasing soluble Mg²⁺ and increasing bicarbonate alkalinity. Combined with continuous carbonic acid dissociation for olivine dissolution, they synergistically enhance dissolved inorganic carbon (DIC) accumulation in soil solution and effluent (~0.4%), promoting soil inorganic carbon (SIC) accrual via carbonate precipitation (~4%). However, rising pH concurrently induces significant SOC losses (~17%), resulting in net C losses of ~13%. As basalt weathering progresses (olivine-depleted), slower H⁺ neutralization and carbonic acid dissociation during less-reactive Ca-bearing mineral dissolution stabilize soil pH, limiting DIC formation. The released Ca²⁺ prioritizes SIC accrual via Ca-carbonate precipitation (~4%). Meanwhile, higher specific surface area (SSA) and exchangeable Ca²⁺ enhance retention and stabilization of both native and straw-derived OC, reducing net C losses (~6%). At both weathering stages, over 95% of total C remaining in soils and effluent exists in organic form. Straw inputs acidify soils by releasing additional free H⁺ during decomposition, competing with carbonic acid for olivine dissolution and reducing bicarbonate alkalinity, which limits the DIC and SIC accrual at both weathering stages. Since soils continuously receive OM input, understanding the balance between these interactive processes is crucial for optimizing long-term carbon sequestration strategies. Therefore, sustaining SOC by minimizing SOC losses should be prioritized for long-term carbon sequestration, besides IC accrual for ERW, particularly as weathering progresses.