Dissolved Inorganic Carbon Destabilizes Mineral-associated Organic Matter

Mineral-associated organic matter (MAOM) constitutes a major fraction of soil organic carbon and plays a critical role in long-term carbon sequestration. While factors like phosphate are known to destabilize MAOM, the role of dissolved inorganic carbon dominated by carbonate and bicarbonate remains underexplored. This study investigates the impact of dissolved inorganic carbon on MAOM stability, hypothesizing that dissolved inorganic carbon promotes desorption and inhibits the accumulation MAOM. In batch experiments, elevated inorganic carbon concentrations (≥0.018 mol/L) significantly desorbed organic matter from mineral surfaces, releasing 10.6% of total MAOM-C—comparable to, though less than, phosphate-induced release (12.8%). FT-ICR-MS and XPS analyses revealed the compositional differences of the organic matter released in inorganic carbon solution compared to that in phosphate solution. In a soil incubation experiment with ¹³C-labeled glucose, increasing dissolved inorganic carbon levels progressively reduced the incorporation of new carbon into MAOM, demonstrating that dissolved inorganic carbon inhibits MAOM accumulation. Meanwhile, at high dissolved inorganic carbon concentrations, glucose addition also reduced total MAOM-C, suggesting an enhanced priming effect. Our results show that dissolved inorganic carbon destabilizes MAOM through competitive adsorption, weakening organo-mineral associations and increasing the lability of soil organic carbon. Given the spatial and temporal variability of dissolved inorganic carbon, these findings highlight the need to integrate inorganic carbon dynamics into soil carbon models and management strategies.