Trade-off between adsorption capacity and binding strength regulates carbon stabilization in mineral-associated organic matter

Sun, R.; Abe, Yukiko; Atarashi-Andoh, Mariko  ; Koarashi, Jun   

The role of minerals in soil carbon sequestration has been increasingly recognized. However, the mechanism by which mineral-associated organic matter (MAOM) resists microbial decomposition remains an open question. We prepared MAOM on amorphous allophane and crystalline kaolinite with different organic matter (OM) proxies, characterized the affinity strength of these associations using isothermal titration calorimetry and X-ray photoelectron spectroscopy, and quantified their microbial mineralization through aerobic incubation. We found that amorphous allophane displayed greater OM adsorption capacity due to its higher specific surface area; in contrast, crystalline kaolinite exhibited stronger binding with OM, consistent with its predominant inner-sphere complexation. Furthermore, lower MAOM mineralization was found corresponding to higher Langmuir affinity. This inverse relationship was more pronounced than the correlation with the amount of OM adsorbed by minerals. These findings suggest that the overall MAOM stability is controlled by a balance between binding strength and adsorption capacity. Namely, stronger binding affinity enhances microbial resistance of MAOM, while the abundance of mineral sites determines the amount of MAOM accessible for microbial decomposition. This study presents a novel mechanistic understanding of the MAOM dynamics and refines the prevailing perspective on MAOM persistence.