Specific surface area of clays affects accumulation of mineral associated organic matter while larger pore volume is crucial for particulate organic matter accumulation

Clay minerals are widely recognized as key regulators of soil organic matter (SOM) stabilization, yet their interactions with litter quality, soil fauna, and plant roots remain insufficiently understood. We investigated carbon (C) storage and partitioning among particulate (POM) and mineral-associated organic matter (MAOM) in substrates dominated by kaolinite, illite, and montmorillonite using two controlled microcosm and pot experiments. In the first experiments litter of contrasting quality (oak vs. alder) was added to clay minerals with and without earthworms, in second mineral organic matter was delivered by growing  plants  (Festuca rubra and Lotus corniculatus), in the same clay minerals.

 Contrary to expectations based expectation that  surface area (S_BET) would be major predictor of SOM storage, illite consistently supported high C storage, particularly through enhanced incorporation of POM, while montmorillonite promoted MAOM accumulation. Earthworms and easily decomposable litter increased total C storage by facilitating transfer of litter-derived POM into mineral soil. Pore size distribution emerged as a critical factor: illite contained a higher proportion of micrometer-sized pores conducive to POM occlusion, whereas montmorillonite was dominated by nanometer-scale pores favoring MAOM formation.