The role of mineral surfaces in soil organic carbon dynamics across soil pores

The spatial distribution of soil organic matter (OM) and its accessibility to microbial decomposers are key regulators of microbial functioning and soil carbon dynamics. However, how the interactions between soil pore sizes and mineral surfaces shape microbial activities remain unclear. In this study, we conducted a 21-day incubation experiment using ceramic microcosms with soil-like pore networks, coated with different mineral surfaces (illite and goethite). We selectively distributed ¹³C-labelled organic matter into distinct pore size classes (<10 μm and >20 μm). By monitoring carbon mineralization and microbial carbon use efficiency (CUE), we elucidated the interplay between microbial microenvironments and metabolic activities. We compared a simple mix of organic low-molecular-weight compounds with water-extractable OM of wheat root, to determine the role of different types of OM on microbial metabolism and OM decomposition. Mean comparisons for microbial respiration, fraction of added carbon respired, microbial biomass, and CUE were performed using linear models with pore size, mineral surface and OM type as fixed factors, allowing us to identify the dominant drivers of microbial OM decomposition across distinct microenvironments. Post-incubation NanoSIMS analyses were used to quantify pore-scale patterns of the incorporation and spatial retention of freshly added OM across mineral surfaces and pore size classes. Our findings provide insights on how localized interactions between microbes and their organo-mineral microenvironments within pores modulate the persistence and turnover of soil organic carbon.