World’s soils store more carbon (C) than both terrestrial vegetation and the atmosphere combined and a significant proportion of the soil C is associated with soil minerals. Although recent studies have shown that the mineral-associated organic C (MAOC) plays an important role in long-term C
sequestration in soils and thus climate change mitigation, the mechanisms behind the microbial resistance of MAOC remain far from fully understood. Here, we explore a chemically mediated process that controls the persistence of MAOC by investigating the microbial resistance of MAOC.
We produced MAOC by the combination of three types of organic compounds (humic acid sodium salt, sodium gluconate, and vanillic acid) and two types of minerals (kaolinite and allophane) through a batch adsorption method. The MAOC was then incubated with a microbial inoculum solution under unsaturated moisture conditions and CO₂ released from the MAOC was measured to evaluate the microbial resistance of MAOC. Results indicate that all organic compounds acquired a pronounced resistance against microbial degradation by forming MAOC. Importantly, the degree of microbial resistance was more closely related to the strength of their binding (affinity) to minerals, rather than the quantity of organic compounds adsorbed on the minerals. The findings of this study suggest that the recalcitrance of MAOC arises from a trade-off between the adsorption capacity of minerals and their affinity for organic compounds.