Microplastic-derived dissolved organic carbon affects soil organic carbon mineralization through shifts in microbial community composition

Microplastics pollution constitutes a global environmental challenge. Although degradable microplastics can alter soil organic carbon (SOC) mineralization through the priming effect, the key drivers governing this process remain poorly understood, especially in the subsoil (> 30 cm). Combining a meta-analysis, an incubation experiment, and a random forest model, we found that dissolved organic carbon (DOC) from microplastics decomposition affected the priming effect by regulating microbial growth. Specifically, the highly biodegradable polyhydroxyalkanoate promoted microbial growth by increasing the DOC released from microplastics decomposition, thereby enhancing SOC mineralization via a positive priming effect (462 to 1198 mg CO 2 -C kg -1 soil). Polylactic acid with low biodegradability reduced carbon availability through the sorptive protection of soil DOC, consequently decreasing SOC mineralization by 149 to 268 mg CO 2 -C kg -1 soil. In the subsoil, fungi primarily used the microplastics-derived DOC as carbon and energy source through enzyme-mediated nutrient mining, which represents the main driving mechanism for the strong positive priming effect. In the topsoil, the preferential absorption and assimilation of microplastics-derived DOC by bacteria promoted the microbial necromass accumulation, thus attenuating the priming effect through microbial and mineral carbon pumps. These findings highlight the role of carbon availability (particularly DOC) in topsoil and subsoil on the microplastics-induced priming effect, and underscore the necessity of incorporating soil carbon status into assessments of microplastics pollution impacts on global carbon budgets and soil health. • Microplastics (MP) input changed dissolved organic carbon (DOC). • Polylactic acid input reduced priming effect intensity by adsorbing soil DOC. • Polyhydroxyalkanoate increased priming effect by increasing MP-derived DOC. • Fungi-mediated nutrient mining dominated priming effect in low DOC subsoil. • Bacterial necromass suppressed priming effect in high DOC topsoil.