Soil microbial-derived carbon accumulation depends on substrate competition between mineral sorption and microbial assimilation

Microbial necromass carbon (MNC) is a major contributor to persistent soil organic carbon storage, yet why chemically distinct plant inputs generate contrasting MNC accumulation remains poorly understood. Current frameworks, particularly the Microbial Efficiency-Matrix Stabilization (MEMS) concept, assume that high-quality substrates universally enhance microbial-derived carbon formation but largely overlook direct competition between mineral sorption and microbial assimilation. Here, we collected dissolved organic matter (DOM) from six plant functional groups (grasses, legumes, sedges, forbs, a mixture, and roots) within a long-term vegetation manipulation experiment on the Qinghai–Tibetan Plateau and incubated them in a controlled model soil system composed of kaolinite and quartz sand to isolate mineral–DOM interactions. We characterized DOM chemistry and quantified microbial carbon use efficiency (CUE), mineral-adsorbed dissolved organic carbon (MADOC), and MNC accumulation during long-term incubation. Plant functional groups generated chemically distinct DOM that differed substantially in mineral interactions and microbial processing. DOM sources exhibiting stronger mineral interactions accumulated greater MADOC but showed lower microbial CUE and reduced MNC formation, whereas DOM with weaker mineral interactions remained more bioavailable and supported greater microbial transformation and MNC accrual. Across all treatments, mineral sorption exerted a strong negative effect on MNC accumulation, while relationships between microbial physiological traits and MNC weakened after accounting for MADOC. Together, our results reveal a previously overlooked “mineral sieve” effect, whereby mineral surfaces selectively intercept substrates before microbial assimilation, thereby regulating substrate accessibility and MNC formation. We demonstrate that the competitive balance between mineral sorption and microbial assimilation, rather than substrate quality alone, is the primary determinant of microbial-derived carbon accumulation. These findings extend the MEMS framework by highlighting the importance of substrate accessibility, in addition to substrate quality, in regulating microbial necromass formation, and sugges that abiotic–biotic competition should be incorporated into Earth System Models.