Mixed forests enhance soil organic carbon sequestration through distinct microbial carbon stabilization pathways across soil aggregates

Mixed forests are increasingly recognized for their potential to enhance soil organic carbon (SOC) sequestration, yet the microbial mechanisms underlying SOC stabilization across soil aggregates remain poorly understood. Soil aggregates provide distinct physical and chemical microhabitats that regulate organic matter accessibility, microbial activity, and mineral interactions, thereby playing a central role in SOC protection and persistence. In particular, the relative roles of microbial necromass carbon (MNC) and extracellular polymeric substances (EPS) in SOC formation under different aggregate microenvironments are unclear. Here, we investigated aggregate-associated organic carbon fractions, microbial-derived organic carbon, microbial properties, and microbial community composition in different aggregate fractions of pure birch ( Betula platyphylla ), pure larch ( Larix gmelinii ), and mixed forests in northeastern China. We demonstrated that mixed forests significantly increased macroaggregate proportion and enhanced SOC accumulation (44.3–65.1%) across aggregate fractions compared with pure stands. Macroaggregates were enriched in MNC, particulate organic carbon, microbial biomass and enzyme activities. In contrast, microaggregates were characterized by higher EPS, mineral-associated organic carbon and Fe-associated organic carbon. In addition, bacterial diversity was more strongly associated with MNC accumulation, whereas fungal diversity showed a stronger association with EPS accumulation, indicating contrasting relationships between microbial community characteristics and microbial-derived carbon pools. Microbial necromass and EPS accumulation was associated with different SOC fraction. Partial least squares path modeling further indicated that the SOC accumulation was primarily regulated by MNC in macroaggregates and the EPS-mediated stabilization in microaggregates. Our results suggested that mixed forests promote SOC sequestration through the modification of microbial carbon stabilization pathways at the aggregate scale. Together, these findings reveal aggregate-specific accumulation and association patterns of microbial-derived carbon pools, highlighting the importance of aggregate microenvironments in shaping microbial contributions to SOC accumulation.