Abstract: Fallen logs are key components of forest biogeochemical processes, regulating carbon and nitrogen dynamics in forest soils. By altering resource availability and environmental conditions, they also influence soil microbial community structure, yet these responses remain poorly understood. To address this gap, we investigated soil microbial communities adjacent to Quercus liaotungensis fallen logs at the mid-decay stage, which is characterized by distinctive decomposition conditions. Soil samples were collected beneath fallen logs (FL) and from paired control sites (CT). Based on whether soil organic carbon (SOC) and total nitrogen (TN) increased (Pos) or decreased (Neg) in log-affected soils relative to controls, the samples were categorized into four microsites to compare microbial communities. Amplicon sequencing of 16S rRNA and ITS regions revealed differences in microbial communities across microsites. Co- occurrence network analysis revealed stronger bacterial–fungal coupling in soils affected by fallen logs, particularly in the log-affected Neg group, which showed increased bacterial– bacterial connections and the most complex network structure. Community assembly analysis showed that bacterial communities in FL-Pos soils were dominated by homogeneous selection and homogenizing dispersal, which indicates stronger environmental filtering, whereas FL-Neg soils showed greater dispersal limitation and drift. Predicted functional profiles indicated enrichment of saprotrophic taxa in soils beneath fallen logs and alterations of bacteria in nitrogen-cycling pathways, with FL-Neg microsite showing lower predicted abundances of nitrification- and denitrification-related functions and higher predicted abundance of nitrogen- fixing taxa. Overall, this study highlights the overlooked ecological importance of mid-decay fallen logs and shows that microbial community reorganization is associated with decomposition-driven forest-floor heterogeneity.

Key words: Fallen logs, Soil microbial community, Co-occurrence network, Microbial functional profile, Soil heterogeneity