Aggregate-size effects on soil physical and chemical properties and microbial communities during wetland-to-upland transition in cold-region ecosystems of northeastern China

Soil aggregates are critical for maintaining soil structure, nutrient cycling, and microbial habitats, but their dynamics under land-use change remain unclear in cold-region ecosystems. This study aimed to examine soil aggregates, physical and chemical properties, enzyme activities, and bacterial communities across three aggregate size classes (mega-, macro-, and micro-aggregates) in wetland and farmland soils in northeastern China. Wetland soils contained larger proportions of coarse aggregates, higher mean weight diameter (MWD), and greater carbon and nitrogen contents than farmland soils, indicating stronger structural stability and nutrient retention. Microbial α- and β-diversity differed by aggregate size, with larger aggregates harboring more diverse bacterial communities. Soil organic carbon, TN, and SWC were the main drivers of microbial composition, especially in larger aggregates. Bacterial taxa also shifted with land use, with Nitrospinota and Desulfobacterota enriched in wetlands, while Actinobacteriota and Gemmatimonadota were more abundant in farmland. These results highlight the role of aggregate stability in shaping microbial diversity and soil functions during wetland-to-upland transitions.