Divergent edaphic filters drive ecosystem-specific microbiome assembly and metabolic strategies in Northern China's...

Soil microbial communities are pivotal drivers of ecosystem processes; however, their responses to pedogenic settings and vertical stratification across ecological gradients remain inadequately characterized. In this study, we integrated soil physicochemical analyses, high-throughput 16S rRNA gene sequencing, and microbial metabolic profiling across seven soil subtypes from forest (SSL) to grassland (SCD) ecosystems in the transitional ecoregions of Northern China. Soil subtypes were sampled at three depths (0-5, 5-10, and 10-15 cm). Our results demonstrated that soil type and depth exerted strong filtering effects on the microbial assembly. Within the surface soil layer (0-15 cm) examined in this study, SSL soils exhibited higher microbial metabolic activity (AWCD) and functional diversity than SCD soils. Carbon metabolism peaked in the surface layers. Bacterial species richness was greater in SCD soils, but the structure of the community present diverged significantly between ecosystems. The Proteobacteria and Acidobacteriota dominated in SSL, whereas the Proteobacteria and Actinobacteriota prevailed in SCD. Soil depth exerted a stronger diversity-reducing effect in SCD than in SSL. Redundancy and Mantel tests identified soil water content, pH, and available phosphorus as key determinants of community variation. Functional potential prediction revealed enhanced aerobic chemoheterotrophy and nitrate reduction in SCD, while SSL was enriched in nitrogen fixation and pathogen-related taxa. These findings demonstrated that ecosystem-specific edaphic properties govern both taxonomic composition and functional attributes, highlighting the prevalence of niche-based assembly processes for soil microbiomes in transitional ecoregions.