Comparison of Soil Microbial Communities Across Three Natural Forest Stands Differing in Canopy Composition and Stand Age in the Greater Khingan Mountains, Northeast China

Cold-temperate forest ecosystems in northeastern China have experienced significant changes due to long-term anthropogenic disturbances. To elucidate how different natural forest stands influence the structure, function, and assembly processes of soil microbial communities, we performed 16S rRNA and ITS amplicon sequencing (n = 3 plots per stand) across three natural stands in Northeast China: a 10-year-old natural poplar forest (A1), a 20-year-old natural larch mixed forest (A2), and a 40-year-old natural spruce-pine mixed forest (A3). The results demonstrated that significant variations in aboveground vegetation were accompanied by significant differences in soil physicochemical properties. Specifically, soil organic carbon (SOC) and total nitrogen (TN) were significantly lower in A1 compared to the coniferous stands (p < 0.05). While α-diversity remained stable across stands (p > 0.05), β-diversity revealed distinct community structures (PERMANOVA, p < 0.05). Neutral Community Model (NCM) analyses indicated a shift in bacterial assembly from a stronger stochastic influence in A1 (R2 = 0.431) to a more prominent deterministic influence in A3 (R2 = 0.188). Because stand age and canopy composition co-vary in our sampling design, we cannot fully disentangle their individual contributions. This comparative observational study reveals distinct soil microbial community structures across the three natural forest types. The transition across forest stands was associated with shifts in dominant microbial taxa and a tendency toward stronger deterministic assembly processes in coniferous stands. These patterns are consistent with differences in litter quality and soil physicochemical properties, providing observational evidence that may inform sustainable management of cold-temperate forests.