Structural and Functional Characteristics of Soil Microbial Communities in Forest–Wetland Ecotones: A Case Study of the Lesser Khingan Mountains

Soil microorganisms play an essential role in vegetation succession, nutrient cycling, and ecosystem restoration. This study investigates the responses of soil microbial communities to ecological transitions from forest to wetland in the Lesser Khingan Mountains, including mixed forest, conifer forest, wetland edge, and natural wetland. The results indicated that natural wetland soils were weakly acidic and contained significantly higher organic matter, total nitrogen, and available phosphorus compared to other soils. Soil bulk density increased with depth. Actinobacteria, Acidobacteriota, and Proteobacteria dominated in mixed forest, wetland edge, and natural wetland soils, respectively, showing minimal variation between depths. Principal component analysis and non-metric multidimensional scaling demonstrated distinct bacterial communities between natural wetlands and wetland edges. Redundancy analysis revealed that soil bacterial communities differed significantly between 15 cm and 30 cm layers, influenced by potassium, bulk density, organic carbon, phosphorus, and nitrogen. Proteobacteria and Bacteroidota abundances correlated positively with nutrients, while Acidobacteriota and Verrucomicrobiota correlated negatively with available potassium. Chemotrophic and aerobic bacteria dominated in forest soils, whereas fermentation-related and anaerobic bacteria were prevalent in wetland soils. The study highlights how ecological transitions and soil properties shape soil microbial communities and their functions.