Slope-induced factors shape bacterial communities in surface soils in a forested headwater catchment

•We examined soil bacterial communities across a forested headwater catchment.•Soil bacterial communities showed rapid turnover along slopes.•Slope-induced soil properties explained bacterial community structures. Despite a rapidly growing body of literature on the spatial variability of soil microbial communities at landscape to regional scales, the community structures of forest soil bacteria at a small catchment scale and their relationships with environmental conditions have been poorly explored. Here, we analyzed soil bacterial communities in the O- and surface mineral horizons at 53 sampling points within a forested headwater catchment in central Japan. Non-metric multidimensional scaling based on the Simpson dissimilarity index was used to analyze compositional changes of soil bacterial communities within the catchment. We evaluated the importance of topographic features, soil chemical properties, and forest type in shaping bacterial communities in each soil horizon using structural equation modeling (SEM). Furthermore, we performed hierarchical clustering and indicator analyses to identify characteristic soil bacterial communities in the catchment. The observed bacterial communities exhibited rapid turnover along slopes. According to the SEM results, the indirect effect of normalized height, a continuous measure of slope position, explains the bacterial community structure in each soil horizon through the effects of pH in the O-horizon and pH and water content in the mineral horizon. Cluster analysis revealed three major spatial units of soil bacterial communities within the catchment in each soil horizon: the lower, middle, and upper slopes. The observed indicators of soil bacterial taxa and metabolic functions differed markedly across slope positions, e.g., ammonifiers were typical of the upper and middle slopes in the mineral horizon, whereas denitrifiers were specific to the lower slopes of the O-horizon. This indicates that the bacterial community functions in this catchment are also constructed to reflect variations in soil pH or moisture along the slopes. This study highlights that the community structure of surface soil bacteria in the forested headwater catchment can be explained by soil chemical properties closely associated with slope positions.