Shifts in Soil Microbial Community Composition along an Alpine Forest-Shrubland Ecotone in the Hengduan Mountains

Forest line is one of the hot spots in ecological research. It is well documented that the dynamics of forest line under climatic warming, yet how soil microbial community composition shifts across forest line ecotone remains unclear. This study investigated the change in microbial communities of rhizosphere and bulk soils at locations above, at, and below the forest line within an alpine forest-shrubland ecotone in the Hengduan Mountains, China. The results showed that the relative abundances of dominant phyla in surface rhizosphere soil changed more significantly along the forest line ecotone, with Proteobacteria and Basidiomycota decreasing but Chloroflexi and Mortierellomycota increasing, compared to bulk soil. Both the network complexity and key nodes of soil microbial communities changed significantly along the forest line ecotone. Specifically, bacterial network complexity between rhizosphere and bulk soils depended on soil depth, whereas fungal network complexity between rhizosphere and bulk soils varied with forest line. Microbial community compositions were co-regulated by both environmental and vegetation factors. Furthermore, microbial community changes were primarily driven by the cover and height of shrub and herb layers, highlighting the importance of shrubs and herbs in shaping soil microbial community composition. Our findings reveal the distribution patterns and underlying shift mechanisms of microbial communities along the forest line ecotone. These insights are crucial for mountain biodiversity conservation and for improving predictions of forest responses to climate change.