Host-specific and environmental core bacteria differentially shape the stability and function of the Sphagnum phyllosphere

Sphagnum mosses maintain peatland ecosystem stability through intimate associations with their microbiomes. As the foundational component of these communities, the core microbiome enables ecosystems to resist, absorb, and recover from environmental changes, yet the roles and processes of Sphagnum core members remain poorly understood, particularly in subtropical ecosystems. Here, we identified different components of core microbiomes and found that host-specific and environmental core microbiomes differentially shape the stability and function of Sphagnum phyllosphere bacteria by examining vertical stratification within a litter–Sphagnum–soil system in a subtropical mountain forest. Sphagnum harbors a microbial community that is significantly distinct from its surrounding environment (i.e. litter and soil), with community assembly primarily driven by deterministic processes, whereas litter and soil communities are more strongly shaped by stochastic processes. Sphagnum host-specific core taxa, enriched in carbon- and nitrogen-cycling lineages (i.e. Ca. Eremiobacterota), stabilized microbial composition, whereas environmental core taxa enhanced interaction strength and network robustness, and these groups responded differently to environmental filters (e.g. pH and elevation). Our framework highlights that core microbiomes are not functionally homogeneous, but instead reflect contrasting strategies that collectively shape ecosystem stability.