Combined Microbiome and Metabolomic Analyses Reveal That Fine-Root Invasion of Rhododendron auriculatum Sapling Enhances Microbial Decomposition of Sphagnum palustre L.

Phenolics in Sphagnum can inhibit its microbial decomposition. Climate warming and drainage have driven vascular plants, such as Ericaceae, to expand into Sphagnum-dominated peatland. However, the impact of fine root invasion by Rhododendron auriculatum Hemsl. on Sphagnum decomposition and changes in phenolic compounds remains unclear. This study compared Sphagnum decomposition in a Sphagnum palustre L.-dominated peatland and an R. auriculatum (Sapling)–S. palustre peatland by examining the microscopic structure of S. palustre and microbial community composition. Decomposition was higher in the R. auriculatum–S. palustre peatland. On this site, bacterial metabolic types such as aerobic chemoheterotrophy and chemoheterotrophy had higher relative abundances, as did fungal trophic modes, including those with combined ectomycorrhizal, ericoid mycorrhizal, and saprotrophic functions. Acid phosphatase, laccase, total nitrogen (TN), C/N ratio (C:N), and pH differed significantly across decomposition stages. Microbial communities are affected by physicochemical factors and enzyme activities. Untargeted metabolomics revealed more downregulated than upregulated phenolics, cinnamic acids, and tannins, indicating loss of phenolic compounds. In summary, R. auriculatum fine root invasion altered enzyme activities and physicochemical properties, driving the restructuring of bacterial and fungal trophic modes and accelerating S. palustre cell wall and hyaline cell decomposition.