Abstract: Peatlands store one-third of the Earth’s carbon. Climate warming-induced peatlands vegetation shifted from Sphagnum to shrub, however, it is controversial whether this change leads to increased carbon losses. Through sequencing of the rhizosphere microbiome (vertically), measuring peat properties (vertically), a 35-day incubation experiment and a 35-day cross-inoculation experiment (only the upper layer), we investigated the ecosystem functions and the role of microbial communities and substrates in influencing the ecosystem functions of Sphagnum- and shrub-dominated peatlands in three locations in south China. The carbon dioxide (CO₂) emission from shrub-dominated peatlands was significantly lower than that from Sphagnum-dominated peatlands. The slow-growing fungi: Archaeorhizomyces, Hyphodiscus and Acidobacteria: Bryobacter, Occallatibacter were identified as keystone taxa in shrub-dominated peatlands, which mainly explained the effects of shrub microbial communities on CO₂ emission. The recalcitrant carbon content was the key substrate associated with CO₂ emission and the community composition of the plant rhizosphere microbiome. Furthermore, microbes fixed carbon in shrub-dominated peatlands was significantly higher than in Sphagnum-dominated peatlands, as the CO₂ emission reversed between Sphagnum- and shrub-dominated peatlands after soil sterilization. Overall, the relative abundance of keystone microbial taxa and nutrient levels decreased with peatland depth. Our study provided new evidence that climate change-induced peatland vegetation shift from Sphagnum to shrub leads to a higher accumulation of recalcitrant carbon, and does not deteriorate ecosystem functions. This study has implications for predicting the future influence of climate change on peatland ecosystems.

Key words: recalcitrant carbon, keystone taxa, ecosystem function, Sphagnum, shrub, peatland