Elevated temperature alters microbial communities, but not decomposition rates, during three years of in-situ peat decomposition
Peatlands store approximately one-third of the global terrestrial carbon and are historically considered carbon sinks due to primary production outpacing microbial decomposition of organic matter. Climate change has the potential to alter the rate at which peatlands store or release carbon, and results from the Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment have shown net losses of organic matter and increased greenhouse gas production from a boreal peatland in response to whole-ecosystem warming. In this study, we utilized the SPRUCE sites to investigate how warming and elevated CO2 impact peat microbial communities and peat soil decomposition. We deployed peat soil decomposition ladders across warming and CO2 treatment enclosures for three years, after which we characterized bacterial, archaeal, and fungal communities through amplicon sequencing and measured peat mass and compositional changes across four depth increments. Microbial diversity and community composition were significantly impacted by soil depth, temperature, and CO2 treatment. Bacterial/archaeal -diversity increased significantly with increasing temperature, and fungal -diversity was significantly lower under elevated CO2 treatment. Trans-domain microbial networks showed higher complexity (nodes, edges, degree, betweenness centrality) of microbial communities in decomposition ladders from warmed enclosures, and the number of highly connected, hub taxa within the networks was positively correlated with temperature. Methanogenic hubs were identified in the networks constructed from the warmest enclosures, indicating increased importance of methanogenesis in response to warming. Microbial community responses were not however reflected in measures of peat soil decomposition, as warming and elevated CO2 had no significant short-term effects on soil mass loss or composition. Regardless of treatment, on average only 4.5% of the original soil mass was lost after three years and variation between replicates was high, potentially masking treatment effects. Many previous studies from the SPRUCE experiment have shown that warming is accelerating organic-matter decomposition and CO2 and CH4 production, and our results suggest that these changes may be driven by warming-induced shifts in microbial communities.