Porewater microbial community dynamics act as an indicator of northern peatland ecosystem change in response to climate drivers.

Northern peatlands store approximately one-third of all terrestrial carbon, making their belowground microbial communities key regulators of peatland carbon cycling and climate feedbacks. We examined how climate drivers alter microbial community composition across peat and porewater habitats over a 0-200 cm depth profile by leveraging the SPRUCE (Spruce and Peatland Responses Under Changing Environments) experiment. SPRUCE uses whole-ecosystem warming to simulate temperature increases from 0°C to +9°C above ambient as well as elevated carbon dioxide (CO₂) treatments. Through small subunit ribosomal ribonucleic acid amplicon sequencing, we found significant differences in taxonomic diversity, abundance, and community composition between attached (peat) and free-living (porewater) microbial assemblages. Porewater communities exhibited significantly higher microbial diversity, lower abundance, and were more responsive to ecosystem-level manipulations (warming and CO₂) than their peat counterparts. The relative abundance of putative methane-cycling microorganisms, methanogens, and methanotrophs was comparable in surface peat and porewater, but at deeper depths, methanotrophs were far more abundant in porewater. Additionally, warming more strongly stimulated putative methanotrophs than methanogens, particularly in porewater, resulting in a more abundant methanotrophic community across all depths. Our results show that peat and porewater habitats harbor distinct microbial communities that respond differently to climate change drivers. The distinct nature of porewater communities, particularly the observed dynamics of methanotrophs, underscores their role in peatland carbon cycling and highlights their potential as sensitive indicators of environmental change, with significant implications for peatland restoration and carbon management in a changing climate.