||Peatlands sequester one-third of all soil carbon and currently act as major sinks of atmospheric CO2. The ability to predict or to simulate the fate of stored carbon in response to climatic disruption remains hampered by our limited understanding of the controls of C turnover and the composition and functioning of peatland microbial communities. Given their global extent and uncertain fate with climatic change, boreal forests are considered a high priority for climate change research. The overall goal of this project is to investigate the lability of soil organic matter and the composition of decomposer microbial communities in response to the climatic forcing of environmental processes that determine carbon storage and sequestration in peatlands. The proposed project will be conducted at the Marcell Experimental Forest (MEF) where ORNL has established a Climate Change Response Scientific Focus Area known as Spruce and Peatland Response Under Climatic and Environmental Change (SPRUCE). Working hypotheses driving the proposed research are: 1) Soil environment (temperature, moisture content, O2, pH, plant community composition and physiology) determines the functional diversity of heterotrophic bacteria and saprobic microfungi which in turn controls or regulates soil carbon storage, 2) Refractory or recalcitrant polyphenolic compounds (plant residues) such as lignin act as a “bottleneck” to organic matter decomposition in peatland forest soils because of their toxicity to microorganisms, 3) Dissolved organic matter (DOM) is the primary intermediate pool regulating carbon turnover in peatland soils, 4) The rate limiting step in peatland soil organic matter decomposition is the degradation of aromatic polymers that is catalyzed by a range of extracellular enzymes produced by a broad diversity of prokaryotic and eukaryotic (fungi) microorganisms, and 5) The presence and relative abundance of condensed aromatic and phenol-type compounds is a molecular signal of DOM lability, and that signal can be quantified using an Aromaticity Index (AI). To address these hypotheses, our objectives are to: 1) conduct a comprehensive interrogation of organic matter recalcitrance and the functional diversity of keystone microbial guilds that are likely to control organic matter decomposition at the ecosystem scale in a boreal peatland forest, 2) validate the molecular-based Aromaticity Index (AI) as an indicatory of DOM recalcitrance, 3) directly link the phylogenetic identity and metabolic function of keystone microbial guilds that mediate soil carbon turnover in response to perturbations in climate change variables (temperature, redox, pH) under controlled conditions in the laboratory, 4) determine the response of DOM recalcitrance, decomposition, and the functional diversity of keystone microbial decomposers to climate change manipulation in the field at the ecosystem scale. The proposed project will leverage existing infrastructure and will be organized around an ecosystem-scale climate change manipulation experiment to be conducted by the SPRUCE program in a black spruce-Sphagnum forest in northern Minnesota. The flux of carbon from peatlands to the atmosphere is projected to increase with climate change, but acceleration of the C cycle does not necessarily mean that peat soils are losing a greater proportion of their large carbon stores to the atmosphere. The proposed research on soil C processes will leverage SPRUCE datasets on ecosystem response to address the question of whether the response of decomposition to climate change is driven by higher carbon inputs to the soil from plants or rather by the mobilization of stored older carbon, thereby constituting a positive feedback loop. Through a close collaboration with SPRUCE investigators at ORNL, these new insights will be embodied in improvements to the land component of Earth system (CCSM) models and in climate projections derived from these improvements. Thus, the project will deliver measurements, experiments, and modeling in support of the long-term performance measure of DOE’s climate change research program.