Hydro-ecological controls of dissolved organic carbon dynamics and greenhouse gas emissions in a temperate peatland: A multi-disciplinary collaboration in the Frasne peatland observatory (Jura Mountains, France)

Peatlands are increasingly recognized as key components of the Critical Zone (CZ) - the thin layer at the surface of the Earth where major biogeochemical reactions occur - , because they tightly integrate, within a single ecosystem, hydrological, biological, and carbon cycle processes that all impact each other. Although they cover only about 3% of the global continental surface, they store over 30% of global soil organic carbon, highlighting their long-term role as carbon sinks, largely due to permanent water saturation and specific vegetation. However, climate change is increasingly disrupting the hydroecological balance of peatlands, potentially converting them from carbon sinks into sources of greenhouse gases (GHGs) and dissolved organic carbon (DOC). In this study, we adopted an approach using innovative techniques developed within the TERRA FORMA initiative of the French OZCAR CNRS research infrastructure. Our work was focused on a temperate 7-hectare peatland (Frasne, French Jura Mountains) hosting a long-term Critical Zone observatory (SNO Tourbières) to unravel the mechanisms underlying the continuum of DOC production, mineralization and export to the atmosphere as GHGs (CO₂ and CH₄). Spatial variability in DOC quality - including aromaticity, molecular weight, and microbial origin - was compared to hydrological gradients, vegetation types and atmospheric GHG concentrations, the latter measured by drone surveys and ground-based accumulation chambers. The results indicate a preferential production of recalcitrant DOC in the upstream part of the peatland, where conifers dominate the vegetation. In contrast, biochemical markers reveal intense microbial decomposition of organic carbon in the more frequently flooded downstream zones, producing DOC that is lower in concentration, less aromatic, and more labile. This area coincides with higher GHG concentrations in the overlying atmosphere, suggesting that the labile DOC is readily transformed into GHGs. This pattern is hypothesized to result from the presence of less aromatic molecules originating from vascular plants and Sphagnum moss exudates formed under anaerobic conditions, in areas where the water table is close to the surface. With declining Water Table Depth (WTD), this more labile carbon becomes exposed to aerobic conditions, enhancing microbial respiration and promoting GHG emissions.Lateral DOC export at the outlet of the peatland is strongly controlled at seasonal scale: export increases in spring and autumn during WTD transitions, with generally higher fluxes in winter when the water table is near the surface. In the context of climate change, with progressively wetter winters and drier summers, this pattern suggests a potential intensification of winter DOC export and higher atmospheric GHG emissions during summer, thus leading both to increased annual organic carbon exports. However, the model still needs to account for changes in vegetation type and productivity to fully capture future dynamics.Overall, this study emphasizes that understanding such a complex environment requires strong integration across scientific disciplines. The integrative framework enabled by the OZCAR research infrastructure provides a robust foundation for a better understanding of peatland carbon dynamics at different spatial scales.