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This study provides a multi‐year (2017–2022) Net Ecosystem Carbon Balance (NECB) of a Pyrenean mountainous peatland through the integration of field data, satellite imagery, and statistical modeling. Fluvial organic carbon export was measured at 30 min frequency, while gaseous (CO2 and CH4) exchanges were measured monthly using closed chambers. These measurements were combined with Sentinel‐2 derived chlorophyll index and in situ high frequency (1 hr) measurements of key environmental variables such as air temperature, photosynthetically active radiation, and water table level, to develop hourly gaseous carbon flux models (R2 = 0.69 for GPP, R2 = 0.84 for ER, R2 = 0.59 for CH4). Over the 2017–2022 period, modeled average GPP (610 ± 39 gC.m−2.year−1) and ER (641 ± 59 gC.m−2.year−1) showed that the peatland acted as a weak source of CO2 to the atmosphere, releasing 31 ± 73 gC.m−2.year−1. Considering fluvial carbon export and CH4 exchanges, the loss of carbon from the peatland increased to 55 ± 73 gC.m−2.year−1. Dissolved organic carbon constituted 8%–106% of the NECB. The estimated long‐term organic accumulation rate indicated a steady carbon accumulation rate of 16.4 gC.m−2.year−1, contrasting with the contemporary NECB, suggesting a recent shift in ecosystem functioning from a carbon sink to a source. The study underscores the role of water availability and air temperature through a drought index (DI), in shaping the NECB. The DI correlated significantly with annual carbon gaseous fluxes, except for 2022, marked by an intense drought. During this year the peatland became a large source of carbon (189 gC.m−2.year−1) to the atmosphere. Plain Language Summary: Peatlands contain large amounts of organic carbon that was accumulated during the Holocene. Mountain peatlands, although very numerous, are still poorly known because they are small and scattered in the landscape in valley bottoms often difficult to reach. Whether these systems currently accumulate carbon and may continue to do so in the future is largely unknown. Here we present a comprehensive assessment of the carbon balance of a mountain peatland in the French Pyrenees, based on field measurements including winter CH4 fluxes and fluvial export of dissolved organic carbon, satellite imagery and statistical models. We show that on average over 2017–2022 the peatland acted as a weak source of carbon to the atmosphere and also lost carbon to the nearby stream. However, this weak source hides strong interannual variability: in 2018 the peatland acted as a carbon sink but in the year 2022 when an intense drought struck SW Europe, the peatland acted as a substantial source of carbon. Our study underscores the vulnerability of mountain peatlands to climatic fluctuations, emphasizing the necessity for comprehensive monitoring and modeling approaches to accurately assess their carbon balance under changing climatic conditions, including extreme events. Key Points: Investigation of the carbon balance of a mountain peatland by integrating carbon flux measurements, satellite imagery, statistical modelsAnnual carbon balances were highly variable and the peatland acted as a source of carbon on average over the periodA large carbon loss occurred during the extreme drought [ABSTRACT FROM AUTHOR]

Peatlands store about 20% of the global soil organic carbon stock and are an important source of dissolved organic carbon (DOC) for inland waters. Recent improvements for in situ optical monitoring revealed that the DOC concentration in streams draining peatlands is highly variable, showing seasonal variation and short and intense DOC concentration peak periods. This study aimed to determine the variables driving stream DOC concentration variations at seasonal and event scales. Two mountainous peatlands (one fen and one bog) were monitored in the French Pyrenees to capture their outlet DOC concentration variability at a high frequency rate (30 min). Abiotic variables including precipitation, stream temperature and water level, water table depth and peat water temperature were also monitored at high frequency and used as potential predictors to explain DOC concentration variability. Results show that at both sites, DOC concentration time series can be decomposed into a seasonal baseline interrupted by many short and intense peaks of higher concentrations. The DOC concentration baseline is driven, at the seasonal scale, by peat water temperature. At the event scale, DOC concentration increases are mostly driven by water table increases within the peat at both sites. Univariate linear models between DOC concentration and peat water temperature or water table increases show greater efficiency at the fen site. Water recession times were derived from water level time series using master recession curve coefficients. They vary greatly between bog and fen but also within one peatland site. They partly explain the differences between DOC dynamics in the studied peatlands, including porewater DOC concentrations and the links between stream DOC concentration and water table rise. This highlights that peatland complexes are composed of a mosaic of heterogeneous peat units distinctively producing or transferring DOC to streams. [ABSTRACT FROM AUTHOR]

Peatlands in mountains are small and patchy. However, they are biogeochemical hotspots forcarbon cycle, and disproportionally contribute to organic carbon transfer to headwaterstreams. In a harsh mountainous climatic and hydrologic context, this study aims atidentifying parameters controlling DOC concentration fluctuations at the outlet ofpeatlands. In the French Pyrenees, the variability of the DOC concentration was investigated at theoutlet of a fen site (1343 m.a.s.l., 4,7 ha) for four years, and at the outlet of a bog site(706 m.a.s.l, 5,3 ha) for a year and a half. High frequency (30 min) in situ fDOM(fluorescence of dissolved organic matter) sensors were deployed as proxies for DOCconcentration. To identify controlling factors on DOC concentration, high frequencydata including meteorological parameters and stream discharge were collected atboth sites. Water table level and the water temperature were also monitored at ahigh frequency in piezometers wells at different representative locations in eachpeatland. High frequency monitoring reveals a strong variability in DOC concentration at bothsites. Most of this variability is related to short (∼30 hours) DOC concentration peaksinitiated by precipitation events. Lower mean DOC concentration (1.8 mg.L−1) but strongerDOC increases (5,1 times mean concentration) are observed at the fen site compared to thebog site (mean concentration of 5.9 mg.L−1, and increases of 1,8 times). At both sites, DOCconcentration increases are not proportional to the discharge increases observed at the outlet.However, peak analysis reveals that DOC concentration increases are linearly correlated tothe averaged water table rises of the whole site. The slope of the linear models differsbetween sites. Master recession curve analysis was used to classify water tablesignals from each piezometer well depending on their hydraulic conductivity. Thishighlights clear differences in hydraulic conductivity between the bog and the fensites, explaining their different DOC concentration peak models. Fen are flashysystems, characterized by low baseline DOC and strong peaks, when bog sustainhigher baseline DOC concentrations but weaker peaks. Whatever peatland types,this study shows that re-wetting of the non-saturated peat layer is a key factor inthe DOC transfer processes from peatland to stream in mountainous watersheds. [ABSTRACT FROM AUTHOR]