Mountain Peatlands and Drought: Carbon Cycling in the Pyrenees Amidst Global Climate Change

• Published in: Journal of geophysical research. Biogeosciences Vol. 129; no. 7
• Main Authors: Garisoain, Raphael, Jacotot, Adrien, Delire, Christine, Binet, Stéphane, Le Roux, Gael, Gascoin, Simon, Rosset, Thomas, Gogo, Sébastien, Granouillac, Franck, Payre‐Suc, Virginie, Gandois, Laure
• Format: Journal Article
• Language: English
• Published: 01.07.2024
• Subjects: CO2 and CH4 exchanges; DOC exports; impact of drought on carbon fluxes; mountainous peatland; multiannual NECB
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2024JG008041

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 models Annual carbon balances were highly variable and the peatland acted as a source of carbon on average over the period A large carbon loss occurred during the extreme drought