Winter fluxes determine the annual carbon balance of an unmanaged subarctic drained peatland

Abstract. Peatlands are critical components of the global carbon (C) cycle, storing large amounts of soil organic carbon (SOC). However, drainage substantially alters their carbon exchange and hydrological functioning, often converting them into net carbon dioxide (CO₂) sources. This study presents the first year-round, ecosystem-scale Eddy Covariance (EC) assessment of CO₂ dynamics from an unmanaged drained peatland in western Iceland, originally drained in the early 1960s. Two years of continuous EC measurements were collected alongside high-resolution environmental data, including solar radiation, air and soil temperatures, soil water content, and groundwater level. Several multispectral drone flights were also conducted during the study period, which provided seasonal NDVI-based estimates of canopy greenness. The two study years differed markedly in annual weather during the growing season (GS), with 2023 GS being unusually warm and dry, while 2024 GS was cold and wet. Despite these contrasts, annual net ecosystem exchange (NEE) remained similar between the two years. The annual NEE was dominated by non-growing-season (NGS) respiration, which highlighted the necessity for year-round measurements. Overall, the site remained a persistent CO₂ source, emitting 4.1–4.4 t CO₂-C ha⁻¹ yr⁻¹ nearly 60 years after drainage. Temperature exerted the strongest control on ecosystem respiration (R_eco), while gross primary production (GPP) responded primarily to seasonal irradiance and NDVI. A compensatory mechanism was observed during the warm year (2023) at this relatively cool site, where warming-induced increases in R_eco were offset by an enhanced GPP, resulting in a relatively stable annual NEE despite meteorological contrasts. Soil moisture and vapor pressure deficit played only minor roles under these cool and moist conditions. These findings highlight the need for continued monitoring of unmanaged drained peatlands to better quantify their contribution to regional greenhouse gas budgets.