Impact of Drought Events on the Greenhouse Gas Balance of a Temperate Mire in the Biebrza Wetlands, Central Europe

Wetlands play a significant role in the climate system due to their ability to store large amounts of carbon, while remaining highly sensitive to hydrometeorological variability. Droughts can profoundly alter these ecosystems, causing them to become significant sources of CO2 and reducing CH4 emissions. However, long-term observational evidence quantifying this response remains scarce. Here, we analyze a 12-year dataset (2013–2024) of CO2 and CH4 fluxes measured using the eddy-covariance method at a site in the Biebrza wetlands of northeastern Poland. The study period included both cool, wet years and hot, dry years characterized by extremely low water table levels. In the warmest and driest year, 2024, the mire acted as a substantial CO2 source with a net emission of 1260 ± 400 g CO2 m−2 y−1. Other drought-affected years, 2019 and 2023, also showed high net emissions of 1020 ± 230 and 840 ± 300 g CO2 m−2 y−1, respectively. Conversely, the wettest year, 2013, exhibited a considerable net uptake of CO2 of −990 ± 250 g CO2 m−2 y−1. During dry years, CH4 emissions declined markedly to values close to measurement uncertainty (1–3 g CH4 m−2 y−1). When expressed as CO2 equivalents, drought conditions consistently transformed the mire into a strong net greenhouse gas source.