Peatland carbon dynamics in a changing climate: A 13-year flux time series of a fen in Northern Finland

Northern peatlands act as a global carbon sink. At the same time, they are a major source of methane. Rising temperatures due to global warming may severely change carbon dynamics. However, long-term studies to evaluate the impact of global warming on northern peatland ecosystems are rare. Here, we monitored carbon dioxide (CO2) and methane (CH4) dynamics at a subarctic/boreal fen in northern Finland throughout 13 years (2007-2019) using the eddy covariance technique accompanied by measurements of abiotic and biotic drivers. Mean yearly CH4 and CO2 exchange were +21.7 g CH4 and –0.14 kg CO2, respectively. The peatland was an average sink of carbon, with a mean annual uptake of –21.4 g C. It remained also a sink during the exceptionally warm summer of 2018 (–52.3 g C y^-1). Soil temperatures strongly drove CH4 emissions whereby summer soil temperatures governed the annual budget (p < 0.001) and summer emissions determined the annual budget (45-57%, p < 0.001). Observed warming in late summer (+2.1 °C, 2007-2019) did not increase CH4 emissions as soil temperatures remained unchanged. Air and soil temperatures during the growing season and the number of snow-free days controlled annual net CO2 exchange (p=0.003). We found an increasing trend in respiration and primary production in the late growing season, keeping net CO2 exchange equally. Warmer temperatures in the late growing season increased respiration, however, primary production responded only positively to warmer temperatures in spring. Instead, warmer late summers and longer extended autumn growing seasons likely delayed autumn senescence and increased greenness, keeping primary production high. Our results suggest that with ongoing global warming and rising summer soil temperatures, methane emissions from boreal peatlands will further increase, feedbacking on the climate through its high global warming potential. The carbon sink potential will be determined by the number of snow-free days and growing season temperatures. Vegetation changes in the late growing season may offset higher respiration due to warming.