Ecosystem respiration during snowmelt and soil thaw leads to a rare annual CO 2 net loss in a boreal fen

Although boreal peatlands play a critical role in the global carbon cycle, their year-round carbon dioxide (CO 2 ) dynamics, and particularly the contribution of the non-growing season, remain poorly constrained in annual balance estimates. Using 17 years (2005–2021) of eddy covariance measurements from a fen in southern Finland, we first quantified the magnitude, timing, and interannual variability of CO 2 fluxes. We then examined in greater detail the non-growing season, specifically the non-productive season defined by the net productivity of the system. We assessed the flux drivers during different periods of the year, with particular emphasis on soil temperature dynamics and the role of thermal legacy effects. On average, the non-productive season accounted for 60 % of the year (226 ± 27 d), ranging from mid-September to late April, and offset 57 % (±33 %) of the subsequent productive season's CO 2 uptake. Emissions declined from autumn to spring, with the highest carbon emissions occurring across September–December and the lowest in January–February. Soil temperature, both concurrent and lagged up to four months, was the main control of CO 2 fluxes during November–December and spring thaw, while photosynthetically active radiation (PAR) dominated during the onset of the non-productive season. Variability in annual CO 2 balances was large, and in two years (2016 and 2018) the fen switched from a net CO 2 sink to a source. Finally, we focused on 2016 in detail: an exceptional six-week CO 2 release during April–May released 84 g C m −2 , offsetting 38 % of the following productive season's CO 2 uptake. This event was linked to unusually warm late-autumn soils, minimal snow insulation, and subsequent rapid surface freezing, which likely enhanced CO 2 accumulation and stimulated CO 2 release during thaw. Our results demonstrate that short-lived but intense events during the non-productive season can determine the annual peatland CO 2 balance and therefore significantly affect the annual carbon budget of boreal peatlands.