Beyond the Growing Season: Variability of 13C-CO2 Fluxes in Temperate Forests and Peatlands

Winter processes are increasingly recognised as important components of ecosystem carbon cycling, yet 13C-CO2 fluxes from temperate forests and peatlands remain poorly quantified. This study quantified cold-season 13C-CO2 fluxes in a Scots pine forest and a temperate fen in western Poland using manual closed chambers coupled with a Picarro G2201-i isotope analyser. Measurements were conducted during the cold half of the year and related to soil temperature, air temperature and, at the forest site, soil moisture. Median 13C-CO2 fluxes were about twice as high in the forest (607 µg·m−2·h−1) as in the fen (290 µg·m−2·h−1), indicating stronger winter respiratory activity in the mineral soil than in the water-saturated peat. In the forest, 13C-CO2 fluxes showed a weak, non-significant tendency to increase with temperature, whereas in the fen they were significantly negatively correlated with soil temperature and tended to peak near 0 °C, pointing to an important role of zero-curtain and freeze–thaw conditions. These plot-scale measurements provide rare constraints on winter 13C-CO2 losses from temperate forest–peatland mosaics and highlight the need to represent cold-season isotopic fluxes in carbon–climate assessments. From a biogeochemical perspective, the findings emphasize that 13C losses during the cold season can occur as transient, high-intensity ‘hot moments’. Such episodic fluxes should therefore be explicitly incorporated into winter carbon accounting and isotopically enabled carbon–climate feedback assessments to improve the fidelity of annual net ecosystem exchange projections.