Peatlands are sinks for carbon dioxide (CO2) because net primary production exceeds decomposition. The contribution of non-growing-season fluxes to the annual C budget of a peatland is, to date, little studied. We therefore measured the changes in the pattern of carbon exchange with seasons in a bog located in the cool temperate climate region. The growing season CO2-C uptake was of −113 g m−2. During the non-growing season, 36 g C m−2 was lost to the atmosphere, resulting in an estimated net ecosystem production of −76 g C m−2. Despite the non-growing-season loss equaling 33 to 40% of the summer uptake, the net annual accumulation of was 3 times the long-term average net accumulation rate usually cited in the literature. The high rate of non-growing-season efflux could be supported directly by temporal concurrent respiration and the release of stored CO2 from prior production. These results indicate the need to revise current models to address peat thermal properties inducing CO2 production at lower temperature ranges.