Boreal forests fix substantial amounts of atmospheric carbon (C). However, the timescales at which this C is cycled through the ecosystem are not yet well understood. To elucidate the temporal dynamics between photosynthesis, allocation and respiration, we assessed the radiocarbon ( 14 C) disequilibrium (D) between different C pools and the current atmosphere to understand the fate of C in a boreal forest ecosystem. Samples of vegetation, fungi, soil and atmospheric CO 2 were collected at the Integrated Carbon Observation System station Svartberget in northern Sweden. Additionally, we analyzed the Δ 14 C-CO 2 from incubated topsoil and forest floor soil respiration (FFSR) collected over a 24-hr cycle, and calculated the Δ 14 C signature of the total ecosystem respiration (Re) using the Miller-Tans method. We found that vegetation pools presented a positive D enriched with bomb 14 C, suggesting a fast-cycling rate (months to years) for living biomass and intermediate (years to decades) for dead biomass. In contrast, mineral soils showed a negative D, indicating minimal incorporation of bomb 14 C. FFSR showed diurnal Δ 14 C variability (mean = 8.5‰), suggesting predominance of autotrophic respiration of recently fixed labile C. Calculations for Δ 14 C in Re (median = 12.7‰) demonstrate the predominance of C fixed from days to decades. Although the boreal forest stores significant amounts of C, most of it is in the soil organic layer and the vegetation, where it is cycled relatively fast. Only minimal amounts of recent C are incorporated into the mineral soil over long timescales despite the current stocks in soils being relatively old.

Plain Language Summary Boreal forests play a key role as an alternative to sequester carbon dioxide (CO 2 ) from the atmosphere and mitigate climate change. However, there is uncertainty on where this CO 2 is stored and the timescales at which it remains. To understand this, we studied the fate of the atmospheric CO 2 since the beginning of the nuclear era (1950s) by measuring the radiocarbon content of vegetation, fungi, soil, forest floor soil respiration and atmospheric air from a boreal forest stand in northern Sweden. We found that vegetation stores carbon from months (in leaves and moss) to decades (in litter layer and wood debris), while the soil organic layer presented carbon even older than a century. In contrast, mineral soils contained little recent carbon along with low content of organic carbon. The analysis of FFSR revealed that carbon comes mainly from recently fixed carbon, while the overall ecosystem respiration reflected a mix of carbon sources, ranging from days to decades. Overall, the boreal forest stores substantial amounts of carbon, but most of it moves relatively fast through the ecosystem. Only a small amount of new carbon is added to the mineral soil where it remains for long timescales.