Radiocarbon isotopic disequilibrium shows little incorporation of carbon in soils and fast cycling in a boreal forest ecosystem

Boreal forests, as critical components of the global carbon (C) cycle, fix annually substantial amounts of atmospheric C. However, the timescales at which this C is cycled through the various ecosystem compartments are yet not well understood. To elucidate the temporal dynamics between photosynthesis, allocation and respiration of C, we assessed the radiocarbon (¹⁴C) to understand the fate of C in a boreal forest ecosystem. Samples from a boreal forest stand at the ICOS station Svartberget (SVB) in northern Sweden were collected, including vegetation, soil cores, atmospheric CO₂ and the ¹⁴CO₂ values from incubated topsoil were used to interpret D in different ecosystem pools. Additionally, we conducted comprehensive analyses of Δ¹⁴CO₂ released from forest floor soil respiration (FFSR) over a 24-hour cycle and calculated the Δ¹⁴C signature of the total ecosystem respiration following the Miller-Tans approach. We show that vegetation pools presented a positive D indicated by the enrichment with bomb ¹⁴C (produced mostly between 1950 and 1964), suggesting a fast-cycling rate (in the order of months to years) for living biomass and intermediate for dead biomass (years to decades). In contrast, soils showed a negative D, indicating minimal incorporation of bomb ¹⁴C. FFSR showed diurnal Δ¹⁴C variability with an average value close to the atmosphere (-2.33‰ in summer 2022 at SVB), suggesting that the output flux is dominated by autotrophic respiration of recently fixed and post-bomb labile C. Calculations for Δ¹⁴C in ecosystem respiration (166 ± 66.2‰), which is enriched in comparison to FFSR, in ecosystem respiration. Although the boreal forest stores significant amounts of C in the soil, , where it is cycled relatively fast. Only minimal amounts of recent C are incorporated and stabilised over long time scales. The potential of the boreal forest to mitigate climate change has to be further studied emphasizing the critical role of soil organic carbon persistence, where the ecosystem-atmosphere ¹⁴C disequilibrium may provide powerful insights.