Radiocarbon Isotopic Disequilibrium Shows Little Incorporation of New Carbon in Mineral Soils of a Boreal Forest Ecosystem

• Main Authors: Peichl, Matthias, Smith, Paul
• Format: Article in Journal/Newspaper
• Language: English
• Published: AMER GEOPHYSICAL UNION 2024
• Subjects: Environmental Sciences (social aspects to be 507); Geosciences; Multidisciplinary; Northern Sweden
• Online Access: https://pub.epsilon.slu.se/35057/; https://pub.epsilon.slu.se/35057/1/tangarife%E2%80%90escobar-a-et-al-20240916.pdf

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 (C14 ${}{14}\mathrm{C}$) 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 CO2 ${\mathrm{C}\mathrm{O}}_{2}$ were collected at the Integrated Carbon Observation System station Svartberget in northern Sweden. Additionally, we analyzed the Delta 14 ${{\Delta }}{14}$C-CO2 ${\mathrm{C}\mathrm{O}}_{2}$ from incubated topsoil and forest floor soil respiration (FFSR) collected over a 24-hr cycle, and calculated the Delta 14 ${{\Delta }}{14}$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 C14 ${}{14}\mathrm{C}$, 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 C14 ${}{14}\mathrm{C}$. FFSR showed diurnal Delta 14 ${{\Delta }}{14}$C variability (mean = 8.5 parts per thousand), suggesting predominance of autotrophic respiration of recently fixed labile C. Calculations for Delta 14 ${{\Delta }}{14}$C in Re (median = 12.7 parts per thousand) 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.Boreal forests play a key role as an alternative to sequester carbon dioxide (CO2 ${\mathrm{C}\mathrm{O}}_{2}$) from the atmosphere and mitigate climate ...