Fate of recently assimilated carbon in the soil-plant system of a boreal Podzol under Vaccinium vitis-idaea and its response to warming

Boreal dwarf shrubs like Vaccinium vitis-idaea L. constitute a significant component of forests in high latitude ecosystems, playing a crucial role in net carbon (C) assimilation and sequestration. Detailed understanding on C allocation patterns in these plant-soil systems and the response to warming is still lacking. Therefore, we conducted a translocation experiment combined with ¹³CO₂ pulse labelling to quantify (I) the allocation of ¹³C in the plant-soils system, (II) the ¹³C distribution in root tips and cells colonized by mycorrhizal fungi, and (III) the impact of warming on C allocation belowground. We translocated cores with soil and V. vitis-idaea from North- to South-Finland (+ 4 °C MAT). After 2.5 years, plants were pulse-labelled with ¹³CO₂. We traced ¹³C in plant and soil compartments and measured its distribution in fine roots. After 7 days, the highest share of assimilated C remained in the leaves, followed by the shoot, and only a small amount was transferred belowground. The ¹³C distribution in tips and cells with mycorrhizal fungi did not show a higher ¹³C-enrichment than other parts of the fine roots. Additionally, warming did not alter the distribution of C in the system, except for a higher uptake of ¹³C by microbes in the mineral soil. Our results indicate that dwarf shrubs in boreal forests allocate only a small share of recently assimilated C to roots and soil, and a short-term exposure to warming did not affect this allocation pattern.