Shrub and tree encroachment alter plant-soil interactions in low Canadian Arctic

Rapid expansion of deciduous shrubs and evergreen trees on the Arctic tundra could induce large losses of soil carbon stocks through increased rhizosphere priming. Through the use of isotopic and molecular techniques, we investigated whether the belowground carbon cycling differed between three plant species that are encroaching Canadian tundra. ¹³CO₂ pulse chase labelling showed that dwarf shrubs (Betula glandulosa) had faster turnover of recent ¹³C-photosynthates belowground than tall shrubs (Alnus viridis) and black spruces (Picea Mariana). Depth-resolved ¹³C flux estimations and partial ¹³C source isolation, both from field and lab measurements, elucidated multiple drivers of the differences in belowground carbon cycling. Turnover rates were strongly dependent on relative belowground carbon allocation, source of respiration and soil depth. Carbon cycling data will be compared with microbial community composition in bulk and rhizosphere soil to disentangle the specific interactions between encroaching plants and their soils. Overall, both plant and soil characteristics were key influences on the fate of recently assimilated carbon belowground. Our work suggests that changing plant communities will influence the belowground carbon cycling of the Arctic tundra. Our data pinpoints towards multiple factors influencing the feedback from northern ecosystems to on-going climate change, which further complicates accurate predictions of soil carbon losses in the northern hemisphere.