Shrub Expansion in Arctic Alaska Alters the Sources (14C) and Magnitudes of Ecosystem Respiration in the Continuous Permafrost Zone

Abstract:

Soils in the Arctic store up to 50% of global soil organic carbon (C) (1,672 Pg C) and more than twice the amount of C currently present in the atmosphere. At the core of this enormous C pool is the large fraction of C that has been frozen in permafrost and disconnected from the active C cycle. As the Arctic undergoes warming and wetting, permafrost C is at risk of being transferred to the atmosphere, resulting in a strong positive feedback to global climate. One of the most commonly observed responses to climate warming in the Arctic is an increase in woody shrub growth and corresponding changes in the surface energy budget and shifts in belowground processes. Increased shrub cover is accompanied by a number of soil physical and chemical changes, including decreased growing season soil temperature due to soil shading, warmer winter soil temperature due to snow accumulation around shrubs, and increased rooting depth. Therefore, shrub expansion may feedback to alter the amount of C stored in arctic ecosystems, but the direction and magnitude of this potential feedback are highly uncertain.

To address this uncertainty, we established four sites across a shrub cover gradient near Toolik Lake, AK, USA, ranging from herbaceous tussock to deciduous shrub tundra. At each site we measured the rate and isotope composition (¹³C/¹²C, ¹⁴C/¹²C) of ecosystem, root and microbial respiration with chambers and incubations, the concentration and isotope composition of soil CO₂ with wells, and the C and N concentration and isotopic composition of bulk organic matter in the active layer and upper permafrost with EA-IRMS. We used the isotope values to partition ecosystem CO₂ emissions into respiration from plants, young surface soil, and old deep soil. We found that rates of ecosystem respiration increase with increasing shrub cover. Furthermore, preliminary results indicate that the proportional contribution of plants and old soil to ecosystem respiration changes with increasing shrub density. Taken together, our results suggest that ongoing shrub expansion will alter permafrost C dynamics in arctic ecosystems. Forthcoming winter measurements will provide insights into the consequences of shrub expansion for the annual C budget of the Arctic.