Seasonal variation of ecosystem respiration delta 13C in response to experimental permafrost thaw and vegetation removal in moist acidic tundra

Abstract:

Permafrost soils store twice as much carbon (C) as is contained in the atmosphere and about one-third of global soil C. Under a warmer future climate, permafrost is expected to thaw and decompose, releasing C to the atmosphere, further amplifying global warming. However, studies show that warmer arctic temperatures promote plant growth, in addition to stimulating losses from the soil C pool. Using delta ¹³C of ecosystem respiration (Reco) during the seasonal cycle of active layer thaw, we seek to understand the effect of permafrost thaw on the relative contributions from microbial decomposition of soil C and more recently fixed, plant-dominated C.

We measured weekly CO₂ flux rates and delta ¹³C of Reco from experimentally warmed plots with rapid permafrost thaw and control thaw. Vegetation removal plots, in un-warmed tundra, were monitored to isolate the seasonal contributions from soil alone. We expected delta ¹³C to be dominated by plant activity in vegetated plots, particularly in areas with greater permafrost thaw because they have highest plant biomass. In vegetation removal plots we expected to see greater contribution from deep soil as seasonal thaw progressed.

From May to July delta ¹³C was extremely variable early in the growing season, but became more uniform as vegetation greened and thaw deepened. In vegetated plots CO₂ fluxes doubled, but remained constant in vegetation removal plots. This indicates that, with thaw, microbes had access to a more spatially uniform C substrate, but this had little effect on the magnitude of CO₂ flux. Overall delta ¹³C in rapidly thawed plots was least enriched (-29.4 ‰), control plots intermediate (-28.9 ‰), and vegetation removal plots were most enriched (-28.5 ‰). This suggests that in vegetation removal plots microbes used more decomposed soil C as substrate, and much of the increase in CO₂ flux in vegetated plots was the result of C recently fixed and contributed by plants.