The contribution of old carbon to respiration from arctic tundra following permafrost thaw

High latitude ecosystems contain up to 950 Pg of soil carbon (C) frozen within permafrost soil. This pool of C has been shown to be readily decomposable when thawed, and represents one of the more likely positive feedbacks to climate change. We hypothesized that sustained transfers of C to the atmosphere that could cause a significant positive feedback to climate change must have a large contribution of old C, which forms the bulk of the soil pool. We used radiocarbon measurements of carbon dioxide fluxes to detect the age and quantity of C respired from moist acidic tussock tundra near Denali National Park, Alaska. At this tundra site, permafrost has been observed to warm and thaw over the past several decades, causing the ground surface to subside as ice volume in the soil decreased. Ecosystem respiration radiocarbon values ranged from 58 permil to 114 permil with values below atmospheric occurring in early spring and late fall, suggesting a larger contribution to respiration from organic matter that was more than 50 years old during these periods. Radiocarbon from gas wells also showed a significant contribution of old organic matter to soil carbon dioxide, but this showed dramatic interannual variation, likely as a result of water table depth. In combination with the release of old C from the soil profile, there was positive net uptake of carbon dioxide during the growing season, but net release when integrated annually and accounting for winter respiration.