Site: Bonanza Creek LTER
Photograph of typical thermokarst feature in the Noatak Valley, Alaska.

At least 1218 Pg (billion tons) of soil carbon (C) are stored in surface permafrost soils in boreal and arctic ecosystems, almost twice as much C than currently contained in the atmosphere (Tarnocai et al. 2009).

Latitudinal gradients of soil C storage, field experiments, and laboratory incubations all show that soil C cycling in these northern ecosystems is likely to be strongly influenced by the effect of cold temperature on rates of decomposition of soil organic matter. This ‘old’ soil C, climatically protected from microbial decomposition in frozen or waterlogged soil, has been accumulating in these ecosystems throughout the Holocene, and for much longer in some unglaciated areas. The BNZ LTER report results from a tundra landscape undergoing permafrost thaw, where net ecosystem C exchange and the radiocarbon age of ecosystem respiration were measured to determine the influence of old C loss on ecosystem C balance (Schuur et al. 2009). Sustained transfers of C to the atmosphere that could cause a significant positive feedback to climate change must come from old C, which forms the bulk of the permafrost C pool that accumulated over thousands of years (Schuur et al. 2008). Areas that thawed over the past 15 years had 40% more annual losses of old C compared to minimally thawed areas, but had overall net ecosystem C uptake as increased plant growth offset these losses. In contrast, sites that thawed decades earlier lost even more old C, a 78% increase over minimally thawed areas, which contributed to overall net ecosystem C release despite increased plant growth. These data document significant losses of soil C with permafrost thaw that, over decadal time scales, overwhelms increased plant C uptake at rates that could make permafrost a large biospheric C source in a warmer world, similar in magnitude in the future to current C fluxes from land use change. At present, increasing greenhouse gases responsible for climate change are largely a result of human activities.

However, climate change may alter the natural cycling of carbon in ecosystems far from direct human influence. This research is key for understanding how terrestrial system feedbacks will interact with human emissions, and may influence policy-driven emission mandates aimed at controlling the overall rate of climate change.

Graph for
Old carbon loss and its relationship to total ecosystem respiration for three sites that differ in the extent of permafrost thaw. Growing-season loss of old C from deeper in the soil profile, based on statistical partitioning estimates of mean proportional old C loss multiplied by ecosystem respiration (Reco) flux measurements. Error bars represent the spatial variability of Reco fluxes. The relationship between total Reco and proportional old C loss for the growing season across sites. Error bars represent the interannual variability in C loss estimates; the regression line is shown for n=3 sites.