Potential carbon release from permafrost soils of Northeastern Siberia

Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and may cause positive feedback to warming as CO2 is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15 degrees C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO2 with radiocarbon (C-14) ages ranging between similar to 21 and 24 ka BP was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q(10) value of 1.9 +/- 0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions of Delta(CO2)-C-14 values in respiration CO2 and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO2 in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5 degrees C, about 1 Pg C will be initially released from labile C pools, followed by respiration of similar to 40 Pg C to the atmosphere over a period of four decades.