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The biological recalcitrance or decomposability of soil organic carbon (SOC) determines its lability and thus C cycling in an ecosystem. Many studies of temperate and tropical soils have reported that SOC from deeper soil is older and more recalcitrant than surface soil. However, it is not clear whether this trend holds for arctic tundra soils. We report results of C isotope measurements and incubation experiments of selected soil samples to determine the relationship between SOC decomposition and its age. Soil organic carbon content was greater in surface soil layers than deeper layers. The 14C age of SOC ranged from modern near the surface to 6,920 years before present at the depth of 87 to 105 cm. However, the SOC of the frozen or deep layer (>20 cm) was also biologically labile compared to the top active layer. Our data reveal that for arctic tundra soils the relationship between the age of SOC and its recalcitrance is opposite to the trend for most temperate and tropical soils, indicating that deep permafrost carbon is highly vulnerable to decomposition in response to climate warming and arctic coastal erosion. [ABSTRACT FROM AUTHOR]