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Abstract. Recent studies have examined temporal fluctuations in the amount and carbon isotope content (δ 13

The magnitude of future emissions of greenhouse gases from the northern permafrost region depends crucially on the mineralization of soil organic carbon (SOC) that has accumulated over millennia in these perennially frozen soils. Many recent studies have used radiocarbon (14 C) to quantify the release of this "old" SOC as CO 2 or CH 4 to the atmosphere or as dissolved and particulate organic carbon (DOC and POC) to surface waters. We compiled~1,900 14 C measurements from 51 sites in the northern permafrost region to assess the vulnerability of thawing SOC in tundra, forest, peatland, lake, and river ecosystems. We found that growing season soil 14 C-CO 2 emissions generally had a modern (post-1950s) signature, but that well-drained, oxic soils had increased CO 2 emissions derived from older sources following recent thaw. The age of CO 2 and CH 4 emitted from lakes depended primarily on the age and quantity of SOC in sediments and on the mode of emission, and indicated substantial losses of previously frozen SOC from actively expanding thermokarst lakes. Increased fluvial export of aged DOC and POC occurred from sites where permafrost thaw caused soil thermal erosion. There was limited evidence supporting release of previously frozen SOC as CO 2 , CH 4 , and DOC from thawing peatlands with anoxic soils. This synthesis thus suggests widespread but not universal release of permafrost SOC following thaw. We show that different definitions of "old" sources among studies hamper the comparison of vulnerability of permafrost SOC across ecosystems and disturbances. We also highlight opportunities for future 14 C studies in the permafrost region.

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Radiocarbon is an exceptionally useful tool for studying soil-respired CO 2 , providing information about soil carbon turnover rates, depths of production, and the biological sources of production through partitioning. Unfortunately, little work has been done to thoroughly investigate the possibility of inherent biases present in current measurement techniques, like those present in δ 13 CO 2 methodologies, caused by disturbances to the soil's natural diffusive regime. This study investigates the degree of bias present in four 14 C sampling chamber methods using a three-dimensional numerical soil-atmosphere CO 2 diffusion model. The four chambers were tested in an idealized, surrogate reality by assessing measurement bias with varying Δ 14 C and δ 13 C signatures of production, collar lengths, soil biological productivity rates, and soil diffusivities. The static and Iso-FD chambers showed almost no isotopic measurement bias, significantly outperforming dynamic chambers, which demonstrated biases up to 200‰ in some modeled scenarios. The study also showed that 13 C and 14 C diffusive fractionation are not a constant multiple of one another, but that the δ 13 C correction still works in diffusive scenarios because the change in fractionation is not large enough to impact measured Δ 14 C values during chamber equilibration.