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Earth system scientists working with radiocarbon in organic samples use a stable carbon isotope (δ13C) correction to account for mass-dependent fractionation, but it has not been evaluated for the soil gas environment, wherein both diffusive gas transport and diffusive mixing are important. Using theory and an analytical soil gas transport model, we demonstrate that the conventional correction is inappropriate for interpreting the radioisotopic composition of CO2 from biological production because it does not account for important gas transport mechanisms. Based on theory used to interpret δ13C of soil production from soil CO2, we propose a new solution for radiocarbon applications in the soil gas environment that fully accounts for both mass-dependent diffusion and mass-independent diffusive mixing.

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 (14C) to quantify the release of this “old” SOC as CO2 or CH4 to the atmosphere or as dissolved and particulate organic carbon (DOC and POC) to surface waters. We compiled ~1,900 14C 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 14C‐CO2 emissions generally had a modern (post‐1950s) signature, but that well‐drained, oxic soils had increased CO2 emissions derived from older sources following recent thaw. The age of CO2 and CH4 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 CO2, CH4, 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 14C studies in the permafrost region., Key Points: We compiled ~1,900 14C measurements of CO2, CH4, DOC, and POC from the northern permafrost region. Old carbon release increases in thawed oxic soils (CO2), thermokarst lakes (CH4 and CO2), and headwaters with thermal erosion (DOC and POC). Simultaneous and year‐long 14C analyses of CO2, CH4, DOC, and POC are needed to assess the vulnerability of permafrost carbon across ecosystems., EC | H2020 | H2020 Priority Excellent Science | H2020 European Research Council (ERC) http://dx.doi.org/10.13039/100010663, Gouvernement du Canada | Natural Sciences and Engineering Research Council of Canada (NSERC) http://dx.doi.org/10.13039/501100000038, National Science Foundation (NSF) http://dx.doi.org/10.13039/100000001

Recent warming in the Arctic, which has been amplified during the winter1–3, greatly enhances microbial decomposition of soil organic matter and subsequent release of carbon dioxide (CO2)4. However, the amount of CO2 released in winter is not known and has not been well represented by ecosystem models or empirically based estimates5,6. Here we synthesize regional in situ observations of CO2 flux from Arctic and boreal soils to assess current and future winter carbon losses from the northern permafrost domain. We estimate a contemporary loss of 1,662 TgC per year from the permafrost region during the winter season (October–April). This loss is greater than the average growing season carbon uptake for this region estimated from process models (−1,032 TgC per year). Extending model predictions to warmer conditions up to 2100 indicates that winter CO2 emissions will increase 17% under a moderate mitigation scenario—Representative Concentration Pathway 4.5—and 41% under business-as-usual emissions scenario—Representative Concentration Pathway 8.5. Our results provide a baseline for winter CO2 emissions from northern terrestrial regions and indicate that enhanced soil CO2 loss due to winter warming may offset growing season carbon uptake under future climatic conditions. Winter warming in the Arctic will increase the CO2 flux from soils. A pan-Arctic analysis shows a current loss of 1,662 TgC per year over the winter, exceeding estimated carbon uptake in the growing season; projections suggest a 17% increase under RCP 4.5 and a 41% increase under RCP 8.5 by 2100.

Recent studies have examined temporal fluctuations in the amount and carbon isotope content (δ13C) of CO2 produced by the respiration of roots and soil organisms. These changes have been correlated with diel cycles of environmental forcing (e.g., sunlight and soil temperature) and with synoptic-scale atmospheric motion (e.g., rain events and pressure-induced ventilation). We used an extensive suite of measurements to examine soil respiration over 2 months in a subalpine forest in Colorado, USA (the Niwot Ridge AmeriFlux forest). Observations included automated measurements of CO2 and δ13C of CO2 in the soil efflux, the soil gas profile, and forest air. There was strong diel variability in soil efflux but no diel change in the δ13C of the soil efflux (δR) or the CO2 produced by biological activity in the soil (δJ). Following rain, soil efflux increased significantly, but δR and δJ did not change. Temporal variation in the δ13C of the soil efflux was unrelated to measured environmental variables, and we failed to find an explanation for this unexpected result. Measurements of the δ13C of the soil efflux with chambers agreed closely with independent observations of the isotopic composition of soil CO2 production derived from soil gas well measurements. Deeper in the soil profile and at the soil surface, results confirmed established theory regarding diffusive soil gas transport and isotopic fractionation. Deviation from best-fit diffusion model results at the shallower depths illuminated a pump-induced ventilation artifact that should be anticipated and avoided in future studies. There was no evidence of natural pressure-induced ventilation of the deep soil. However, higher variability in δ13C of the soil efflux relative to δ13C of production derived from soil profile measurements was likely caused by transient pressure-induced transport with small horizontal length scales.

Radiocarbon is an exceptionally useful tool for studying soil-respired CO2, 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 δ13CO2 methodologies, caused by disturbances to the soil's natural diffusive regime. This study investigates the degree of bias present in four 14C sampling chamber methods using a three-dimensional numerical soil-atmosphere CO2 diffusion model. The four chambers were tested in an idealized, surrogate reality by assessing measurement bias with varying Δ14C and δ13C 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 13C and 14C diffusive fractionation are not a constant multiple of one another, but that the δ13C correction still works in diffusive scenarios because the change in fractionation is not large enough to impact measured Δ14C values during chamber equilibration.

Stable carbon isotopes have become a critical and often used tool in understanding ecological and physical processes affecting gas production and emissions in soil. While the insights gained using chamber based flux methods have been significant, it is known now that many of these chamber methods have an inherent bias that complicates the interpretation of their measurements. Here we present a new chamber method that uses diffusive membranes to control CO2 flow into and out of the chamber, and can measure the isotopic composition of soil flux without inducing a bias. We present numerical modeling, followed by laboratory calibration and field measurements using this new method coupled to a Cavity Ring Down Spectrometer (CRDS). Simulations, as well was lab and field results showed that the method is both robust over a range of environmental conditions and can be unbiased, unlike other chamber approaches. Finally, we discuss possibilities for future improvements and variations on the measurement approaches we used.