Subheading: Reduced carbon uptake by soils during the 21st century One Sentence Summary:Global radiocarbon observations show that Earth system models, lacking carbon stabilization mechanisms, overestimate the 21st century soil carbon sink by almost two-fold. Abstract:Soil is the largest terrestrial carbon reservoir and may influence the sign and magnitude of carbon cycle-climate feedbacks. Changes in soil carbon-the largest terrestrial carbon reservoir-may influence the sign and magnitude of climate-carbon cycle feedbacks. Many Earth system models (ESMs) estimate a significant soil carbon sink by 2100, yet the underlying carbon dynamics determining this response have not been systematically tested against observations. Using We used  14 C data from 157 globally distributed soil profiles sampled to 1 m depth, we to show that ESMs underestimated the mean age of soil carbon by more than six-fold (430±50 years vs. 3100±1800 years). Consequently, ESMs overestimated the carbon sequestration potential of soils 21 st century soil carbon sequestration by nearly two-fold (40 ± 27%). These biasesinconsistencies suggest that ESMs must better represent carbon stabilization processes and the turnover time of slow and passive reservoirs when simulating future atmospheric CO 2 dynamics.To improve simulations of future atmospheric CO 2 and carbon storage, ESMs must better represent stabilization processes and turnover times for soil carbon pools.Keywords: soil carbon, earth system models, carbon-concentration feedback, mean age, Rapid rates of carbon sequestration in ESMs contrast with findings from CO 2 and warming experiments (7, 8) as well as multiple theoretical and observational constraints indicating slow (millennial) rates of soil organic carbon accrual and turnover (9-14). Model uncertainty-as measured by inter-model spread-is high for soil carbon turnover time (τ) and exceeds the uncertainty estimated for carbon uptake through gross primary production (GPP) (15, 16).In coupled model simulations, the relative sink strength (i.e. percentage change in soil carbon) depends on the responses of net primary production (NPP) and soil carbon dynamics to increasing atmospheric CO 2 concentrations and to a lesser extent climate change (5). Elevated warms the climate, which tends to accelerate soil carbon turnover and reduce carbon storage (the carbon-climate feedback) (17,18). Although these feedbacks oppose one another, the carbonconcentration feedback is more than 4 times greater on average than the carbon-climate feedback in current ESMs at the global scale (3). Differences in the representation of elevated CO 2 versus climate effects on ecosystem processes result in substantial variation in soil carbon sequestration estimates (19) ( Table S1).Without a strong carbon-concentration feedbacks, ESMs would likely project smaller gains or larger losses of soil carbon over the 21 st century. Our aim was to constrain the magnitude of the soil carbon-concentration feedback with soil radiocarbon observations. Radiocarbon content can b...

Abstract. Radiocarbon is a critical constraint on our estimates of the timescales of soil carbon cycling that can aid in identifying mechanisms of carbon stabilization and destabilization and improve the forecast of soil carbon response to management or environmental change. Despite the wealth of soil radiocarbon data that have been reported over the past 75 years, the ability to apply these data to global-scale questions is limited by our capacity to synthesize and compare measurements generated using a variety of methods. Here, we present the International Soil Radiocarbon Database (ISRaD; http://soilradiocarbon.org, last access: 16 December 2019), an open-source archive of soil data that include reported measurements from bulk soils, distinct soil carbon pools isolated in the laboratory by a variety of soil fractionation methods, samples of soil gas or water collected interstitially from within an intact soil profile, CO2 gas isolated from laboratory soil incubations, and fluxes collected in situ from a soil profile. The core of ISRaD is a relational database structured around individual datasets (entries) and organized hierarchically to report soil radiocarbon data, measured at different physical and temporal scales as well as other soil or environmental properties that may also be measured and may assist with interpretation and context. Anyone may contribute their own data to the database by entering it into the ISRaD template and subjecting it to quality assurance protocols. ISRaD can be accessed through (1) a web-based interface, (2) an R package (ISRaD), or (3) direct access to code and data through the GitHub repository, which hosts both code and data. The design of ISRaD allows for participants to become directly involved in the management, design, and application of ISRaD data. The synthesized dataset is available in two forms: the original data as reported by the authors of the datasets and an enhanced dataset that includes ancillary geospatial data calculated within the ISRaD framework. ISRaD also provides data management tools in the ISRaD-R package that provide a starting point for data analysis; as an open-source project, the broader soil community is invited and encouraged to add data, tools, and ideas for improvement. As a whole, ISRaD provides resources to aid our evaluation of soil dynamics across a range of spatial and temporal scales. The ISRaD v1.0 dataset is archived and freely available at https://doi.org/10.5281/zenodo.2613911 (Lawrence et al., 2019).

Arctic wetlands are currently net sources of atmospheric CH4 . Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH4 emissions and gross CH4 processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH4 production, oxidation, and surface emissions in Arctic polygon tundra, across a wet-to-dry permafrost degradation gradient from low-centered (intact) to flat- and high-centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH4 and CO2 fluxes, concentrations and stable isotope compositions of CH4 and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH4 emissions, a different primary methanogenic pathway, and greater CH4 oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH4 flux decreased from 64 nmol m(-2)  s(-1) in intact polygons to 7 nmol m(-2)  s(-1) in degraded polygons, and stable isotope signatures of CH4 and DIC showed that acetate cleavage dominated CH4 production in low-centered polygons, while CO2 reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH4 emissions.