Radiocarbon constraints imply reduced carbon uptake by soils during the 21st century

Abstract

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...

A vertical representation of soil carbon in the JULES land surface scheme (vn4.3_permafrost) with a focus on permafrost regions

“…However, this then limit the usefulness of the soil carbon observations for model evaluation. Alternative methods of model evaluation need to be developed such as those discussed by He et al (2016) and Carvalhais et al (2014).…”

“…However, this deep soil carbon now consists of both "original" permafrost carbon and "newer" active carbon which, in reality, are likely to have different qualities (Harden et al, 2012). This tracking of soil carbon could also be used for more detailed model evaluation -see, for example, He et al (2016).…”

A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils

“…However, a portion of the soil organic carbon (SOC) reservoir may not contribute significantly to the net exchange of CO 2 and CH 4 between atmosphere and land during the next century because its residence time exceeds 100 years and its rate of carbon inputs is low (Trumbore, 1997;He et al, 2016). The size of this centennially persistent SOC pool is presumed to be large (i.e.…”

“…The importance of better information on the size of the centennially persistent SOC pool has been emphasized recently (International Soil Carbon Initiative, 2011;Bailey et al, in press;Bispo et al, 2017;Harden et al, in press), stressing the need for operational and standardized metrics or proxies to accurately quantify SOC persistent at the centennial timescale. The general lack of information on the size and turnover rate of measurable SOC pools hampers the 25 initialization of SOC pools in dynamic models, questioning their predictions of the evolution of the global SOC reservoir (Falloon and Smith, 2000;Luo et al, 2014;Feng et al, 2016;He et al, 2016;Sanderman et al, 2016). Luo et al (2016) and He et al (2016) recently claimed that optimizing parameter estimation with global data sets on SOC pools and fluxes was the highest priority to reduce biases among Earth system models.…”

“…The general lack of information on the size and turnover rate of measurable SOC pools hampers the 25 initialization of SOC pools in dynamic models, questioning their predictions of the evolution of the global SOC reservoir (Falloon and Smith, 2000;Luo et al, 2014;Feng et al, 2016;He et al, 2016;Sanderman et al, 2016). Luo et al (2016) and He et al (2016) recently claimed that optimizing parameter estimation with global data sets on SOC pools and fluxes was the highest priority to reduce biases among Earth system models.…”