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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 12 | Copyright
Biogeosciences, 15, 3659-3671, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 18 Jun 2018

Research article | 18 Jun 2018

Field-warmed soil carbon changes imply high 21st-century modeling uncertainty

Katherine Todd-Brown1, Bin Zheng1, and Thomas W. Crowther2 Katherine Todd-Brown et al.
  • 1Pacific Northwest National Laboratory, Richland, WA 99354, USA
  • 2Institute of Integrative Biology, ETH Zürich, Univeritätstrasse 16, 8006, Zürich, Switzerland

Abstract. The feedback between planetary warming and soil carbon loss has been the focus of considerable scientific attention in recent decades, due to its potential to accelerate anthropogenic climate change. The soil carbon temperature sensitivity is traditionally estimated from short-term respiration measurements – either from laboratory incubations that are artificially manipulated or from field measurements that cannot distinguish between plant and microbial respiration. To address these limitations of previous approaches, we developed a new method to estimate soil temperature sensitivity (Q10) of soil carbon directly from warming-induced changes in soil carbon stocks measured in 36 field experiments across the world. Variations in warming magnitude and control organic carbon percentage explained much of field-warmed organic carbon percentage (R2 = 0.96), revealing Q10 across sites of 2.2 [1.6, 2.7] 95% confidence interval (CI). When these field-derived Q10 values were extrapolated over the 21st century using a post hoc correction of 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth system model outputs, the multi-model mean soil carbon stock changes shifted from the previous value of 88±153Pg carbon (weighted mean±1SD) to 19±155Pg carbon with a Q10-driven 95%CI of 248±191 to −95±209Pg carbon. On average, incorporating the field-derived Q10 values into Earth system model simulations led to reductions in the projected amount of carbon sequestered in the soil over the 21st century. However, the considerable parameter uncertainty led to extremely high variability in soil carbon stock projections within each model; intra-model uncertainty driven by the field-derived Q10 was as great as that between model variation. This study demonstrates that data integration should capture the variation of the system, as well as mean trends.

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The temperature sensitivity of soil carbon loss is a critical parameter for projecting future CO2. Isolating soil temperature response in the field is challenging due to difficulties isolating root and microbial respiration. We use a database of direct-warming soil carbon changes to generate a new global temperature sensitivity. Incorporating this into Earth system models reduces projected soil carbon. But it also shows that variation due to this parameter is as high as all other causes.
The temperature sensitivity of soil carbon loss is a critical parameter for projecting future...