Multiple models and experiments underscore large uncertainty in soil carbon dynamics
Soils contain more carbon than plants or the atmosphere, and sensitivities of soil organic carbon (SOC) stocks to changing climate and plant productivity are a major uncertainty in global carbon cycle projections. Despite a consensus that microbial degradation and mineral stabilization processes control SOC cycling, no systematic synthesis of long-term warming and litter addition experiments has been used to test process-based microbe-mineral SOC models. We explored SOC responses to warming and increased carbon inputs using a synthesis of 147 field manipulation experiments and five SOC models with different representations of microbial and mineral processes. Model projections diverged but encompassed a similar range of variability as the experimental results. Experimental measurements were insufficient to eliminate or validate individual model outcomes. While all models projected that CO2 efflux would increase and SOC stocks would decline under warming, nearly one-third of experiments observed decreases in CO2 flux and nearly half of experiments observed increases in SOC stocks under warming. Long-term measurements of C inputs to soil and their changes under warming are needed to reconcile modeled and observed patterns. Measurements separating the responses of mineral-protected and unprotected SOC fractions in manipulation experiments are needed to address key uncertainties in microbial degradation and mineral stabilization mechanisms. Integrating models with experimental design will allow targeting of these uncertainties and help to reconcile divergence among models to produce more confident projections of SOC responses to global changes.
KeywordsSoil organic carbon Warming Modeling Meta-analysis Litter addition Decomposition
This study emerged from two INTERFACE RCN (US NSF DEB-0955771) workshops held in 2016. C. Averill was supported by the National Oceanographic and Atmospheric Administration Climate and Global Change Postdoctoral Fellowship Program, administered by Cooperative Programs for the Advancement of Earth System Science (CPAESS), University Corporation for Atmospheric Research (UCAR), Boulder, Colorado, USA. A. Classen and J. Moore were supported by U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Sciences program under Award Number DE-SC0010562. R. Abramoff, W.J. Riley, and J. Tang were supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research as part of the Terrestrial Ecosystem Science Program under Contract No. DE-AC02-05CH11231. B. Sulman was supported by award NA14OAR4320106 from the National Oceanographic and Atmospheric Administration, U.S. Department of Commerce. G. Wang and M. Mayes were supported by the U.S. Department of Energy Office of Biological and Environmental Research through the Oak Ridge National Laboratory (ORNL) Terrestrial Ecosystem Science Scientific Focus Area. ORNL is managed by the University of Tennessee-Battelle, LLC, under contract DE-AC05-00OR22725 with the US DOE. W. Wieder was supported by the National Institute of Food and Agriculture, Grant/Award Number: 2015-67003-23485. K. Georgiou was supported by the U.S. Department of Energy Office of Science Graduate Student Research program (contract DE-SC0014664) and the USDA National Institute of Food and Agriculture postdoctoral program. Individual litter addition and warming experiments that produced data for the publications we analyzed were supported by funding sources too numerous to list here. We thank K. Lajtha for helpful feedback on the study. Thanks to Diana Swantek for graphic design work on Fig. 1. Thanks to two anonymous reviewers for their helpful comments on the manuscript.
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