The Millennial model: in search of measurable pools and transformations for modeling soil carbon in the new century
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Soil organic carbon (SOC) can be defined by measurable chemical and physical pools, such as mineral-associated carbon, carbon physically entrapped in aggregates, dissolved carbon, and fragments of plant detritus. Yet, most soil models use conceptual rather than measurable SOC pools. What would the traditional pool-based soil model look like if it were built today, reflecting the latest understanding of biological, chemical, and physical transformations in soils? We propose a conceptual model—the Millennial model—that defines pools as measurable entities. First, we discuss relevant pool definitions conceptually and in terms of the measurements that can be used to quantify pool size, formation, and destabilization. Then, we develop a numerical model following the Millennial model conceptual framework to evaluate against the Century model, a widely-used standard for estimating SOC stocks across space and through time. The Millennial model predicts qualitatively similar changes in total SOC in response to single factor perturbations when compared to Century, but different responses to multiple factor perturbations. We review important conceptual and behavioral differences between the Millennial and Century modeling approaches, and the field and lab measurements needed to constrain parameter values. We propose the Millennial model as a simple but comprehensive framework to model SOC pools and guide measurements for further model development.
KeywordsModeling Soil carbon Organic matter Microbial activity Decomposition Global change
The Millennial model code, model inputs, and the model output used in this manuscript are archived at a GITHUB Repository (https://github.com/email-clm/Millennial) that is publicly accessible. The authors would like to thank the Carbon Cycle Interagency Working Group, via the US Carbon Cycle Science Program under the auspices of the US Global Change Research Program, for providing funding for the “Celebrating the 2015 International Decade of Soil – Understanding Soil’s Resilience and Vulnerability,” workshop held at the University Corporation for Atmospheric Research in Boulder, CO, USA on 14–16 March 2016. We would also like to thank the University Corporation for Atmospheric Research for providing meeting space, as well as the 36 workshop participants, William J. Riley, and three anonymous reviewers for helpful comments and discussion. Lawrence Berkeley National Laboratory is managed and operated by the Regents of the University of California under Contract DE-AC02-05CH11231 with the US Department of Energy. Argonne National Laboratory is managed by UChicago Argonne, LLC, under contract DE-AC02-06CH11357 with the US Department of Energy. Oak Ridge National Laboratory is managed by the University of Tennessee-Battelle, LLC, under Contract DE-AC05-00OR22725 with the US Department of Energy.
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Conflict of Interest
The authors declare that they have no conflict of interest.
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