Beyond clay: towards an improved set of variables for predicting soil organic matter content
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Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.
KeywordsSoil organic matter Biogeochemistry Carbon cycle
This work was conducted as a part of the “What Lies Below? Improving quantification and prediction of soil carbon storage, stability, and susceptibility to disturbance” Working Group supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the U.S. Geological Survey. Additional support was provided by NSF EAR-1331408 and EAR- 1123454 to Rasmussen, NSF CAREER BCS-1349952 to Marin-Spiotta, US Department of Agriculture NIFA 2015-67003-23485 and US Department of Energy TES DE-SC0014374 to Wieder, and USDA-NIFA Hatch project HAW01130-H to Crow. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
This work is the result of two workshops sponsored by the USGS John Wesley Powell Center for Analysis and Synthesis in May of 2016 and May of 2017. The motivation and ideas for this work were generated collaboratively among all authors during these workshops. Rasmussen led manuscript development, data compilation, and analysis. All authors contributed to writing/editing, statistical analyses, and figure development.
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