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Long residence times of rapidly decomposable soil organic matter: application of a multi-phase, multi-component, and vertically resolved model (BAMS1) to soil carbon dynamics

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  • Accurate representation of soil organic matter (SOM) dynamics in Earth system models is critical for future climate prediction, yet large uncertainties exist regarding how, and to what extent, the suite of proposed relevant mechanisms should be included. To investigate how various mechanisms interact to influence SOM storage and dynamics, we developed an SOM reaction network integrated in a one-dimensional, multi-phase, and multi-component reactive transport solver. The model includes representations of bacterial and fungal activity, multiple archetypal polymeric and monomeric carbon substrate groups, aqueous chemistry, aqueous advection and diffusion, gaseous diffusion, and adsorption (and protection) and desorption from the soil mineral phase. The model predictions reasonably matched observed depth-resolved SOM and dissolved organic matter (DOM) stocks and fluxes, lignin content, and fungi to aerobic bacteria ratios. We performed a suite of sensitivity analyses under equilibrium and dynamic conditions to examine the role of dynamic sorption, microbial assimilation rates, and carbon inputs. To our knowledge, observations do not exist to fully test such a complicated model structure or to test the hypotheses used to explain observations of substantial storage of very old SOM below the rooting depth. Nevertheless, we demonstrated that a reasonable combination of sorption parameters, microbial biomass and necromass dynamics, and advective transport can match observations without resorting to an arbitrary depth-dependent decline in SOM turnover rates, as is often done. We conclude that, contrary to assertions derived from existing turnover time based model formulations, observed carbon content and Δ¹⁴C vertical profiles are consistent with a representation of SOM consisting of carbon compounds with relatively fast reaction rates, vertical aqueous transport, and dynamic protection on mineral surfaces.
  • Keywords: Microbial communities, Subsoil horizons, Mineral soils, Spatial variability, Grassland soils, Climate change
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  • Riley, W. J., Maggi, F., Kleber, M., Torn, M. S., Tang, J. Y., Dwivedi, D., and Guerry, N. (2014). Long residence times of rapidly decomposable soil organic matter: application of a multi-phase, multi-component, and vertically resolved model (BAMS1) to soil carbon dynamics, Geoscientific Model Development, 7, 1335-1355. doi:10.5194/gmd-7-1335-2014
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  • 7
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  • 4
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  • This research was supported by the Director,Office of Science, Office of Biological and EnvironmentalResearch of the US Department of Energy under Contract #DEAC02-05CH11231 as part of the Terrestrial Ecosystem ScienceProgram including the Next-Generation Ecosystem Experiments(NGEE-Arctic) project. Federico Maggi and Nathan Guerry werepartly supported by the 2012–2013 International Program DevelopmentFund of the University of Sydney, Australia. The contributionsof Markus Kleber were supported by a research fellowship fromthe Institute of Soil Landscape Research, Leibniz-Center forAgricultural Landscape Research (ZALF), 15374 Müncheberg,Germany.
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