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

To the best of our knowledge, one or more authors of this paper were federal employees when contributing to this work. This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Copernicus Publications on behalf of the European Geosciences Union. The published article can be found at: http://www.geoscientific-model-development.net/home.html.Accurate representation of soil organic matter\ud (SOM) dynamics in Earth system models is critical for future\ud climate prediction, yet large uncertainties exist regarding\ud how, and to what extent, the suite of proposed relevant\ud mechanisms should be included. To investigate how various\ud mechanisms interact to influence SOM storage and dynamics,\ud we developed an SOM reaction network integrated in\ud a one-dimensional, multi-phase, and multi-component reactive\ud transport solver. The model includes representations of\ud bacterial and fungal activity, multiple archetypal polymeric\ud and monomeric carbon substrate groups, aqueous chemistry,\ud aqueous advection and diffusion, gaseous diffusion, and adsorption\ud (and protection) and desorption from the soil mineral\ud phase. The model predictions reasonably matched observed\ud depth-resolved SOM and dissolved organic matter\ud (DOM) stocks and fluxes, lignin content, and fungi to aerobic\ud bacteria ratios. We performed a suite of sensitivity analyses\ud under equilibrium and dynamic conditions to examine the\ud role of dynamic sorption, microbial assimilation rates, and\ud carbon inputs. To our knowledge, observations do not exist\ud to fully test such a complicated model structure or to test the\ud hypotheses used to explain observations of substantial storage\ud of very old SOM below the rooting depth. Nevertheless,\ud we demonstrated that a reasonable combination of sorption\ud parameters, microbial biomass and necromass dynamics, and\ud advective transport can match observations without resorting to an arbitrary depth-dependent decline in SOM turnover\ud rates, as is often done. We conclude that, contrary to assertions\ud derived from existing turnover time based model formulations,\ud observed carbon content and Δ¹⁴C vertical profiles\ud are consistent with a representation of SOM consisting\ud of carbon compounds with relatively fast reaction rates, vertical\ud aqueous transport, and dynamic protection on mineral\ud surfaces