#L03 Formulation, implementation, and testing of an intermediate complexity model for robust soil biogeochemistry modeling

Abstract

Robust soil biogeochemical (BGC) modeling is essential for credible analyses of carbon-climate feedbacks. We previously identified a numerical inconsistency in the formulation of ALM-v0 soil BGC, which has resulted in a misinterpretation of the carbon-nitrogen coupling, leading to an uncertainty that has a magnitude comparable to that associated with model parameterizations and climate forcing. This inconsistency has been inherited by ALM-v1, and cannot easily be fixed given the non-modular ALM-v1 formulations. We here propose and implement a modular intermediate complexity soil BGC formulation that rectifies this numerical inconsistency systematically, while also improving representation of (1) the litter chemistry effect on SOM decomposition and stabilization, (2) woody decomposition dynamics, (3) nitrification and denitrification, and (4) mineral surface, plant, and phosphorus interactions. The new formulation includes a re-interpretation of the CENTURY-like BGC formulation by linking the fast and intermediate soil organic matter (SOM) pools with non-phenol-like and phenol-like dissolved organic matter (DOM) pools, and explicitly considers the effect of lignin content on structural litter carbon decomposition. Three different pools are introduced to respectively represent woody carbon input from dead fine woody branches, dead large woody branches, and dead coarse woody roots, enabling a better coupling to the ecosystem demographic model (FATES) that will be used in ALM.  The DOM pools are then used to fuel the denitrification process, with explicit consideration of sorptive interaction with mineral soils. The overall biogeochemistry is then recast into the Peterson matrix form, and is solved with the newly designed stoichiometry based flux-limiting ordinary differential equation solver to ensure consistent substrate limitation to all involved biogeochemical processes. The overall BGC model is implemented in the ALMv1 reactive transport solver BeTR, with a coherent treatment of various transport pathways for potential linking with the atmospheric and river components of ACME. We also show some preliminary simulations and discuss how to use a set of simple tests to check the robustness of this BGC formulation.