A process-based modeling of soil organic matter physical properties for land surface models – Part 1: Soil mixture theory

Abstract. Numerous studies have highlighted the critical role of soil organic matter (SOM) physical properties in simulating hydrological and energy exchanges within Earth system models. However, current approaches in their land surface model (LSM) components typically rely on empirically derived parameterizations that lack physical consistency and often fail to distinguish between soil organic carbon (SOC) and total SOM. This conceptual simplification leads to inaccurate estimates of the volumetric organic fraction of soils and, consequently, of their physical properties as highlighted in this study. A process-based framework grounded in soil mixture theory is thus proposed to provide a physically consistent representation of the effects of SOM on soil behavior. The volumetric fraction of SOM is derived using mass-volume relationships, combined with an SOC-to-SOM conversion based on recent pedotransfer functions. For LSMs using the Brooks and Corey model to simulate soil water retention and hydraulic conductivity, new parameterizations are proposed for SOM hydrodynamic properties as functions of bulk density and depth, informed by recent observational datasets. Validation against experimental binary mixtures and large in situ datasets shows significant improvements over conventional methods. Designed for compatibility with global soil databases, the framework enables more physically consistent SOM representation in LSMs without requiring additional inputs or calibration.