Accounting for empirical global soil organic characteristics and moisture heterogeneities in soil organic decomposition scheme of land surface models

Abstract. Below ground soil organic carbon (OC) decomposition is governed by several biophysical drivers, causing difficulties to accurately capture the spatial patterns of soil OC stock and of CO₂ flux in Earth System models (ESMs). These biophysical drivers influence soil OC decomposition due to the respiration of heterotrophic organisms. Formulation in global scale process-based models of these processes consist of functions that modify the soil OC decay rate and therefore the soil heterotrophic respiration (HR) which modify global soil OC stock estimated by models. Current soil HR modifiers employed in models are a single relationship between soil moisture and the rate of decomposition that are employed for all the ecosystem types. Observational database meta-analysis relationships of SOIL MOISTURE and soil HR has been established considering observed soil physical properties. These relationships serve to define an empirical model that consists of a collection of different relationships based on soil organic carbon content, clay fraction and bulk density in order to uniquely substitute SOIL MOISTURE control on soil HR with a function modifier that reflects soil HR spatial heterogeneity.

In the present study, this empirical model has been embedded in the land surface model Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE). The effect of the multivariate approach on simulation results has been assessed on soil OC stock and soil HR estimations at global scale. Results show that global soil OC stocks are nearly doubled in the modified model version, which is closer to observations-based products compared with the initial version, while CO₂ emissions, due to soil HR, are unchanged. The latitudinal soil OC distribution is maintained, displaying as much soil OC stock in tropical regions as under higher latitudes. This study demonstrates the significance of secondary drivers in the relationship between SOIL MOISTURE and the soil HR response to enable accounting for soil OC stock and CO₂ fluxes heterogeneous spatial pattern.