A Microbial-Explicit Soil Organic Carbon Decomposition Model (MESDM) Coupled with Noah-MP: Development and Testing in Semiarid Grasslands

Explicit representation of soil microbial activity and respiration for use in earth system models has received increasing attention, since it remains one of the greatest uncertainties for predicting climate-carbon feedbacks. Microbial-explicit models have been increasingly used at both site- and global-scale studies at from hourly to daily timescales. However, it is still a challenge to improve parameterization of soil heterotrophic respiration response to drying-rewetting cycle in these models. Moreover, few models have been incorporated into land surface model with high soil vertical resolution to validate the simulation of soil respiration further rigorously using observations at field site under natural drying-rewetting cycle. In this study, we develop a microbial-explicit SOC decomposition model (MESDM) and incorporate MESDM into Noah-MP land surface model (LSM) instead of the first-order decay model, and test against half-hourly observed soil CO₂ concentration at various depths and net ecosystem exchange (NEE) as well as derived soil CO₂ efflux at the Santa Rita Experimental Range in semiarid ecosystem characterized by the pulsed precipitation and high temperature. In MESDM, we parameterize two main aspects: 1) soil moisture controls on enzyme-catalyzed soil organic carbon (SOC) depolymerization to dissolved organic carbon (DOC) and microbial uptake of DOC that is limited by O₂ diffusion and 2) both soil moisture and substrate controls on microbial dormancy and transformation between dormant and active microbial biomasses. A vertically-layered MESDM model are constructed for incorporation into Noah-MP, with coupling with O₂ and CO₂ gas transport models to calculate O₂ and CO₂ concentrations at various depths and CO₂ efflux. The results show that the Noah-MP coupled with the MESDM can successfully simulate the observed soil respiration pulses of various sizes in response to precipitation, thereby improving the simulation of NEE at Santa Rita site in semiarid grassland.