Microbial dynamics and organo-mineral interactions strongly regulate soil organic matter (SOM) cycling. However, prevailing Earth system model (ESM) land models usually do not explicitly represent microbial processes and their interactions with abiotic processes, making the SOM predictions highly uncertain. To better represent soil-microbe-plant interactions, we integrated a microbe- and mineral-surface-explicit model, ReSOM (the Reaction-network-based model of SOM and microbes), into the Energy Exascale Earth System Model (E3SM) land model (ELM) via the multi-phase and multi-substrate reactive transport solver BeTRv2 (the Biogeochemical Transport and Reaction model).

With ELM-ReSOM, we first demonstrate the critical role of microbe-mineral interactions on subsurface SOM dynamics under different warming scenarios. Then, we show that model simulations of CO2 efflux and soil carbon stocks along soil depth agree well with observations at the Blodgett montane conifer forest in California, including its long-term (9-year) whole-soil warming experiment. The ELM-ReSOM simulation results reasonably captured the observed increase in respiration resulting from long-term whole soil warming at the heated plots. We also explore the difference between microbially implicit and explicit models regarding microbial decomposition and respiration response to temperature changes, highlighting the need to represent microbial dynamics and coupled biotic and abiotic interactions in ESM soil biogeochemistry models. Finally, by demonstrating the uncertainty associated with model representations of microbial temperature sensitivity, mineral-surface adsorption capacity, affinity parameters, and substrate transport, we discuss the challenges that microbe-explicit models face to accurately predict the soil carbon cycle response to future warming across space and time.