Using a new soil biogeochemical model to understand processes controlling SOC dynamics: an application to the LBNL TES SFA Warming Experiment

Soil organic carbon (SOC) is the largest actively cycling terrestrial carbon pool. However, accurately assessing how this large carbon pool responds to climate change, such as warming, is difficult. Most soil biogeochemical (BGC) models simulate SOC dynamics based on decay rates of conceptually defined organic matter pools and lack explicit representation of microbial processes. Further, previous studies have demonstrated that soil minerals affect SOC dynamics through interactions with microbes. Ignoring microbial and mineral-surface processes can result in unrealistic BGC responses to both sudden and gradual changes in environmental drivers. To provide realistic simulations of dynamic soil physical, chemical, and biological processes and their responses to environmental change, we report here on a new microbe and mineral-surface explicit soil BGC submodel integrated in the E3SM land model using the BeTR reactive transport framework. We applied the new model to both control and heated (+4℃) plots of a whole-profile soil-warming experiment at Blodgett forest in the foothills of the Sierra Nevada, California. We found good quantitative predictive skill for SOC stocks, microbial biomass, soil-gas CO₂ concentrations, and CO₂ surface fluxes. Seasonal cycles of CO₂ surface fluxes and vertical patterns of soil-gas CO₂ concentration profiles from field observations were also accurately captured. Further, based on how they affect the model-data agreement in sensitivity analysis, we inferred the relative importance of microbial and mineral processes to SOC dynamics, and how they control SOC dynamics. We expect our results will help better understand the sensitivities of SOC stocks to temperature change and organo-mineral interactions.