Coupling vegetation dynamics, soil microbial processes, and physicochemical stabilization mechanisms within the new NOAA/GFDL Global Integrated Microbial Interactions with Carbon in Soil (GIMICS) model

The large carbon storage in terrestrial soils underscores the need for mechanistic soil process representations in Earth System Models (ESMs) aimed at simulating carbon-climate feedbacks under changing climate and land use. However, ESMs participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) still exhibit large uncertainties in simulating historical and future soil carbon stocks, reflecting incomplete process understanding and discrepancies between model assumptions and emerging empirical knowledge. 

We present the recently developed NOAA/GFDL Global Integrated Microbial Interactions with Carbon in Soil (GIMICS) model, which integrates key advances in soil biogeochemistry and addresses limitations of the previous soil component, CORPSE, in GFDL ESM4.1. GIMICS has been incorporated into the CMIP7-class GFDL ESM4.5. GIMICS explicitly represents soil microbial dynamics and physicochemical stabilization mechanisms, as well as interactions among microbes, minerals, and vegetation, for both aboveground organic materials (leaf and coarse wood litter) and belowground, vertically resolved mineral soils (rhizosphere and bulk soil). GIMICS also accounts for the effects of spatially and vertically varying soil temperature and moisture on microbial processes and organic matter turnover, and represents redistribution and transport via bioturbation, diffusion, advection, and runoff to rivers. 

GIMICS has been integrated with the GFDL Land Model LM4.2, which includes dynamic vegetation and wildfire processes. The resulting LM4.2-GIMICS configuration is evaluated in a stand-alone mode against a widely used, observationally derived global soil inventory dataset (Harmonized World Soil Database, HWSD), reported global synthesis estimates, and other global model results. Results show improved simulations of global and regional soil carbon stocks relative to models that do not explicitly represent microbial processes and mineral-associated organic matter (MAOM) stabilization. Analyses of pool–specific carbon stocks and fluxes highlight the critical role of soil mineral–microbe–vegetation interactions in regulating terrestrial carbon persistence.