STEM is a modified implementation of Hartig et al.'s (2019) Very Simple Ecosystem Model (VSEM). The vegetation part of the model has two stocks of biomass carbon (C): aboveground C and belowground C. The soil part of the model has a single stock of soil organic C. Carbon flows into the biomass C stocks via net primary productivity (NPP). Carbon flows out of these stocks and into the soil organic C stock via the loss of aboveground/belowground C through senescence (i.e., abscission of dead leaves and roots). SOC loss is due to heterotrophic respiration of the soil organic matter.
Reference:
Hartig, F., Minunno, F., and Paul, S. (2019). BayesianTools: General-Purpose MCMC and SMC Samplers and Tools for Bayesian Statistics. R package version 0.1.7. https://CRAN.R-project.org/package=BayesianTools