At present, scientific interest is focused on studying the carbon (C) balance in the biosphere due to climate change processes. Therefore advancing soil organic matter (SOM) dynamics modelling is critical for quantification of the belowground C cycle to terrestrial ecosystems. Most soil C models do not include stable SOM resulting from the activities of soil biota. We proposed a modelling approach based on the stoichiometric relations among the processes of SOM formation that are mediated by soil biota (Romul_Hum model. Ecol. Mod. 2017. 345, 113-140). Soil respiration, as a core rate variable in all SOM models, was associated with the production of micro-and meso-faunal excrement and necromass in soil food webs and earthworm casts as precursors of stable SOM. A food-web based module was developed, using a synthesis of published data. An anecic earthworm module was developed, with representation of processes in fresh casts. The contributions of these modules into stable SOM formation was integrated into the structure of the existing ROMUL model (Chertov et al., 2001). Testing and application of the new Romul_Hum model for a post-agrogenic soil chronosequence with mull Luvisols showed that about 15% of total C and 30-40% total N in SOM of the Ah horizon is fauna derived. In a Podzol, these values were 11% and 22%, respectively, in the Ae horizon, and 8% and 33%, respectively, in the mor forest floor. These results show that soil fauna are significant agents in stabilization of SOM and should be included in soil C models.