New decomposition rate functions based on volumetric soil water content for the ROMUL soil organic matter dynamics model

•ROMUL was applied with decomposition rate functions derived from a CO2 flux model.•A two-storage soil water model was developed with volumetric water content.•New elements enhance the description of the soil water limitation on decomposition.•New model improves soil carbon stock prediction in comparison with measured data.

The ROMUL decomposition model describes the flux of soil organic matter through the soil decomposition process separately for the organic layer (forest floor) and mineral soil. In the original ROMUL the effect of soil moisture on soil organic matter decomposition was described by using gravimetric soil water content, a parameter that is often difficult to obtain when the model is applied using continuous weather data. The new decomposition rate functions based on a soil respiration model use volumetric soil water content, replacing the original moisture functions with new ones. This paper also describes the development of a simple volumetric soil water model, with separate water storage compartments for the organic and mineral soil layers to better correspond with the structure of the ROMUL model. A volumetric soil model is also easier to adapt to various locations than the gravimetric soil water model.The new decomposition rate functions employing the new soil water model were tested by re-simulating a previous application of the ROMUL model to a forest site representing conditions in Southern Finland. Whereas the original ROMUL model underestimated the steady-state soil carbon stock of the site, the new model structure considerably reduced the decomposition rates, and the revised soil carbon storage estimates are now in good agreement with the measured data.