Simulating trends of soil organic carbon in seven long-term experiments using the SOMM model of the humus types

Using the SOMM model of the humus types (Mor, Moder, Mull), simulations of SOM dynamics at seven long-term experiments were attempted as a part of a model evaluation exercise. The model comprises three compartments (undecomposed litter, partially humified litter, humus of mineral topsoil) and considers six processes of mineralization and humification as influenced by litter nitrogen and ash contents, soil C/N ratio, temperature and moisture. Results of simulations performed without any site-specific calibration mostly underestimate soil organic matter (SOM). Despite this lack of precision, general trends in SOM over time were often captured. Success of simulation was limited by difficulties in obtaining direct information about soil moisture regime, litter quantity and litter quality (nitrogen and ash content). Inadequacies in the model, such as no consideration of soil properties, may also account for some of the errors. Despite the limited success of the evaluation, it is encouraging that the model conceived for forested ecosystems, with additional development, may be applicable to non-forested systems. The exercise underlines the necessity of constant direct contact between modeller and data holder for a success of an evaluation.