Calibration and validation of the soil organic matter dynamics of the Daisy model with data from the Askov long-term experiments

Daisy is a semi-mechanistic model that simulates crop production, soil water and C and N dynamics in agro-ecosystems. In this study, the soil organic matter (SOM) submodel of Daisy was calibrated and validated with data from a series of long-term (30–100 yr) experiments dominated by cereal cropping systems. The experiments were conducted under temperate conditions at the Askov Experimental Station, Denmark. The recalibration of the Daisy model improved the model simulations considerably. The RMSE between simulations and observations of the experiments used for calibration improved from 33 to 19, whereas this measure improved from 32 to 9.0 for the experiments used for validation. The original calibration of the model appears to be based upon too low an input of organic matter (especially because rhizodeposition is ignored), too low an efficiency with which added organic matter is converted into more stable forms of SOM, and too low an overall decomposition rate. Our study suggests that the fraction of SOM involved in medium-term turnover is substantially larger than previously thought and inferred by most SOM turnover models. This may warrant a recalibration of current simulation models, and stresses the importance of using long-term experiments with widely differing treatments for model evaluation. Long-term treatments with widely contrasting initial C contents and annual C inputs, especially bare fallow treatments, appear to be valuable resources in this respect.