Modelling nitrogen dynamics in soil-crop systems with HERMES

Model runs with HERMES were performed over entire crop rotation cycles for two experimental sites on loamy and sandy soils in Germany with differently managed plots. The model was able to simulate soil water and nitrogen contents on the sandy plots of Muncheberg with an index of agreement (IA) > 0.8 and > 0.69. Crop growth and N-uptake was simulated well with IA values > 0.89 and > 0.75, respectively. For the loess site in Bad Lauchstadt model results for above-ground biomass, storage organ and N-uptake agreed well with observations over a 4-year rotation with IA values of 0.93, 0.94 and 0.71, respectively. Soil mineral nitrogen was significantly overestimated on the cropped plot (IA = 0.45) as well as on the black fallow plot (IA = 0.65) using the default initialization of the decomposable nitrogen pools from the organic matter content. Equilibration of the pools, using data from a neighbouring long term experiment, improved soil mineral nitrogen simulation to an IA of 0.72 for the cropped and 0.91 for the fallow plot. The long term model performance was investigated using data from 1903 to 2002 of four differently managed plots in Bad Lauchstadt. Soil organic carbon, derived from simulated nitrogen pools, showed acceptable results for the unfertilized plot, but a distinct underestimation on plots with farmyard manure application. Simulated historical winter wheat and potato yields were distinctly overestimated during the initial 50 years. Therefore, an adoption of crop parameters for older varieties is necessary. The index of agreement of 0.9 indicates that the annual weather impact on yield fluctuations was correctly reflected.