Simulation of water and nitrogen flows and plant growth for a winter wheat stand in central Germany

Two linked simulation models were applied to an experimental loam site at Neuenkirchen, Germany, for the years 1989 and 1991, using a data set with soil and plant properties for winter wheat. Simulations of soil water and heat dynamics were performed with the SOIL model. Both years were rather dry. For the first year, soil water content and soil water tension were calibrated by tuning the saturated hydraulic conductivity. The simulations showed good correspondence with the measurements during the first six months down to 130 cm depth. However, the dynamics of the simulated water tension in deeper layers was poor compared to the measurements. Using the same parameterization for 1991, the model simulated soil moisture dynamics reasonably well in the upper 70 cm. Below this depth, the water content was underestimated. Soil and plant nitrogen dynamics, litter decomposition and plant biomass production were simulated with the SOILN model. The model was tested against measurements of leaf, stem and grain biomass and nitrogen, leaf area and soil nitrate and ammonium content. It was calibrated for the first year (1989) and validated in 1991. Parameters related to translocation of assimilates and nitrogen from different tissues to grain were estimated by calibration. The simulated plant properties in 1989 explained about 95% of the variations in measured values. A similar agreement was obtained for the validation year. The corresponding value for soil mineral nitrate in 1989 was only 38%, mainly due to problems in reproducing the dynamics in the upper 30 cm in connection to fertilization applications. In deeper layers up to 90% of the variation was explained. For nitrate in 1991 and ammonium the agreement between simulated and measured values was poor.