Winter wheat biomass and nitrogen dynamics under different fertilization and water regimes: application of a crop growth model

Growth, nitrogen uptake and nitrogen allocation between roots, stems, leaves and grains were measured and simulated in winter wheat on a clay soil in three treatments including daily irrigation and fertilization. Special emphasis was placed on biomass and nitrogen allocation within the crop and on the availability of soil nitrogen for crop growth. The model used for the simulations of growth (SOILN-CROP), which was driven by a hydrological model, is based on the light interception concept and empirical allometric functions. Growth is the driving force for nitrogen uptake, which is limited by the availability of mineral N in the soil. The model was calibrated for one treatment. Thereafter, the same parameter set was used to simulate the other two treatments. Frequent irrigation in combination with single-dose fertilization increased crop growth and N leaching compared with the non-irrigated but single-dose fertilized control, whereas irrigation together with daily fertilization increased crop growth and N uptake but not N leaching. Simulated soil mineral N levels agreed well with measurements on a 1-year time scale. Assimilate allocation to roots decreased logarithmically with total crop biomass in all treatments. Allocation to leaves decreased linearly with total above-ground crop mass. The crop availability of mineral N differed considerably between treatments. The model parameter defining the proportion of soil mineral N available for plant uptake had a strong influence on model behaviour. This proportion is indicated to depend on soil water content and the mechanisms of this relation need to be considered in future work to improve our predictions of N uptake.