Simulated cereal nitrogen uptake and soil mineral nitrogen after clover-grass leys

Simulations were made to test the effects of age and composition of red clover (Trifolium pratense) based leys on yield of two subsequent spring cereal crops, as well as nitrogen (N) uptake and soil mineral N content. The experimental plots in two trials were cropped for 2-3 years with spring cereals, or 1-, 2- or 3-year-old red clover based leys, followed by spring wheat and subsequent spring oats. CoupModel, a process oriented ecosystem model, was calibrated with measured values of above ground N uptake and soil mineral N contents from plots of cereal monoculture. Cereal N uptake was simulated for a 2 year period in cereals after leys. The calculations of N inputs in incorporated plant material of leys were also tested. Simulated N uptake in the above ground biomass generally agreed with the field data with default values of the model. Some parameters were increased in order to improve plant N uptake and keep the soil mineral N contents at the measured levels. The simulated soil mineral N contents were close to the measured values for surface layers and were more accurate than for deeper layers in the profile. However, the high simulated mineral N increase after harvest in one trial was not seen in field measurements, which remains difficult to explain. Most probably the C: N estimate for crop residues was set too low in the model, but calculated N input was on a reasonable level. These results show that further testing and adjusting of N dynamics in organic farming system using CoupModel should be continued.