Modelling biomass, water and nitrogen in grass ley: Estimation of N uptake parameters

The N concentration over the 2-year life cycle of grass swards was modelled assuming plant total N uptake to be the lowest value of the plant N demand and the availability of soil mineral N. The variability in the N demand and N availability parameters was estimated for seven site conditions in Central and Southern Sweden, at three N addition levels (0, 14 and 20 g N m−2 year−1, respectively) by calibration to measured above ground N concentration. The N demand was a function of plant growth, plant current N concentration and day length. Plant growth and biomass were taken from growth model predictions calibrated to measured aboveground biomass. The N availability was assumed to be a fraction of the soil mineral N within the root zone, which was simulated with soil water and nitrogen models calibrated against previous soil model applications.The plant N maximum concentration (nMax) was successfully modelled as a decreasing function of increasing biomass and decreasing day length in combination. The fraction of available soil mineral N taken up per day under N limiting conditions (cUptake) decreased after cutting. The parameters varied between sites, cuts and N treatment levels. The effects of these variations on predicted seasonal N uptake was most pronounced for cUptake and sites, and least for nMax and cuts. It is concluded that cUptake needs to be calibrated for new site conditions in order to accurately predict plant N uptake.