SPN: A model for the study of soil-plant nitrogen fluxes in silage maize cultivation

The aim of this study was to adapt the Soil-Plant-Nitrogen (SPN) model to silage maize, as an explorative tool for evaluating the possibility of expanding maize cultivation to regions which historically have been too cold for maize production, and for estimating the effects of management practices as fertiliser and manure application on C and N fluxes. The model was validated using data from a 5-year maize experiment conducted in northern Germany. Available observations included shoot biomass and N yield, soil mineral N (0–90 cm) sampled in spring and autumn, and nitrate in the soil solution during winter; occasionally also the soil water content and ground water level were recorded. The SPN crop module, originally developed for barley production in Norway, was successfully adapted to maize. The strategy adopted was to assess the maize-specific algorithms for the radiation use efficiency (RUE) and the leaf area index (LAI) by statistical analysis of selected subsets of the available observations. In addition, parameters were modified with respect to crops characteristics. The baseline mineralisation of the SPN soil module was inspected with respect to the ability to predict both the observed plant N uptake and soil mineral N of the unfertilised plots. In order to predict the time course of plant N uptake it was necessary to incorporate a ploughing effect, as a transient mineralisation burst. The modified SPN was evaluated with respect to the whole range of mineral N and cattle slurry treatments. Nitrogen and biomass yields were successfully predicted. The dynamicity of the soil mineral N prediction was reasonable, but a statistical analysis of the model performance was hampered by the selective timing of the soil samples. The good fit of the time course of the plant N uptake under contrasting fertiliser treatments suggests that the predictions of soil N immobilisation and losses can also be assumed to be reasonable. The SPN model seems a suitable tool for the prediction of the effects of climate and management on the yield and on the soil-plant N economy of silage maize. However, under extreme combination of high radiation and low temperature or vice-versa, RUE estimation may be a source of uncertainty and more empirical studies are recommended.