The value of simulating the soil water and nitrogen dynamics in decision support systems for plot-scale agro-environmental management

Nitrogen N losses from vegetable fields, primarily due to nitrate leaching, represent one of the most challenging problems that modern agriculture faces. Decision Support Systems (DSS) developed to guide plot-scale fertilization and minimize N losses are based on estimates of crop water/N potential requirements during the growth cycle. Irrigation/fertilization strategies can be designed based on these estimates, assuming that all applied water/N is used by the crop. Losses due to deep drainage or leaching are overlooked, but could be significant in open-field crop systems. In that case, water/N availability in the root zone could be short compared to crop requirements and production could fall below potential levels. To avoid the caveats of requirement-based systems, this study presents a system that uses the water and N available in the root zone as control variables to design irrigation/fertilization schedules. The DSS uses a modified version of the VegSyst crop model that explicitly simulates the dynamics of water/N in the soil profile and crop growth under water/N stress. The modified VegSyst model was calibrated and validated against observations collected during a series of field trial experiments conducted in Granada, Spain. The agreement indexes in these exercises were, in general, above commonly accepted limits in environmental modeling. On average, availability-based DSS could reduce water/N use by 18% and 15%, respectively, compared to requirement-based DSS. Global efficiency indexes for fertilization/irrigation strategies designed using availability-based DSS were 20 points higher than those designed using requirement-based DSS.