An integrated model for the management of nitrogen fertilization in leafy vegetables.

Optimal nitrogen (N) supply is crucial for high yield, produce quality and crop sustainability of leafy vegetables. In spinach (Spinacia oleracea L.), N availability in the root zone is positively correlated with biomass production, tissue minerals and antioxidants content, and other commercial quality attributes such as leaf colour and wrinkledness. An erroneous N supply may cause crop N deficiency or overfertilization, which in turn results in waste of fertilizers with increased production costs and environmental impact. Nevertheless, growers very often supply N at constant rate based on empirical protocols, without taking into consideration climate variables and crop rotations. During a four-year long study (2007-2011), we assembled, calibrated and validated an integrated model (DSS, decision support system) that simulates N concentration in the root zone (daily basis) and gives information on the amount of N that should be delivered to the crop for maintaining the desired (i.e. optimal) N mineral content in the root zone. Simulations run on the basis of climate parameters, chemical-physical soil characteristics, and crop characteristics. Main outputs of the DSS are: crop cumulated biomass and N uptake, soil water balance and nitrogen balance. The DSS was tested on spinach grown in open field, in sandy-loam soil, under Mediterranean climate conditions. A significant linear correlation was found between simulated and measured data. The DSS was effective to support the management of N fertilization: in some cases, it allowed a reduced N supply in comparison to standard protocols applied by local growers, with no significant reduction in yield and quality.