Sustainable management of nitrogen fertilization in irrigated forage systems in nitrate vulnerable zones

The research was conducted in an intensive dairy farming Nitrate Vulnerable Zone (NVZ) under irrigated Mediterranean conditions focusing on the irrigated maize-Italian ryegrass cropping system. The aims of study were i) quantification of the cropping system nitrogen use efficiency (NUE) of organic fertilizers in relation to the prescriptions imposed by the Nitrate Directive (91/676/CEE), ii) assessment of the nitrate leaching dynamics under the maize-ryegrass cropping system in relation to contrasting fertilization systems, iii) assessment of the relations, at territorial scale, between N surplus and NO3 concentration in groundwater.Four fertilization systems were compared at field scale: slurry+mineral, slurry, farmyard manure and mineral at a target rate of 315+130 kg ha-1 N for maize and ryegrass respectively. Fertilization rates of organic fertilizers varied according to their variable N content. NUE of organic fertilizers was not significantly different from that of mineral for maize if N rates were near to target. Organic fertilizers had very low NUE for the ryegrass.The soil water nitrate concentration was very variable in time with minimum in spring and maximum in autumn-winter. At district area, the average N surplus was closely correlated with the N input.The study provided sufficient evidence that N pollution of groundwater occurs in autumn-winter and that there are margins to increase the crop N use efficiency by reducing the actual N inputs without significant reduction of crop yield, but with increased costs for the disposal of effluents outside the NVZ.