Replacing organic with mineral N fertilization does not reduce nitrate leaching in double crop forage systems under Mediterranean conditions

This research evaluated the impact of four nitrogen (N) fertilizer management systems on nitrate losses in an irrigated forage system under Mediterranean conditions within a Nitrate Vulnerable Zone (NVZ). The experiment was conducted from June 2009 to May 2012 in an intensive dairy cattle farm that produces silage maize and Italian ryegrass in a double cropping system. A monthly monitoring of the nitrate concentrations in the soil solution was carried out using 10cm diameter disk lysimeters. The N fertilization systems had a target N application of 316 and 130kgha-1 for maize and Italian ryegrass, respectively. Four systems were compared: cattle manure (MA); cattle slurry (SL); cattle slurry+mineral N fertilizer (SM); mineral N fertilization (MI). A clear seasonal dynamics of nitrate concentration was observed in the three years and was similar among treatments, with maximum occurring in autumn-winter. On average, at soil depths between 50 and 90cm, nitrate concentrations in the soil solution were intermediate for the MI treatment (146±10.4mgL-1), in between those of SL or SM (202±11.3 and 164±9.4mgL-1, respectively) and MA (78±4.9mgL-1). Despite the high average concentrations, only in some sampling dates the nitrate concentration was significantly higher than 50mgL-1. The estimated annual N leaching losses below 90cm soil depth ranged from 42 (MA) to 110 (SL)kgNha-1. These findings highlighted that, under Mediterranean conditions and with the N input rates, nitrate leaching in autumn-winter cannot be easily controlled through N fertilizer management because it is mainly associated to the natural water surplus and the low N uptake from the winter crop. The cattle manure has proved to be the most conservative in terms of N leaching, while replacing organic with mineral sources of N did not reduce nitrate leaching.