Nitrate content of soil water under forage crops fertilised with dairy slurry in nitrate vulnerable zone

Three contiguous plots established on highly permeable soil in the Po valley (Italy), and equipped with tensiometers and ceramic cup samplers, have been cropped respectively with lucerne (Medicago sativa), tall fescue (Festuca arundinacea), and their mixture. Dairy slurry was applied three times during the second year (at the end of the winter and after the 1st and 4th cut), with total amounts of 272 kg N ha-1 for tall fescue and 136 kg N ha-1 (half rate) for the mixture. Lucerne was not fertilised with nitrogen. Total yields for the 5 cuts were 17.8 t DM ha-1 for lucerne (average protein content 18.2% DM), 10.9 for tall fescue (protein 12.6 % DM) and 19.2 for the mixture (protein 17.0 % DM). Soil water nitrate content was steadily low under each of the three forage crops, with average values ranging from 5 (tall fescue plot) to 10 (lucerne plot) mg NO3-N l-1, without significant negative impact on groundwater. Activities are currently being carried out to verify the effect of repeated application rates higher than the 170 kg N ha-1 year-1 limit fixed for NVZ in Nitrate Directive 91/676.