Effects of repeated dairy slurry applications on forage crops in a nitrate vulnerable zone of Northern Italy

In the spring of 2005 three different swards of lucerne (Medicago sativa L.), tall fescue (Festuca arundinacea Schreb.) and a mixture of the two were grown on three contiguous 30x50 m plots on highly permeable soil in the Po valley (Northern Italy). Each plot was equipped with tensiometers to measure soil potential at depths of up to 180 cm and ceramic cup samplers to collect soil water up to 500 cm. Dairy slurry was applied three times during both 2006 and 2007 (at the end of the winter and after theist and 4th cut), with total amounts significantly exceeding the Nitrate Directive limit (about double) for tall fescue and close to the same limit for the mixture. Lucerne was not fertilised with nitrogen. Five cuts were effected in 2006 and four in 2007. Production was very good for lucerne and the mixture, both in quantity and quality, for both years. Tall fescue in the mixture was overwhelmed by lucerne after the first cut. Pure tall fescue production was markedly lower. Soil water nitrate content was steadily low under each of the three forage crops, with average values ranging from 5 (tall fescue plot) to 11 (lucerne plot) mg NO3-N L-1 without significant negative impact on groundwater quality. The results obtained confirm that forage crops, in particular some grasses, fertilised with nitrogen from slurry applied at rates of more than 170 kg N ha 1 year-1, are able to deplete nitrates in the soil and in the soil water in vulnerable soils. Such crops can thus be qualified as useful in protecting groundwater from infiltration by these pollutants.