NITRATE LEACHING FROM SANDY LOAM SOILS UNDER A DOUBLE-CROPPING FORAGESYSTEM ESTIMATED FROM SUCTION-PROBE MEASUREMENTS

Nitrate leaching from a double-cropping forage system was measured ove r a 2-year period (June 1994-May 1996) in the Northwest re ion of Port ugal using ceramic cup samplers. The crops were grown for silage makin g and include maize (from May to September) and a winter crop (rest of the year) consisting of a mixture of cereals and Italian ryegrass. Th e experiment was performed on two different sites with a history of ma ny years under the same crop and fertiliser management, but differing in the amounts of N applied as fertiliser and by regular cattle slurry applications. The annual nitrate leaching losses measured ranged from 154 to 338 kg N ha(-1). These amounts lead to annual mean concentrati ons between 22 and 41 mg NO3--NL-1 in the drained water. The coarse te xtured soils (sandy loams) and the climatic conditions of the region w ith more than 600 mm of drainage concentrated between October and Marc h, tended to promote the leaching of all the nitrate-N left in the soi l after the maize crop plus the N released by mineralization during th e winter period. On these soils, the minimum amount of drainage (neces sary to provide the complete leaching of all the nitrate-N in the soil profile in the end of summer), seems to be between 300 and 400 mm. Th e winter crops removed important quantities of N (83-116 kg N ha(-1)) but, due to their late establishment in autumn they did not succeed in taking up the nitrate-N left in the soil after the maize crop. Approa ches for reducing the nitrate leaching losses in this particular syste m are discussed.