Nitrate leaching from a maize × oats double-cropping forage system fertilized with organic residues under Mediterranean conditions

Further information about the environmental effects and best management options for agricultural use of organic residues in Mediterranean areas is required. A field experiment was carried out over two winter seasons in central Portugal, to evaluate NO3-N leaching losses originating from the soil application of cattle slurry (CS), sewage sludge (SS) and urban waste compost (UWC) to a double-cropping system producing oats and maize. A mineral fertilizer treatment (MIN) and a Control were included. The use of a nitrification inhibitor (DCD) with MIN (MIN + I) and CS (CS + I), an organic residue with high C/N ratio (pulp mill sludge – PMS) together with mineral fertilizer (PMS + M), and split application of residues (SS and UWC), were evaluated as measures to mitigate N leaching. Total fertilizer N applied in treatments were 80 and 170 kg N ha−1 to oats (Avena sativa L.) and maize (Zea mays L.), respectively. Water samples were collected at 0.70 m deep, using porous ceramic cups. The highest annual NO3 losses were measured in MIN + I, PMS + M and MIN (185, 152 and 145 kg N ha−1, respectively) and with the split application of SS (145 kg N ha−1). In Control the maximum loss was 50 kg N ha−1. The use of DCD in spring fertilization is not recommended. The N lost by leaching and runoff following the application of organic residues was less than to 30% of N applied (IPCC factor). Compared to mineral N application, the use of organic residues in agriculture with an appropriate management can contribute to lower nitrate losses.

► An adequate use of organic residues in traditional agriculture can contribute to reduce nitrate losses.
► Application of DCD in spring can promote N leaching losses in autumn.
► The use of residues with high C/N ratio with mineral N fertilizers is not a durable solution to reduce N leaching.
► To reduce N leaching losses sewage sludge should be used in a single soil application in spring.
► Nitrate leaching from a maize × oats double-cropping forage system fertilized with organic residues under Mediterranean conditions.