Risk assessment of nitrate leaching in autumn after incorporation of catch crops in spring

In Flanders, the current environmental policy allows farmers to apply manure after harvest of winter cereals at a maximum rate of 60 kg N ha-1, if a catch crop is sown before September. The N uptake by the catch crop is assumed to prevent nitrate leaching during autumn and winter. After incorporation of the catch crop, its easily decomposable N is mineralized and is expected to become available for the next spring crop. N mineralization was measured through incubation experiments at 15°C for white mustard (Sinapsis alba), Italian ryegrass (Lolium multiflorum), black oat (Avena strigosa) and a grass-clover mixture (Lolium perenne, Trifolium repens, Trifolium pratense). The relative amount of N mineralized after 3 months was negatively correlated with the C:N-ratio of the catch crops. Immobilization was important and long lasting for black oat and to a lesser extent for white mustard. N release from these catch crops could therefore occur late in the season and thus delay the risk of nitrate leaching. To assess effects of sowing date and manure application rates, soil mineral N content in summer and autumn will be monitored for corn grown after catch crop incorporation in spring. Both black oat and white mustard were sown on 2 dates and pig slurry was applied at 3 different rates. Fallow treatments were included. Soil mineral N content and aboveground N were determined before incorporation in April. Mineral N profiles will be used to assess whether delayed risks of nitrate leaching exist with these specific catch crops.