Evolution of soil mineral nitrogen under different catch crops fertilized with pig slurry after harvest of winter cereals in northern Belgium

In temperate humid climates, catch crops have been proven to be a useful tool in the abatement of soil erosion, nutrient leaching and soil organic carbon losses. In Flanders (northern Belgium), the environmental policy allows farmers to apply manure after harvest at a rate of 60 kg N/ha, if they sow a catch crop before the 1st of September (on light textures) or 15th of October (on heavy clay). This will only remain possible if nitrate leaching losses are not larger then when no additional manure is applied. To this end, field experiments were set up on 4 locations with different soil textures. Winter cereals were followed by 4 different catch crops: white mustard (Sinapis alba), Italian ryegrass (Lolium multiflorum), black oat (Avena strigosa) and a grass-clover mixture (Lolium perenne, Trifolium repens, Trifolium pratense), sown at 2 different dates, 2-4 weeks apart. Before sowing, pig slurry was applied in 2 rates, corresponding to about 60 and 120 kg N/ha (60N, 120N), and a control without manure application was included (0N). Wheather conditions were exceptionally favorable for catch crop growth. Soil mineral nitrogen in the 0-90cm layer (Nmin) was monitored during autumn 2011, winter and early spring 2012. From October to January, Nmin was significantly larger (p < 0.05) on the fallow plots compared to the plots with early sown catch crops for all cases; this was not always the case for late sown catch crops: in particular Italian ryegrass and the grass-clover mixture had relatively high Nmin contents. The effect of manure application depended strongly on sowing date too: under early sown catch crops Nmin showed no significant differences between 0N, 60N and 120N, with the exception of 1 single case. For late sown catch crops, Nmin was in some cases significantly different between 0N and 120N treatments, but never for white mustard. In early spring, Nmin was only significantly different between the fallow plots and the plots with frost resistant catch crops (Italian ryegrass and grass-clover mixture). In conclusion, an additional manure application of 60 kg N/ha after harvest of winter cereals did not significantly increase Nmin under catch crops, allowing the assumption that it neither increased nitrate leaching losses.