Influence of set-aside on the nitrate content of soil profiles

In 1992, the European Union created the current set-aside regime. The risks that the introduction of this new "crop" into the three-courses crop rotat ion might cause to the environment were unknown especially after the destru ction of the set-aside cover. Over a period of four years, nitrate quantiti es were measured in the soil profile to a depth of 1.5 m in order to evalua te the pollution risks to groundwater. The swards studied were perennial ry egrass, red clover, perennial ryegrass/red clover and spontaneous cover. Th ese experiments were carried out before, during and after the set-aside per iod, i.e. also during the subsequent crop. Results show that the sowing of a set-aside cover before winter leads to a reduction in the leaching risks. Little nitrate generally remains in the sown plots; the highest quantities are measured after red clover: 26 +/- 10 kg NO3-N ha(-1). As far as sponta neous covers are concerned, the quantities can be high and highly variable: 43 +/- 48 kg NO3-N ha(-1). At the sowing of sugar beet, i.e. 4 months afte r the set-aside cover is destroyed, mineralisation of the incorporated cove rs has started: 17 +/- 11 kg NO3-N ha(-1) for the perennial ryegrass and 96 +/- 19 kg NO3-N ha(-1) for the red clover sown before winter. However, in each case, nitrate is mainly located in the upper part of the profile. When sugar beet is harvested, taking into account a nitrogen fertilisation of 6 5 or 75 N kg ha(-1), little nitrate remains in the profile. Therefore, ther e is little risk of groundwater pollution if a set-aside cover is sown and if it is destroyed after 15 November before a spring crop.