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.