Article révisé par les pairs
Résumé : 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 rotation might cause to the environment were unknown especially after the destruction of the set-aside cover. Over a period of four years, nitrate quantities were measured in the soil profile to a depth of 1.5 m in order to evaluate the pollution risks to groundwater. The swards studied were perennial ryegrass, red clover, perennial ryegrass/red clover and spontaneous cover. These experiments were carried out before, during and after the set- aside period, 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 spontaneous 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 after the set-aside cover is destroyed, mineralisation of the incorporated covers 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 65 or 75 N kg ha-1, little nitrate remains in the profile. Therefore, there 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.