L'épandage d'engrais de ferme avant le semis d'une culture intermédiaire (CIPAN) présente-t-il un risque important de lixiviation de nitrate ?

Do manures spread before the sowing of a catch crop (CIPAN) present an excessive risk of nitrate leaching?. Description of the subject. The implementation of the Nitrates Directives in the Walloon region (in the south of Belgium) allows the summer spreading of any kind of manure on soils in preparation for the sowing of a catch crop. Objectives. This paper aims to assess whether the spreading of a manure before sowing a catch crop is more risky for groundwater quality than planting a cereal followed by another crop without any action being implemented between the harvesting of the cereal and the sowing (winter or spring) of the next crop. Method. This study is based on measurements of the nitrate nitrogen content of soils, at the beginning of the leaching period, performed on reference parcels and on controlled parcels. Results. Three thousand six hundred soil analyses performed in relation to controlled parcels between 2008 and 2013 showed that the presence of a catch crop (information about the possible application of a manure was not known) led to a nitrate nitrogen content (median) lower (with a very highly significant difference) than in a situation where no action was implemented between the two main crops. For the same period, the analysis of 600 results in reference parcels led to the same conclusion. Moreover, no significant difference was found between the two kinds of manure ("fast action", such as pig slurry or "low action", such as bovine manure). Conclusions. After the harvesting of a cereal, the spreading of a manure before sowing a catch crop creates no higher risk for water quality than the succession of a cereal by a crop sowed in the autumn.