Water Pollution Monitoring Of Nitrates Leaching From Agricultural Fields With Industrial Crop Harvesting Practices. The Belgian Case Of Onsite Lysimeters As A Monitoring Tool.

The European Nitrate Directive (91/676/CEE) aims at moderating or preventing water pollution from nitrates released by agricultural practices. The Belgian Walloon region transposed it into law and initiated a Sustainable Nitrogen Management Program in 2002 (also called PGDA in French). The non-profit association NITRAWAL amongst others, is in charge of providing support to farmers to implement and respect this program. GRENeRA is one of the organizations designated for scientific studies. In this frame, GRENeRA and its partners implemented six lysimeters in agricultural open fields, operated under real conditions of industrial harvest crops, especially vegetables. The lysimeters are used as follow up tools to monitor and quantify the amount of nitrates leaching beyond the root zone towards underground water reserves. This study demonstrates that lysimeters are efficient and permit the collection of representative samples of seepage water. On a quality stand point, nitrate content combined with data related to agricultural practices, fertilisation rates applied and advised, climate, the defined environmental indicator (APL) provide a global approach to link agricultural practices and underground water protection from nitrate pollution. The lysimeters validate the reliability of the environmental indicator (APL) with the nitrate content in leaching water, which will not be uptaken by the next crop. Unfortunately, it is also demonstrated that even when the nitrogen management is respectful of the current legislation, the goal to comply with the targeted standards of water quality is not ensured. The lysimeter tool clearly show that the respect of seepage water quality standards issued from industrial agriculture does not have to be considered per crop but rather from an integrated and global perspective of the complete crop sequence and rotations, in respect with best agricultural practices, reasonable fertilisation rates, inset crops, etc., for each individual crop being part of the overall rotation. Some favourable and non favourable crop sequences with respect to nitrates content of underground water are summarized. Experiments are in progress on crop sequence evaluation and fertilisation levels and recommendations for Belgian Walloon industrial crops and their associated impacts on the leaching water quality and nitrate content collected by lysimeters.