Influencing factors on the nitrate residue levels in Flemish agricultural soils: a statistical analysis of 8 years of nitrate measurements

In Flanders, nitrate residues in the soil profile are used as an indicator for the risk of nitrate leaching from agricultural soils to surface and ground water. The nitrate residue is defined as the amount of nitrate-N present in the soil profile (0-90cm) during the period October 1st to November 15th. They are measured annually, on one hand in a (more or less) directed selection of agricultural parcels commissioned by the Manure Bank) and on the other hand in all the parcels having an agro-environmental agreement “Water” (AEA Water). This produces datasets of 18 000 up to more than 30 000 nitrate residue measurements per year. In the framework of the assignment of the Flemish Government to evaluate and differentiate the current nitrate residue standard, an extensive review of the historical nitrate residue measurements was made. This descriptive analysis had to provide an answer to the following questions: - which factors have a significant influence on the nitrate residue levels? - what are the impacts of policy measures such as stricter fertilisation standards, agro-environmental agreements, etc.? The extensive analysis of the available nitrate residue measurements in Flanders since 2001 demonstrates the importance of fertilisation practices and policy measures. Parcels with an AEA Water have to meet stricter fertilisation standards and as a consequence show lower nitrate residue levels. Moreover, in all the measurements a significantly decreasing trend is observed over the years, parallel to the evolution of fertilisation standards becoming stricter and farmers adopting more the principles of good fertilisation practices, such as the use of catch crops.