Impact of application of nitrogen from livestock manure on water quality: derogation in Flemish agriculture

In the commission decision of 21 June 2011, the Commission of the European Communities approved the Belgian request, with regard to the region of Flanders, to allow a higher application of livestock manure than provided in Nitrogen Directive 91/676/EEC. In this derogation decision a number of specific conditions were imposed on farms applying derogation as well on the competent authorities with regard to monitoring, control and reporting. The objective of the present research is the follow-up of a monitoring network of at least 150 farms to assess the impact of this derogation on nitrogen losses from the soil and on water quality during 2011-2014. If derogation would have a significant impact on water quality in Flanders, it is of great importance to identify the underlying causes and to determine the precise impact. The basis for the set-up of the derogation monitoring network is the existing monitoring network for phreatic groundwater. It is a dense network of 2,107 measuring points with short well screens at 3 depths, distributed over Flanders. In a previous study the infiltration area and the travel time for water from the root zone to the uppermost well screen was calculated. Only monitoring wells where the infiltration area was completely located in a single agricultural parcel and had a travel time less than 3 years were selected. In this way, the measured water quality in a monitoring well could be linked to one agricultural parcel. One hundred and twenty one parcels linked to monitoring wells were selected, less than the required 217 parcels. Additional parcels were selected from farmers who volunteered to participate in the network and extra monitoring wells were installed to measure the water quality on their parcels. Each hydrological year a soil sample was taken before and after winter from 0-90 cm in three layers to measure the amount of nitrate in the soil. This provides information about the residual soil nitrate after harvest and the nitrate leaching out towards the surface- and groundwater. To investigate the quality of the surface and groundwater, water samples from the phreatic monitoring wells, the extra monitoring wells, drains, ditches and canals were taken. In order to measure the water quality on parcels with a water level deeper than 1.5 m, a soil sample was taken from 90-120 cm. ANOVA tests (P ≤ 0.05) were carried out to verify statistical differences of measured nutrients between derogation and non-derogation parcels for all crops and soil types, as well as for the most common combinations of cultivated crop (grass and maize) and soil type in Flanders (sand and sandy loam). Based on the extensive information from the monitoring network in 2011 and 2012, it is possible to conclude that application under specific derogation conditions of a higher amount of nitrogen originating from livestock manure than described in Nitrogen Directive 91/676/EEC has no significant negative impact on water quality in Flanders.