The level of nitrate in the upper groundwater in sandy soils and arable land in the Netherlands is very high. The reason why this is the case, will be studied in this project.
Arable land on sandy soils in The Netherlands is still associated with a high nitrate concentration in the upper groundwater, particularly where livestock manures have been applied. This situation fuels the debate to further limit the use of manure. However, it is as yet unclear whether N leaching simply results from assigning too low legal defaults to the N fertilizer replacement value to these manures (stimulating the application of unnecessarily high mineral fertilizer supplements) or from the very nature of manures i.e. the lack of synchrony between crop N demand and N supply from manures through mineralization. If the latter holds, livestock manures with little organically bound N (Norg) relative to ammoniacal N (e.g. liquid fractions) should at least in the long run lead to less N leaching per unit total N applied than manures with a large share of Norg (e.g. farm yard manures). To find out which of these factors determines the nitrate concentration, field trials were carried out in 2010 and 2011. In this trial silage maize was grown (with and without a winter cover crop) on different rates and types of livestock manures and on mineral fertilizer N. The results will be used for the underpinning of the 5th EU Nitrates Directive Action Program of The Netherlands, including requests to extend the current derogation.The results will be used for the underpinning of the 5th EU Nitrates Directive Action Program of The Netherlands, including requests to extend the current derogation.