Hooijboer AEJ; de Koeijer TJ; Prins H; Vrijhoef A; Daatselaar CHG (Rijksinstituut voor Volksgezondheid en Milieu RIVM, 2017-07-07)
Dutch manure policy tries to limit the harmful environmental impact of agriculture. This is in line with international agreements on fertilizer use. The European Nitrates Directive prescribes Member States to limit the use of animal manure to 170 kg nitrogen per hectare. Farms with at least 80 percent of grassland may, under certain conditions, use more manure from grazing animals such as cows and sheep (derogation). Over the last 10 years, nitrate leaching from the manure to the upper groundwater has decreased or remained the same for these farms. By 2015, on average, the concentration is in all regions below the EU standard of 50 milligrams of nitrate per liter. This is according to the annual report by RIVM and Wageningen Economic Research. They follow agricultural practices and the effects on water quality at 300 derogation farms and report their results to the EU annually. This report describes the situation in 2015 and the development between 2006 and 2016 (trend). Management. The permissible amount of nitrogen from grazing manure is, depending on the soil and region, 250 kilograms per hectare (in the Clay region. Peat region and northern part of the Sand region) or 230 kg/ha (in the Loess region and the rest of the Sand region). On average, derogation companies have used 238 kilograms of nitrogen from animal manure per hectare in 2015. The amount of nitrogen that can leached as nitrate to groundwater is determined, among others, by the so-called nitrogen soil surplus. This is the difference between the input of nitrogen (such as fertilizers) and their output (including through grass and maize). The average nitrogen surplus over the regions has decreased over the period considered. Groundwater quality. By 2015, the average nitrate concentration in the groundwater in Sand 250 was 26 milligrams per liter (mg/l). The highest concentration is measured in the Loess region (42 mg/l) and in Sand 230 (45 mg/l). On the average, farms in the Clay region and the Peat region had lower nitrate concentrations in leaching water (22 and 13 mg/l respectively). The difference between the regions can be explained by the proportion of soils prone to nitrate leaching. Especially in Sand 230 and in the Loess region there are grounds for which nitrate is reduced in a lesser extent, and therefore can leach more to groundwater.
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