Presentations

Nitrogen is one of the most important nutrients for crop production. Excessive use of nitrogen in agriculture in the previous century has led to widespread pollution of groundwater with nitrate. Agricultural use of nitrogen decreased since the mid-80s in the Netherlands, as it did in many other countries of the European Union. In the Netherlands, nitrogen use and nitrogen surplus in agriculture decreased in the last decades, resulting in a decrease in nitrate concentrations in the upper groundwater of farms in the sandy regions from an average of about 140 mg/l in 1992 to about 75 mg/l in 2006. Nevertheless, the EU nitrate standard for groundwater of 50 mg/l is exceeded on about 60 percent of the farms in the sandy region. To calculate environmentally safe nitrogen use standards for Dutch agriculture, two empirically derived relationships are used. First, a relationship between nitrogen use and nitrogen surplus on the soil surface balance, derived from data from field experiments. And, secondly, a relationship between nitrogen surplus and nitrogen leaching, derived from data from farms in the LMM. This study focused on deriving a relationship between nitrogen surplus and nitrogen leaching. We calculated nitrate leaching from the root zone of agricultural land as a fraction of the nitrogen surplus on the soil surface balance. Nitrate nitrogen leaching fractions were calculated for grassland and arable land in the sand region of the Netherlands, which is the most vulnerable to nitrate leaching. Farm practice data and water quality data from dairy farms and arable farms, participating in the Minerals Policy Monitoring Program (LMM) in the period 1991-2005 were used in these calculations. LMM farms in the sand region have other soil types than vulnerable sandy soils. Dairy farms have grassland as well as arable land (mainly silage maize). We developed a procedure to account for the presence of other soil types and different land use on LMM farms in nitrate leaching calculations. The results show that nitrate leaching decreases as nitrogen surpluses decrease. The decrease in nitrate leaching is smaller for dairy farms with mainly grassland (0.67 kg per kg surplus) than for arable farms (1.01 kg per kg surplus). The nitrate nitrogen leaching fraction is almost twice as high for arable land (0.89, standard deviation 0.15) than for grassland (0.46, standard deviation 0.09). No clear relationship is found between the leaching fraction and the nitrogen surplus or between the leaching fraction and the precipitation surplus. Therefore, the nitrate nitrogen leaching fractions used to calculate nitrogen use standards were not differentiated for these factors