Results of 10 years of monitoring nitrogen in the sandy regions in the Netherlands

Nitrogen surplus on dairy farms decreased by 100 kg/ha (30%) in the 1991-1999 period due to demonstration projects and the implementation of MINAS, limiting farm gate balance surplus. Nitrogen surplus on arable farms has slightly decreased (30 kg/ha, i.e. 20%), probably because MINAS became mandatory for arable farms from 2001 onwards. About 25% of the farm-gate balance N surplus leached as nitrate into the upper metre of the groundwater. The NO3-N concentration decreased in the 1992-2001 period, but the decrease was larger for dairy farms (16 mg/l, 46%) than for arable farms (7.7 mg/l, 30%). Of the decrease in NO3-N concentration on dairy farms, 23% is caused by a decrease in N surplus, and 27% by environmental factors, such as a higher precipitation surplus, more wet soils and more peat soils in the second part of the monitoring period. However, unknown factors are responsible for 50% of the decrease.