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

The effect of policy measures on agriculture and water quality with respect to nitrogen have been monitored on farms in the sandy regions since the early 1990s. 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.