Influence of P-status and hydrology on phosphorous losses to surface waters on dairy farms in the Netherlands Caroline van der Salm (1), Christy van Beek(1), and Rikje van de Weerd(2) (1) Alterra, Wageningen University and Research Centre, PO. Box 47, 6700AA Wageningen, the Netherlands. (2) Arcadis, PO. Box 673, 7300 AR Apeldoorn, The Netherlands Caroline.vandersalm@wur.nl Introduction The contribution of agriculture to the contamination of Dutch surface waters has increased from 43 % in 1985 to 57 % in 2002 (ref). Dairy farming is the largest producer of animal manure in the Netherlands (circa 75% of the total annual manure production). Information on nutrient budgets and leaching of nutrients from dairy farms was, however, limited. To mitigate this problem, monitoring programs were set-up at three dairy farms, namely one in the sandy area of the Netherlands, one in the peat-district and one on a river-clay soil. The three farms together represent the environmental conditions encountered in the Dutch dairy region. This study focuses on the losses of phosphorus from grazed grasslands on a clay soil a sandy soil and a peat soil, with different soil P-status and different hydrological pathways. Experimental design Phosphorous losses to ground- and surface waters were measured for a period of two to three years on a site with a heavy clay soil, a peat soil and a sandy soil. All sites were almost level and were draining on a dead-end ditch. At the end of each ditch a weir was placed with a flow meter connected to a sampling device for flow proportional sampling. The sandy site was well drained and drain-pipes were absent. About 3.5 m below the surface, a confining loam layer prevents exchange of water and solutes with deeper groundwater. The clay site was drained by tile-drains and trenches. The subsurface drains were located at a depth of 80 cm below the surface. The ditches were shallow (50 cm depth) and located at intervals of 46 m. Due to the low permeability of the heavy clay there was no seepage and groundwater recharge. The peat site has a man-made topsoil (0-40 cm) consisting of organic matter rich sandy clay. The subsoil consisted of woody peat. At 3 m below the soil surface a dense clay layer prevented extensive groundwater recharge. At the clay site the phosphorus losses to surface water were determined by flow proportional measurement of the discharge of trenches and drains. For sand and peat soil, the soil solution in the unsaturated zone was sampled using porous suction cups, which were placed in transects perpendicular to the ditches. Phosphate leaching fluxes at these sites were based on measured soil solution concentrations and simulated water fluxes. Surface run-off was assessed based on measurements using catchment plates and simple balance models for the peat soil (Van Beek et al., 2003a and was modelled for the sand soil (Torenbeek and Voskamp, 2003). At the clay site surface runoff was collected by the trenches, direct runoff form the field to the ditch was negligible (van der Salm et al., 2006, 2007). Hydrological pathways, phosphate saturation of the soils and phosphate leaching losses. The three sites differed considerably with respect to the main hydrological pathways. The clay site had a very low hydraulic conductivity and large part of the discharge took place by runoff or interflow through the upper soil layers to the trenches (Table 1). The peat and sand sites were better drained and most of the water was conducted through the soil matrix.