Water and Nutrient Transport On a Heavy Clay Soil in a Fluvial Plain in the Netherlands.

Phosphorus (P) losses from agricultural soils have led to surface water quality impairment in The Netherlands. To develop measures to reduce P enrichment of surface waters, a thorough understanding of the transport pathways and the forms in which P is transferred is needed. Transport of dissolved nutrients by water through the soil matrix to groundwater and drains is assumed to be the dominant pathway for nutrient losses to ground- and surface water in level areas as in The Netherlands. In 2003, a study was started to investigate nutrient losses from a grassland site on a Dutch heavy clay soil in a fluvial plain. The site was drained by drains and trenches. Average annual N losses to surface water amounted to 16.6 kg ha^-1 yr^-1 and annual P loses were 3.4 kg ha^-1 yr^-1. Rapid discharge by means of the trenches was the dominant pathway (60-90%) for water and nutrient transport. Discharge to the groundwater was negligible. The contribution of the drains to the discharge of the plot was dependent on the existence of shrinkage cracks in the clay soil. At the end of a dry summer, cracks were abundant and discharge was equally divided to drains and trenches. After prolonged wet periods, cracks were absent and discharge by drains was almost negligible.