Abstract.The influence of intensively fertilized short‐rotation forest plantations on nitrogen concentrations in groundwater was studied by piezometer readings and water sampling over a two‐year period in a sandy field growing willow (Salixspp.) and other species. The mineral‐N content of the unsaturated zone was measured in soil samples collected to 0.9 m depth. Although piezometer readings suggested that deep groundwater could be affected, the concentrations of nitrate‐N and ammonium‐N were usually less than 1 mg per litre. There was also little mineral‐N in the unsaturated zone, except for occasional peaks in the topsoil (0–30 cm) after application of fertilizer. We conclude that there is little risk of nitrogen contamination of groundwater in intensively cultured tree stands receiving up to 150 kg N/ha/yr as fertilizer. This is probably because willow can take up water and nitrogen from deep parts of the