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Abstract:  Three-year-old Norway spruce trees were planted into a low-nitrogen mineral forest soil and supplied either with two different levels of mineral nitrogen (NH4NO3) or with a slow-release form of organic nitrogen (keratin). Supply of mineral nitrogen increased the concentrations of ammonium and nitrate in the soil solution and in CaCl2-extracts of the rhizosphere and bulk soil. In the soil solution, in all treatments nitrate concentrations were higher than ammonium concentrations, while in the soil extracts ammonium concentrations were often higher than nitrate concentrations. After 7 months of growth, 15N labelled ammonium or nitrate was added to the soil. Plants were harvested 2 weeks later. Keratin supply to the soil did not affect growth and nitrogen accumulation of the trees. In contrast, supply of mineral nitrogen increased shoot growth and increased the ratio of above-ground to below-ground growth. The proportion of needle biomass to total above-ground biomass was not increased by mineral N supply. The atom-% 15N was higher in younger needles than in older needles, and in younger needles higher in plants supplied with 15N-nitrate than in plants supplied with 15N-ammonium. The present data show that young Norway spruce plants take up nitrate even under conditions of high plant internal N levels.