Short-term N uptake by Norway spruce rootswas investigated after tracer application both in the field and in the laboratory. In the forest the influence of wood ash or liquid fertiliser treatments on the uptake was studied. A rapid uptake of the ¹⁵NH₄¹⁵NO₃ tracer into the fine roots was observed. Within 1 day of application about 50% of the maximum δ¹⁵N value observed was measured and within 1 week as much as 70–90%. The ¹⁵N enrichment in fine roots was not affected by wood ash and liquid fertiliser treatments applied in the previous year. δ¹⁵Ν increased continuously until 2 months after ¹⁵N application and decreased to 60% of its maximum value within 1 year. Nine months after the tracer application, an analysis of δ¹⁵N distribution throughout the root system was conducted. The δ¹⁵N values were highest in roots of the topsoil compared to roots at deeper soil layers, and higher in fine roots than in larger roots. The 15N was not translocated within the root system. The laboratory experiment, using Norway spruce seedlings, aimed at describing short-term ¹⁵NO₃ uptake and assimilation into free amino acids with 2 mM or 20 mM nitrate application. The ¹⁵N was detected within 4 h to 1 day in the amino acids Glu, Gln, Asp of roots, while in Asn, Ala and Ser and in the shoots, the ¹⁵N was retrieved at the earliest after 1 day. The enrichment factor in the amino acids increased to a maximum within 3–7 days, depending on the nitrate concentration applied.