A ¹⁵N-tracer experiment was carried out in a stand of adult spruce trees [Picea abies (L.) Karst.] located on the Swiss Plateau in order to investigate the effects of wood ash treatment on seasonal nitrogen fluctuations in fine roots and needles. Treatments included irrigation (W), liquid fertilization (LF) and wood ash (A) application. ¹⁵N fluctuation in fine roots and current to 3-year-old needles was studied after one ¹⁵N pulse for 2 consecutive years (1999, 2000). ¹⁵N tracer was rapidly incorporated into the fine roots of adult trees, and δ¹⁵N values reached similar levels in all treatments 2 months after the pulse. In the needles, the largest increase in δ¹⁵N was observed in those of the current year. Following the initial peak during spring growth, δ¹⁵N values in needles of control trees showed an oscillating pattern through the season. This oscillation is attributed to the increased use of internal N sources, as soon as the roots can no longer meet the increased N demand during the sprouting phase. However, W-, LF- and A-treated trees no longer showed the oscillation in δ¹⁵N. Additional water (W and LF) as well as fertilizer (A and LF) may have induced shifts in the microbial flora, thus increasing the unlabelled N release from the soil. The strongest dampening was observed for the A treatment, indicating sufficient N availability from the soil, and making intensive use of the internal N sources unnecessary. Treatment with wood ash thus resulted in a similar fertilizer response to liquid fertilization.