VARIATIONS IN N-15 ABUNDANCE IN A FOREST FERTILIZATION TRIAL - CRITICAL LOADS OF N, N-SATURATION, CONTAMINATION AND EFFECTS OF REVITALIZATION FERTILIZATION

N-15 abundances of current needles of Norway spruce collected during 23 yrs of a forest fertilization experiment were studied in order to follow ecosystem gains and losses of N. Unlabelled ammonium nitrate at four rates (N0-N3), phosphorus at three rates (P0-P2), and potassium plus other elements including micronutrients at two rates (K0-K1), had been applied to plots in a complete factorial design. Nitrogen had been applied annually at average rates of 0, 34, 68 and 102 kg N ha-1 yr-1. Tree growth had responded positively to additions of N, but the response was remarkably more positive to the N2P2K1 treatment. In N1 treatments, delta-N-15 (parts per thousand) declined over time. This was consistent with an earlier study, and should reflect a change in N-15 abundance towards that of fertilizer N (minus discrimination during uptake), which in turn means accretion of most of the N added. As in the earlier study, in which N3 plots lost most of the N added, the present N3 plots showed an increasing delta-N-15 (parts per thousand). This pattern was not significantly affected by additions of P and K plus other elements, although a weak negative effect of P on N accretion was indicated, i.e. there was a tendency delta-N-15 (parts per thousand) to be higher when P was added. This, and another recent result based on an N budget, shows that so-called revitalization fertilization may well increase growth of trees, but also promotes losses of N from the ecosystem. As in the previous study, a decline in delta-N-15 (parts per thousand) on control plots provided evidence of contamination. Given a removal of 100 kg N ha-1 at stem harvest and a leaching of 2 kg N ha-1 yr-1, our data on N-15 suggest that a load of 9 kg N ha-1 yr-1 would saturate the ecosystem after 100 years. This load is only about twice the annual deposition at the site.