NITROGEN NUTRITION AND ISOTOPE DIFFERENCES AMONG LIFE FORMS AT THE NORTHERN TREELINE OF ALASKA

Natural abundances of nitrogen isotopes, delta(15)N, indicate that, in the same habitat, Alaskan Picea glauca and P. mariana use a different soil nitrogen compartment from the evergreen shrub Vaccinium vitis-idaea or the deciduous grass Calamagrostis canadensis. The very low delta(15)N values (-7.7 %) suggest that (1) Picea mainly uses inorganic nitrogen (probably mainly ammonium) or organic N in fresh litter, (2) Vaccinium (-4.3 %) with its ericoid mycorrhizae uses more stable organic matter, and (3) Calamagrostis (+0.9 %) exploits deeper soil horizons with higher delta(15)N values of soil N. We conclude that species limited by the same nutrient may coexist by drawing on different pools of soil N in a nutrient-deficient environment. The differences among life-forms decrease with increasing N availability. The different levels of delta(15)N are associated with different nitrogen concentrations in leaves, Picea having a lower N concentration (0.62 mmol g(-1)) than Vaccinium (0.98 mmol g(-1)) or Calamagrostis (1.33 mmol g(-1)). An extended vector analysis by Timmer and Armstrong (1987) suggests that N is the most limiting element for Picea in this habitat, causing needle yellowing at N concentrations below 0.5 mmol g(-1) or N contents below 2 mmol needle(-1). Increasing N supply had an exponential effect on twig and needle growth. Phosphorus, potassium and magnesium are at marginal supply, but not interaction between ammonium supply and needle Mg concentration could be detected. Calcium is in adequate supply on both calcareous and acidic soils. The results are compared with European conditions of excessive N supply from anthropogenic N depositions.