N-15 natural abundances and N use by tundra plants

Plant species collected from tundra ecosystems located along a north-south transect from central Alaska to the north coast of Alaska showed large and consistent differences in N-15 natural abundances. Foliar delta(15)N values varied by about 10 parts per thousand among species within each of two moist tussock tundra sites. Differences in N-15 contents among species or plant groups were consistent across moist tussock tundra at several other sites and across five other tundra types at a single site. Ericaceous species had the lowest delta(15)N values, ranging between about -8 to -6 parts per thousand. Foliar N-15 contents increased progressively in birch, willows and sedges to maximum delta(15)N values of about +2 parts per thousand in sedges. Soil N-15 contents in tundra ecosystems at our two most intensively studied sites increased with depth and delta(15)N values were usually higher for soils than for plants. Isotopic fractionations during soil N transformations and possibly during plant N uptake could lead to observed differences in N-15 contents among plant species and between plants and soils. Patterns of variation in N-15 content among species indicate that tundra plants acquire nitrogen in extremely nutrient-poor environments by competitive partitioning of the overall N pool. Differences in plant N sources, rooting depth, mycorrhizal associations, forms of N taken up, and other factors controlling plant N uptake are possible causes of variations in delta(15)N values of tundra plant species.