N-15 NATURAL ABUNDANCES AND N USE BY TUNDRA PLANTS

Plant species collected from tundra ecosystems located along a north-s outh transect from central Alaska to the north coast of Alaska showed large and consistent differences in N-15 natural abundances. Foliar de lta(15)N values varied by about 10 parts per thousand among species wi thin each of two moist tussock tundra sites. Differences in N-15 conte nts 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, ran ging between about -8 to -6 parts per thousand. Foliar N-15 contents i ncreased progressively in birch, willows and sedges to maximum delta(1 5)N values of about +2 parts per thousand in sedges. Soil N-15 content s in tundra ecosystems at our two most intensively studied sites incre ased with depth and delta(15)N values were usually higher for soils th an for plants. Isotopic fractionations during soil N transformations a nd possibly during plant N uptake could lead to observed differences i n N-15 contents among plant species and between plants and soils. Patt erns of variation in N-15 content among species indicate that tundra p lants acquire nitrogen in extremely nutrient-poor environments by comp etitive partitioning of the overall N pool. Differences in plant N sou rces, rooting depth, mycorrhizal associations, forms of N taken up, an d other factors controlling plant N uptake are possible causes of vari ations in delta(15)N values of tundra plant species.