Variation in nitrogen-15 natural abundance and nitrogen uptake traits among co-occurring alpine species: do species partition by nitrogen form?

In the N-limited alpine tundra, plants may utilize a diversity of N sources (organic and inorganic N) in order to meet their nutritional requirements. To characterize species-level differences in traits related to N acquisition, we analyzed foliar δ15N, nitrate reductase activity (NRA) and mycorrhizal infection in co-occurring alpine species during the first half of the growing season and compared these traits to patterns of N uptake using a 15N (15N-NH4+, 15N-NO3-) or 13C,15N ([1]-13C-15N-glycine) tracer addition in the greenhouse. 13C enrichment in belowground tissue indicated that all species were capable of taking up labeled glycine, although only one species showed uptake of glycine potentially exceeding that of inorganic N. Species showing the most depleted foliar δ15N and elevated NRA in the field also tended to show relatively high rates of NO3- uptake in the greenhouse. Likewise, species showing the most enriched foliar δ15N also showed high rates of NH4+ uptake. The ratio of NO3-:NH4+ uptake rates and growth rate explained 64% and 72% of the variance in foliar δ15N, respectively, suggesting that species differ in the ability to take up NO3- and NH4+ in the field and that such differences may enable species to partition soil N on the basis of N form.