| Variation among species in the uptake of inorganic (NH4 +, NO3-) and organic (glycine) N were investigated in the dry meadow alpine tundra community using a combination of stable isotope techniques, in order to evaluate the potential for differentiation among species in the uptake of different chemical forms of N. The species differed in rooting depth and mycorrhizal infection rates, both of which may influence plant N uptake capacities. This work had two primary objectives: (1) to determine whether species differed in the capacity to take up NH4+, NO3- and glycine in the field, and (2) to examine the effect of competition for N on species' N uptake characteristics.; Variation among species in the capacity to take up inorganic and organic N, corroborated by variation in foliar delta15N and nitrate reductase activity, indicated that individual species show differentiation in N use in the field. Co-occurring species differed in the capacity to take up NH4+ and NO3- in spite of substantial differences in the relative pool sizes of these two N forms, and while all species took up glycine, no species appeared to utilize glycine to a greater extent than the inorganic N forms.; Species likewise varied in their N uptake characteristics in response to neighbor presence and identity, and no single species was the best competitor for all forms of N. Such variation in species' competitive abilities (competitive ranks) for different forms of N is suggestive of competitive tradeoffs for N that may facilitate differentiation in N use within this and other N-limited communities. |
