Assimilation and isotopic fractionation of nitrogen by mycorrhizal and nonmycorrhizal subarctic plants

Discrimination between N-14 and N-15 isotopes during assimilation of nitrogen (N) from inorganic (NO3, NH4) and organic (glutamic acid, glycine) sources by subarctic, ecotypes of Eriophorum vaginatum, Luzula wahlenbergii. Betula nana and Vaccinium vitis-idaea was studied under axenic laboratory conditions. Ectomycorrhizal B. nana and ericoid mycorrhizal V. vitis-idaea assimilated significant amounts of all the N sources, whereas nonmycorrhizal E. vaginatum and L. wahlenbergii used mineral N but had very limited abilities to use amino acids. Root, shoot and whole-plant N-15 abundance frequently differed from the sources and were influenced by mycorrhizas, indicating that plant N-15 abundance is not a reliable guide to that of the N source used. This study establishes that differences in leaf N-15 abundance between coexisting plants may result from species-specific differences in isotopic fractionation of N. This challenges the view that N-15 abundance in shoots of different plant functional types directly reflects their utilization of isotopically distinct soil N pools.