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

Abstract

Discrimination between 14N and 15N 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. vaginaturn and L. wahlenbergii used mineral N but had very limited abilities to use amino acids. Root, shoot and whole-plant 15N abundance frequently differed from the sources and were influenced by mycorrhizas, indicating that plant 15N abundance is not a reliable guide to that of the N source used. This study establishes that differences in leaf 15N abundance between coexisting plants may result from species-specific differences in isotopic fractionation of N. This challenges the view that 15N abundance in shoots of different plant functional types directly reflects their utilization of isotopically distinct soil N pools. © New Phytologist (2001).