Assimilation and isotopic fractionation of nitrogen by mycorrhizal fungi

Abstract

The patterns of nitrogen (N) utilization and of N isotope fractionation were determined when two ecto-(ECM) and an ericoid (ERM) mycorrhizal fungus were grown with inorganic (ammonium or nitrate) or organic (glutamic acid or glycine) N sources of predetermined N isotope composition. All N sources were readily utilized by each of the fungi but substantial differences in the pattern of N isotope fractionation were observed both between the fungi and the N sources. Whereas several of the ECM-N source combinations exhibited significant net fractionation in favour of 15N, no such effect was seen in the ERM fungus, where, on ammonium, there was preferential assimilation of 14N. It is concluded that isotopic fractionation during N uptake and metabolism can cause significant shifts in the 15N abundance of mycorrhizal fungi and that, as a result, any attempt to use the tissue 15N abundance as a means of identifying the substrates being exploited by mycorrhizal fungi, or their plant partners, in nature, are likely to be unrealistic. © New Phytologist (2001).