Assimilation and isotopic fractionation of nitrogen by mycorrhizal fungi

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 N-15, no such effect was seen in the ERM fungus, where, on ammonium, there was preferential assimilation of N-14. It is concluded that isotopic fractionation during N uptake and metabolism can cause significant shifts in the N-15 abundance of mycorrhizal fungi and that, as a result, any attempt to use the tissue N-15 abundance as a means of identifying the substrates being exploited by mycorrhizal fungi, or their plant partners, in nature, are likely to be unrealistic.