Arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi affect plant nitrogen (N) dynamics. Plant N isotope patterns have been used to characterise the contribution of ECM fungi to plant N uptake. By quantifying and comparing the effects of an AM and an ECM fungus on growth, N uptake and isotopic composition of one host plant grown at different relative N supply levels, the aim of this study was to improve the mechanistic understanding of natural 15N abundance patterns in mycorrhizal plants and their underlying causes. Grey alders were inoculated with one ECM fungus or one AM fungus or left non-mycorrhizal. Plants were grown under semi-hydroponic conditions and were supplied with three rates of relative N supply ranging from deficient to luxurious. Neither mycorrhizal fungus increased plant growth or N uptake. AM root colonisation had no effect on whole plant δ15N and decreased foliar δ 15N only under N deficiency. The roots of these plants were 15N-enriched. ECM root colonisation consistently decreased foliar and whole plant δ15N. It is concluded, that both mycorrhizal fungi contributed to plant N uptake into the shoot. Nitrogen isotope fractionation during N assimilation and transformations in fungal mycelia is suggested to have resulted in plants receiving 15N-depleted N via the mycorrhizal uptake pathways. Negative mycorrhizal growth effects are explained by symbiotic resource trade on carbon and N and decreased direct plant N uptake.
Keywords: Alnus incana (grey alder); Arbuscular mycorrhiza; Ectomycorrhiza; Isotope fractionation; Nitrogen uptake; δ(15)N.
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