Leaf 15N abundance of subarctic plants provides field evidence that ericoid, ectomycorrhizal and non-and arbuscular mycorrhizal species access different sources of soil nitrogen

Abstract

The natural abundance of the nitrogen isotope 15, δ¹⁵N, was analysed in leaves of 23 subarctic vascular plant species and two lichens from a tree-line heath at 450 m altitude and a fellfield at 1150 m altitude close to Abisko in N. Sweden, as well as in soil, rain and snow. The aim was to reveal if plant species with different types of mycorrhizal fungi also differ in their use of the various soil N sources. The dwarf shrubs and the shrubs, which in combination formed more than 65% of the total above-ground biomass at both sites, were colonized by ericoid or ectomycorrhizal fungi. Their leaf δ¹⁵N was between-8.8 and-5.5‰ at the heath and between-6.1 and -3.3‰ at the fellfield. The leaf δ¹⁵N of non- or arbuscular mycorrhizal species was markedly different, ranging from -4.1 to -0.4‰ at the heath, and from -3.4 to+2.2‰ at the fellfield. We conclude that ericoid and ectomycorrhizal dwarf shrubs and shrubs utilize a distinct N source, most likely a fraction of the organic N in fresh litter, and not complexed N in recalcitrant organic matter. The latter is the largest component of soil total N, which had a δ¹⁵N of -0.7‰ at the heath and +0.5‰ at the fellfield. Our field-based data thus support earlier controlled-environment studies and studies on the N uptake of excised roots, which have demonstrated protease activity and amino acid uptake by ericoid and ectomycorrhizal tundra species. The leaves of ectomycorrhizal plants had slightly higher δ¹⁵N (fellfield) and N concentration than leaves of the ericoids, and Betula nana, Dryas octopetala and Salix spp. also showed NO _inf3^sup- reductase activity. These species may depend more on soil inorganic N than the ericoids. The δ¹⁵N of non- or arbuscular mycorrhizal species indicates that the δ¹⁵N of inorganic N available to these plants was higher than that of average fresh litter, probably due to high microbial immobilization of inorganic N. The δ¹⁵N of NH _inf4^sup+ -N was +12.3‰ in winter snow and +1.9‰ in summer rain. Precipitation N might be a major contributer in species with poorly developed root systems, e.g. Lycopodium selago. Our results show that coexisting plant species under severe nutrient limitation may tap several different N sources: NH _inf4^sup+ , NO _inf3^sup- and organic N from the soil, atmospheric N₂, and N in precipitation. Ericoid and ectomycorrhizal fungi are of major importance for plant N uptake in tundra ecosystems, and mycorrhizal fungi probably exert a major control on plant δ¹⁵N in organic soils.

Keywords: Arctic mycorrhiza; Legume and lichen N2 fixation; Nitrate reductase activity; Plant; Stable isotopes; soil and precipitation δ15N.