Determinants of ¹⁵N Natural Abundance in Leaves of Co-Occurring Plant Species and Types within an Alpine Lichen Heath in the Northern Caucasus

Several factors may have interactive effects on natural ¹⁵N abundance of plant species. Some of these effects could be associated with different plant functional types, including mycorrhizal association type. Due to its high taxonomic and functional diversity, the alpine heath community in the Caucasus is a suitable object for studying ¹⁵N natural abundance of plants in relation to different functional/mycorrhizal groups, contrasting with the limited numbers of plant groups or species considered in previous studies of individual communities. The N concentration and δ¹⁵N were determined in leaves of 25 plant species from 8 functional/mycorrhizal groups from an alpine lichen heath in the Teberda Reserve, Northern Caucasus, Russia. Functional groups were represented by ericoid mycorrhizal species (ERI), ectomycorrhizal species (ECT), arbuscular mycorrhizal forbs (AM—FORB), arbuscular mycorrhizal grasses (AM—GRA), arbuscular mycorrhizal nodulated legumes (FAB—N), non-mycorrhizal graminoids (sedges and rushes) (NOM—GRA), non-mycorrhizal hemiparasites (NOM—SP), and orchids (ORC). We can summarize our results in two rankings for leaf N concentration (FAB—N > ORC > AM—FORB, ECT > NOM—SP, ERI ≥⃒ NOM—GRA, AM—GRA) and leaf δ¹⁵N signature (ORC > NOM—GRA, FAB—N > ECT ≥⃒ ERI ≥⃒ AM—FORB, NOM—SP, AM—GRA) of alpine heath species. We conclude that, within the alpine lichen heath in the Northern Caucasus, the δ¹⁵N signature of plant foliage is a relevant indicator of plant functional groups with relatively high ¹⁵N content (ORC, FAB-N, NOM-GRA), while the absence of a significant difference between relatively ¹⁵N-depleted groups (AM, ERI, and ECT species) isn't clear and may result from both processes, as the increased N isotope fractionation by arbuscular mycorrhizal fungi as the decreased role of ecto- and ericoid mycorrhizal fungi in the flux of N.