Nitrogen Isotopes in Soils and Plants of Tundra Ecosystems in the Khibiny Mountains

Resumo

The isotopic composition of nitrogen in soils and plants may be an indicator of transformation of its compounds and sources of N nutrition of plants. Natural ¹⁵N abundance (δ¹⁵N) was determined in soils (the total, ammonium, and nitrate nitrogen) and in plant leaves and roots of four tundra ecosystems in the Khibiny Mountains. The studied soils (Folic Leptic Entic Podzol and Leptosols) significantly differ in N availability, and plants are represented by the species, forming ectomycorrhiza, ericoid mycorrhiza, and arbuscular mycorrhiza, as well as by the species, which usually do not form a mycorrhiza. The range of δ¹⁵N in soil inorganic compounds is from –16.2 ‰ in nitrates to +6.4‰ in ammonium, which reflects the correlation between the activities of N-mineralization and nitrification and δ¹⁵N-\({\text{NH}}_{4}^{ + }\), as well as a potentially strong effect on the isotopic composition of nitrogen in plants. The value of δ¹⁵N in plant leaves and roots changes in a narrower range (from –7.3 to +2.4‰), which may be related to N uptake from different sources and to fractionation of N isotopes during N assimilation. Roots are ¹⁵N-enriched in comparison with leaves in most of the studied plant species, which corresponds to the concept of mycorrhiza participation in N nutrition of plants. Regardless of the type of mycorrhizal symbiosis, the difference in δ¹⁵N between roots and leaves of most plant species decreases contrary to N availability in soils.