Abstract <p>Partial mycoheterotrophy (mixotrophy, obtaining organic substances from mycorrhizal fungi) is widespread in mycorrhizal flowering plants. We hypothesized that alpine members of the Gentianaceae family, which contain fully mycoheterotrophic species with arbuscular mycorrhiza, may be largely mixotrophic. The isotopic (δ¹³С, δ¹⁵N, δ²H, and δ¹⁸O) and elemental (C and N) composition of leaves and roots of 13 pairs of high-mountain plant species (Gentianaceae species and nearby reference plants – dicotyledons with arbuscular mycorrhiza) of three Holarctic Mountain systems (Alps, Caucasus, and Tibet) were studied. Contrary to our hypothesis, the organic matter of the leaves and roots of Gentianaceae was not enriched in deuterium (²H) compared to reference plants, indicating the absence or low significance of mycoheterotrophy in the studied species. Leaves and roots of gentian plants had higher values of δ¹⁵N and δ¹⁸O, but lower values of δ¹³C and δ²H compared to the leaves and roots of reference plants. The leaves and roots of gentian plants contained more C, but the N content in the leaves was higher and, in the roots, it was lower than in the corresponding organs of the reference plants. We hypothesized that these results could be explained by more intense nitrogen fixation by prokaryotes of the leaf microbiome, providing better nitrogen nutrition to leaves and their more intense photosynthesis and transpiration.</p>