Abstract: Ectomycorrhizal trees obtain nitrogen (N) through direct root uptake from soil (root pathway) and via ectomycorrhizal fungal mycelia translocated to roots (mycelia pathway). However, estimates of the contribution of the mycelial pathway to tree N acquisition (f_fungi) remain highly uncertain, and the key influencing factors are not well defined. In this study, we employed the ¹⁵N natural abundance method to quantify the relative contribution of the mycelial pathway to N acquisition in ectomycorrhizal trees (Abies fargesii var. faxoniana and Betula utilis) on the eastern Qinghai–Tibetan Plateau. We further analyzed the influencing factors of f_fungi. This study revealed that the contribution of mycelia pathways to total N uptake was comparable to that of root pathways in both A. fargesii var. faxoniana and B. utilis. Specifically, the mycelia pathway contributed to 33.6% ~ 71.8% of total N uptake in A. fargesii var. faxoniana and 41.6% ~ 63.0% in B. utilis. Moreover, the soil C/N, mean annual temperature and soil pH were identified as a key influencing factor for f_fungi. This study provides novel perspectives and insights for understanding N acquisition strategies and their influencing factors in ectomycorrhizal trees within natural ecosystems.

Key words: nitrogen stable isotope, ectomycorrhizal tree, mycelia pathway, nitrogen uptake, subalpine forest