Andrea Polle

Summary Roots of forest trees are colonized by a diverse spectrum of ectomycorrhizal (EM) fungal species differing in their nitrogen (N) acquisition abilities. Here, we hypothesized that root N gain is the result of EM fungal diversity or related to taxon‐specific traits for N uptake. To test our hypotheses, we traced 15 N enrichment in fine roots, coarse roots and taxon‐specific ectomycorrhizas in temperate beech forests in two regions and three seasons, feeding 1 mM NH 4 NO 3 labelled with either 15 NH 4 + or 15 NO 3 − . We morphotyped > 45 000 vital root tips and identified 51 of 53 detected EM species by sequencing. EM root tips exhibited strong, fungal taxon‐specific variation in 15 N enrichment with higher NH 4 + than NO 3 − enrichment. The translocation of N into the upper parts of the root system increased with increasing EM fungal diversity. Across the growth season, influential EM species predicting root N gain were not identified, probably due to high temporal dynamics of the species composition of EM assemblages. Our results support that root N acquisition is related to EM fungal community‐level traits and highlight the importance of EM diversity for tree N nutrition.