Nutrient cycling in temperate forests is driven by carbon allocation of trees to soil via ectomycorrhizas (EM). The sink activities of different fungal taxa for host resources are unknown. Aboveground dual labeling of young beech with ¹⁵N and ¹³C was used to trace resource transport to ectomycorrhizal root tips. Isotope enrichment in EM correlated with that in the corresponding EM-attached lateral root, supporting that EM drive taxon-specific N- and C-fluxes. The enrichments with ¹³C and ¹⁵N in EM increased with decreasing C/N ratio of the symbiotic association.  Abundances of EM species were positively correlated with ¹³C enrichment, demonstrating higher fitness of stronger than of less C-demanding symbioses. Overall, our results imply that differences among the resource traits of EM species regulate the supply of the symbioses with host-derived C and N.

Here we provide the data sets containing information on the identities of fungal species colonizing roots tips of European beech and for N, C, 15N and 13C contents in bark, coarse roots, fine roots, very fine lateral roots, ectomycorrhizal species and rhizosphere soil and for biomass of the different compartments. The data were collected 5 and 20 days after labelling.

Young, healthy beech (Fagus sylvatica) trees (n = 40) with fully open leaves were excavated in May in a beech forest. The trees were carefully removed and placed with intact soil layers individually in pots (diameter: 183 mm, height 255 mm, corresponding to about 6.5 L soil volume) and were kept under common garden conditions in Göttingen until labelling. Three adjacent top, three middle and three bottom leaves were slightly abraded and exposed for 3 days to ¹⁵NH₄Cl (99% NH₄Cl, Campro Scientific GmbH, Berlin, Germany). The labelled leaves were removed and the plants were exposed in gas-tight chambers for two days to ¹²CO₂, then for three days to ¹³CO₂ and then for two days to ¹²CO_2.  The experimental exposures were conducted in two independent runs, each with ten trees. Ten non-labelled trees were used as controls.

Harvests took place immediately after removal and 15 days after removal of the trees from the chambers, corresponding to 5 and 20 days since the start of the ¹³CO₂ treatment. Rhizosphere soil, bark, roots and ectomycorrhizas were harvested, used for stable isotope analyses and mycorrhizal species identification by internal transcribed spacer (ITS) sequencing. The mycorrhizal sequences were blasted in NCBI GenBank (www.ncbi.nlm.nih.gov) and UNITE (unite.ut.ee) databases and deposited in NCBI Genbank under the accession numbers MK430999 to MK431014. The morphotypes, species assignments and abundances are listed in Table 1.

The labelled and non-labelled soil, plant and fungal samples were analyzed in an isotope ratio mass spectrometer (IRMS; Delta C, Finnigan MAT, Bremen, Germany) in the Center for Stable Isotopes (KOSI, University of Goettingen, Göttingen, Germany) to obtain C, N, 13C and 15N contents. The data are shown in Table 2.