Patterns of nitrogen and carbon stable isotope ratios in macrofungi, plants and soils in two old-growth conifer forests.

•  To further assess the usefulness of stable isotope ratios for understanding elemental cycling and fungal ecology, we measured δ N and δ C in ectomycorrhizal and saprotrophic macrofungi, plants, woody debris and soils from two old-growth conifer forests in Olympic National Park, Washington, USA. •  Ecosystem isotope patterns were similar at the two forests, but differences existed that appear to reflect soil nitrogen availability and C allocation within the ectomycorrhizal symbioses. δ N and δ C of ectomycorrhizal and saprotrophic fungi differed in both forests, and a dual δ N/δ C plot provided the best means of distinguishing them. Within both groups, δ N and δ C differed among genera and species, and the difference in species composition was an important determinant of the different overall δ N of the ectomycorrhizal fungi at the two forests. •  Variation in multiple ecophysiological traits such as organic N use, mycelial morphology and transfer of N to phytobionts appears to underlie the variation in the isotope signatures of ectomycorrhizal fungi. •  The varied isotope signatures of ectomycorrhizal fungi suggest considerable functional diversity among them. Life-history strategies could provide a framework for interpreting these patterns.