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 delta(15)N and delta(13)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. delta(15)N and delta(13)C of ectomycorrhizal and saprotrophic fungi differed in both forests, and a dual delta(15)N/delta(13)C plot provided the best means of distinguishing them. Within both groups, delta(15)N and delta(13)C differed among genera and species, and the difference in species composition was an important determinant of the different overall delta(15)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.