N and C natural abundance of autotrophic and myco-heterotrophic orchids provides insight into nitrogen and carbon gain from fungal association.

•  Whereas mycorrhizal fungi are acknowledged to be the sources of nitrogen (N) and carbon (C) in achlorophyllous (myco-heterotrophic) orchids, the sources of these elements in autotrophic orchids are unknown. We have determined the stable isotope abundance of N and C to quantify their gain from different sources in these two functional groups and in non-orchids of distinctive mycorrhizal types. •  Leaves of each plant were collected from four forest and four grassland sites in Europe. The N and C isotope abundance, and total N concentrations of their tissues and of associated soils were determined. •  Myco-heterotrophic orchids were significantly more enriched in N (ɛ = 11.5‰) and C (ɛ = 8.4‰) than co-occurring non-orchids. δ N and δ C signatures of autotrophic orchids ranged from values typical of non-orchids to those more representative of myco-heterotrophic orchids. •  Utilization of fungi-derived N and C probably explains the relative N and C enrichment in the myco-heterotrophs. A linear two-source isotopic mixing model was used to estimate N and C gain of autotrophic orchids from their fungal associates. Of the putatively autotrophic species, Cephalanthera damasonium obtained the most N and C by the fungal route, but several other species also fell into the partially myco-heterotrophic category.