15 N and 13 C natural abundance of autotrophic and myco-heterotrophic orchids provides insight into nitrogen and carbon gain from fungal association.

Abstract

• 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 15 N (ɛMHO-R =11.5‰) and 13 C (ɛMHO-R =8.4‰) than co-occurring non-orchids. δ15 N and δ13 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 15 N and 13 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.