Fungal carbon sources in a pine forest: evidence from a 13C-labeled global change experiment

We used natural abundance 13C:12C (δ13C) and 8 yr of labeling with 13C-depleted CO2 in a Pinus taeda Free Air CO2 Enrichment (FACE) experiment to investigate carbon sources of saprotrophic fungi, ectomycorrhizal fungi, and fungi of uncertain life history. Sporocarp δ13C identified Sistotrema confluens as ectomycorrhizal, as suspected previously from morphological characteristics. Saprotrophic δ13C declined by 2 ‰-13 ‰ between ambient to elevated CO2 treatments and corresponded to different carbon sources, including surface litter (Rhodocollybia, Mycena), pine cones (Baeospora), wood (Gymnopilus, Pholiota), and soil (Ramariopsis). Ectomycorrhizal fungi, foliage, and surficial litter declined 12 ‰ in δ13C between ambient and elevated treatments, confirming that these fungi depend on recent photosynthate. The δ13C of ectomycorrhizal genera correlated between treatments with a slope (4.3 ± 1.2) greater than the expected value of one. This suggested that Inocybe and Cortinarius incorporated some pre-treatment, soil-derived carbon (presumably from amino acids) whereas Lactarius and Russula only incorporated current-year photosynthate or recent, litter-derived carbon. Combining natural abundance and tracer 13C measurements proved a powerful technique to examine carbon sources of different fungi.