Using radiocarbon to determine the mycorrhizal status of fungi

Measurements of C-13 in fungal sporocarps are useful in assessing mycorrhizal or saprotrophic status. Because C-14 measurements can indicate the age of fungal carbon (C) and mycorrhizal fungi depend closely on recent photosynthate, C-14 may provide additional insight into possible mycorrhizal status. Sporocarps, needles, and litter from Woods Creek, OR, USA together with archived sporocarps were measured for C-14 content by accelerator mass spectrometry. Known mycorrhizal fungi resembled current-year needles (Amanita, Cantharellus and Gomphidius) or atmospheric CO₂ (Tuber) in C-14 and indicated an average age of 0-2 yr for incorporated C, whereas saprotrophic genera (Pleurocybella , Lepiota and Hypholoma) were composed of C at least 10 yr old. Of genera tentatively considered mycorrhizal from previous work with C-13, only Otidia and Sowerbyella appeared mycorrhizal from C-14 measurements, whereas Aleuria, Clavulina, Paurocotylis and Ramaria (sensu lato) consisted of older carbon and were presumably saprotrophic. C-14 clearly separated known mycorrhizal or saprotrophic fungi and indicated C-13 measurements should be interpreted cautiously on species of unknown status. C-14 results for needles and mycorrhizal fungi suggested that C sources other than atmospheric CO₂ may contribute small amounts of C. Possible sources include storage of carbohydrates and amino acids, organic nitrogen uptake, and incorporation of soil-respired CO₂ by anaplerotic or photosynthetic pathways