Nitrogen and carbon isotopes and concentrations in ectomycorrhizal and saprotrophic sporocarps from the Swiss FACE experiment, 2001-2005
We analyzed %C, %N, δ13C, δ15N, and C: N stoichiometry of species of ectomycorrhizal and saprotrophic sporocarps (fungi) collected from the Swiss FACE experiment between 2001 and 2005 near Basel, Switzerland. In the experiment, canopies of 14 broad-leaved trees in a 550-m2-cluster were supplied with carbon dioxide through a network of plastic tubing (webFACE). The 13C of CO2 used to elevate CO2 levels had a distinct 13C signature useful for tracing carbon dynamics through the system both spatially and temporally. Here, although data for both saprotrophic and ectomycorrhizal fungi are in the data set, we focused on what the 13C and C/N of ectomycorrhizal sporocarps could tell us about the belowground movement of carbon in the plant-mycorrhizal system. We traced the movement of that carbon for four growing seasons into ectomycorrhizal sporocarps collected under CO2-labeled trees (elevated treatment), and at 0-6 m (elevated transition, ET), 6-12 m (ambient transition, AT), and greater than 12 m distance (ambient) from the labeled trees. We could then use patterns of 13C:12C ratios (expressed as 13C) and C/N in sporocarps across these treatments to infer whether amino acids or carbohydrates were primarily transported within mycorrhizal networks. Fungal carbohydrates (CHO) and protein (pro) will differ in 13C depending on source region, with 13Cpro-amb > 13CCHO-amb > 13Cpro-elev > 13CCHO-elev and C/NCHO > C/Npro. Results/Conclusions. Sporocarps derived 54±4%, 24±5%, and 12±5% of their carbon from labeled trees in the elevated, 0-6 m, and 6-12 m distances, respectively. However, low plant productivity in the cool and cloudy summer of 2004 reduced carbon transport so that carbon from labeled trees was not detected in the 6-12 m distance. In this study, sporocarp 13C correlated positively with C/N within the elevated CO2 distance and negatively elsewhere, reflecting that high-13C carbohydrates from the surrounding ambient zone contributed to sporocarps under elevated CO2. We calculated that 52% to 65% of carbohydrates and 26% to 0% of protein carbon was derived from trees from outside the elevated CO2 zone. These patterns accordingly indicated that 1) carbohydrates rather than amino acids were preferentially transferred between regions differing in source 13C, 2) this transfer can be quantified using the natural 13C depletion of fossil fuel-derived CO2, and 3) the average transport distance of carbon was decreased with lower plant productivity. This approach provided a powerful tool in 13C labeling studies to examine belowground fungal networks and their spatial extent.