Rates and quantities of carbon flux to ectomycorrhizal mycelium following C-14 pulse labeling of Pinus sylvestris seedlings: effects of litter patches and interaction with a wood-decomposer fungus

We used a novel digital autoradiographic technique that enabled, for the fi rst time, simultaneous visualization and quantification of spatial and temp oral changes in carbon allocation patterns in ectomycorrhizal mycelia. Myco rrhizal plants of Pinus sylvestris L. were grown in microcosms containing n on-sterile peat. The time course and spatial distribution of carbon allocat ion by Fl sylvestris to mycelia of its mycorrhizal partners, Paxillus invol utus (Batsch) Fr. and Suillus bovinus (L.): Kuntze, were quantified followi ng C-14 pulse labeling of the plants. Litter patches were used to investiga te the effects of nutrient resource quality on carbon allocation. The wood- decomposer fungus Phanerochaete velutina (D.C.: Pers.) Parmasto was introdu ced to evaluate competitive and territorial interactions between its myceli al cords and the mycelial system of S. bovinus. Growth of ectomycorrhizal mycelium was stimulated in the litter patches. Ne arly 60% of the C transferred from host plant to external mycorrhizal mycel ium (> 2 mm from root surfaces) was allocated to mycelium in the patches, w hich comprised only 12% of the soil area available for mycelial colonizatio n. Mycelia in the litter patch most recently colonized by mycorrhizal mycel ium received the largest investment of carbon, amounting to 27 to 50% of th e total C-14 in external mycorrhizal mycelium. The amount of C transfer to external mycelium of S. bovinus following pulse labeling was reduced from a maximum of 167 nmol in systems with no saprotr oph to a maximum of 61 nmol in systems interacting with P. velutina. The C- 14 content of S. bovinus mycelium reached a maximum 24-36 h after labeling in control microcosms, but allocation did not reach a peak until 56 h after labeling, when S, bovinus interacted with mycelium of fl velutina. The myc elium of S. bovinus contained 9% of the total C-14 in the plants (including mycorrhizae) at the end of the experiment, but this was reduced to 4% in t he presence of P. velutina. The results demonstrate the dynamic manner in which mycorrhizal mycelia dep loy C when foraging for nutrients. The inhibitory effect of the wood-decomp oser fungus P. velutina on C allocation to external mycorrhizal mycelium ha s important implications for nutrient cycling in forest ecosystems.