Transfer of 13C between paired Douglas-fir seedlings reveals plant kinship effects and uptake of exudates by ectomycorrhizas

Processes governing the fixation, partitioning, and mineralization of carbon in soils are under increasing scrutiny as we develop a more comprehensive understanding of global carbon cycling. Here we examined fixation by Douglas-fir seedlings and transfer to associated ectomycorrhizal fungi, soil microbes, and full-sibling or nonsibling neighbouring seedlings. Stable isotope probing with 99% C-13-CO2 was applied to trace C-13-labelled photosynthate throughout plants, fungi, and soil microbes in an experiment designed to assess the effect of relatedness on C-13 transfer between plant pairs. The fixation and transfer of the C-13 label to plant, fungal, and soil microbial tissue was examined in biomass and phospholipid fatty acids. After a 6 d chase period, c. 26.8% of the C-13 remaining in the system was translocated below ground. Enrichment was proportionally greatest in ectomycorrhizal biomass. The presence of mesh barriers (0.5 or 35 mu m) between seedlings did not restrict C-13 transfer. Fungi were the primary recipients of 13 C-labelled photosynthate throughout the system, representing 60-70% of total C-13-enriched phospholipids. Full-sibling pairs exhibited significantly greater C-13 transfer to recipient roots in two of four Douglas-fir families, representing three-and fourfold increases (+ c. 4 mu g excess C-13) compared with nonsibling pairs. The existence of a root/mycorrhizal exudation-hyphal uptake pathway was supported.