How soil carbon balance will be affected by plant–mycorrhizal interactions under future climate scenarios remains a significant unknown in our ability to forecast ecosystem carbon storage and fluxes. We examined the effects of soil temperature (14, 20, 26 °C) on the structure and extent of a multispecies community of arbuscular mycorrhizal (AM) fungi associated with Plantago lanceolata. To isolate fungi from roots, we used a mesh-divided pot system with separate hyphal compartments near and away from the plant. A 13C pulse label was then used to trace the flow of recently fixed photosynthate from plants into belowground pools and respiration. Temperature significantly altered the structure and allocation of the AM hyphal network, with a switch from more vesicles (storage) in cooled soils to more extensive extraradical hyphal networks (growth) in warmed soils. As soil temperature increased, we also observed an increase in the speed at which plant photosynthate was transferred to and respired by roots and AM fungi coupled with an increase in the amount of carbon respired per unit hyphal length. These differences were largely independent of plant size and rates of photosynthesis. In a warmer world, we would therefore expect more carbon losses to the atmosphere from AM fungal respiration, which are unlikely to be balanced by increased growth of AM fungal hyphae.
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