Temperature dependence of respiration rates of mycorrhizal fungi: Acclimation or limitation?

Background/Question/Methods

Fungi are key mediators of terrestrial carbon cycling via their role as symbionts, saprotrophs and pathogens.  Forest ecosystems are large storage pools of carbon, with the largest component of that carbon stored in the soil. Whether carbon is accumulated in or lost from ecosystems is determined by the balance between gross photosynthesis and ecosystem respiration, of which soil respiration is a major component. The relative contribution of roots, mycorrhizal fungi and free-living heterotrophs to soil respiration is poorly understood.  Given the relative abundance of fungi in most higher-latitude soils, we expect fungal respiration to be a significant fraction of soil respiration, but good estimates of fungal respiration are notoriously hard to find. We have been working to get a better estimate of the mycorrhizal component of soil respiration, and of the temperature-dependence of that respiration. We have measured respiration of fungal sporocarps as well as arbuscular mycorrhizal and ectomycorrhizal fungal mycelia in the field and in the laboratory.

Results/Conclusions

Respiration increases with temperature. In pure culture experiments, respiration rate increases with a relatively constant Q₁₀. However, in symbiosis field respiration rates of mycorrhizal fungi deviate from a fixed Q₁₀ model at ambient temperatures, appearing to approach an asymptote as temperature rises, indicating either acclimation or resource limitation of respiration, or both. We discuss the likelihood of alternative mechanisms of respiratory limitation. This constraint on temperature response of mycorrhizal fungal respiration has significant implications for modeling the fungal contribution to ecosystem respiration.