Non-destructive field method reveals diel hysteresis in ectomycorrhizal fungal temperature-respiration relationships

Background/Question/Methods Soil respiration is one of the largest C loss terms from terrestrial ecosystems, and its response to climate change could alter the terrestrial C balance, yet our ability to measure and model the contributions of different functional and phylogenetic groups to soil respiration is still primitive. Fungal respiration is a major contributor to soil respiration, yet good field estimates of fungal respiratory response to temperature are hard to find. We estimated respiration rates of sporocarps of the ectomycorrhizal fungus Laccaria laccata in a young pine plantation adjacent to the Houghton Rhizotron facility. We used a novel approach to gain in situ estimates of fungal sporocarp respiration. Using an LI-8100 soil respiration system with a custom chamber, we measured the diel patterns of fungal sporocarp respiration non-destructively. The chamber clamps around the stem of the sporocarp, allowing automated determination of rates at set intervals, with exposure to ambient conditions between measurements. Harvest at the end of this period permitted estimation of mass-specific respiration rates. Over the course of 24 hour measurement periods temperatures varied from just below 0°C to about 10°C.

Results/Conclusions We found that respiration of L. laccata exhibited a repeatable pattern of counterclockwise hysteresis in respiration over diel temperature cycles. Estimated Q₁₀ of respiration increased between the first and second half of both the falling and rising limb of these diel temperature cycles, resulting in a right-leaning D-shaped temperature-respiration relationship, with the falling limb approximately linear, and the rising limb steeply exponential. Estimated Q₁₀ values increased approximately 1.5 fold going down the descending limb, and approximately 3.6 fold going up the rising limb. Causes of these diel cycles are unclear, but could arise from circadian rhythms, cold tolerance or other thermal adaptation mechanisms, or resource availability dynamics arising from diel patterns of host C or water flux. Further investigation is required to determine the generality and causes of these patterns across fungal taxa and functional groups, especially comparing saprotrophs and mycorrhizal fungi. If these patterns are driven by cyclic resource limitation related to C fixation or hydraulic redistribution patterns, and are hence limited to mycorrhizal fungi, the hysteresis signal might provide a tool for partitioning saprotrophic and mycorrhizal respiration in soils.