The role of fungi in mediating ecosystem responses to global change

Background/Question/Methods
In this synthesis of published studies, we address two global change scenarios (global warming and anthropogenic N deposition) in which fungi can form feedbacks on carbon cycling that are counterintuitive to prevailing predictions and models. In each case, a consideration of fungal ecology, physiology, or evolutionary constraints could lead to changes in the sign or magnitude of carbon storage within soils, compared to current predictions. In the case of global warming, the majority of global carbon models that address this issue are parameterized so that decomposition rates increase in response
to warming, and that this increase is sustained in the long-term. Indeed, the majority of field studies have confirmed this response, at least within the first years after onset of warming. Nevertheless, longer-term studies have reported reductions in the warming effect over time. One mechanism for this decline is thermal adaptation of fungal communities, in which mass-specific respiration rates decrease following long-term exposure to warming, possibly owing to changes in efficiencies of enzymes. In addition, evolutionary trade-offs could reduce the ability of warming-adapted strains to use particular carbon substrates or to tolerate drier conditions that often coincide with warming. In respect to N deposition, recent coupled C-N models either (1) allow inorganic N availability to increase decomposition rates overall, or (2) do not include a direct effect of N availability on decomposition. However, meta-analyses indicate that microbes typically become less abundant following N enrichment, with corresponding reductions in respiration rates. In addition, turnover rates of recalcitrant material can decrease. These declines in microbial abundance and activity could result from sensitivity of microbial taxa (particularly white rot fungi) to toxins produced via “browning” of organic compounds by nitrogenous compounds. Moreover, mycorrhizal fungi decrease under N enrichment owing to reduction in investments by host plants. Any decomposition of soil organic matter by mycorrhizal fungi could likewise be
reduced.

Results/Conclusions
Microbial ecologists can support the efforts of modelers by examining and quantifying relevant microbial mechanisms that provide feedbacks on global change, especially if these feedbacks are counter-intuitive. For large-scale models, global syntheses of empirical data would be especially valuable in determining the extent of these feedbacks.

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