Experimental warming selects for fungal decomposers that use recalcitrant carbon

Background/Question/Methods

Boreal ecosystems contain large stocks of soil organic compounds that can be used for growth by microorganisms. Warmer temperatures may facilitate breakdown of the substrates, potentially leading to greater carbon release to the atmosphere. However, it is unclear how elevated temperatures will affect microbial contributions to biogeochemical cycling in natural boreal forests. We measured the changes in fungal community structure using Illumina sequencing and bioinformatics of the Fungal ITS2 after years of exposure to warming and compared results to unwarmed controls in Alaskan surface soils. We also tested how temperature changes may be linked to functional traits. In particular, we conducted an in situ enrichment trial to categorize the degree to which the fungal taxa used labile, intermediate, and recalcitrant carbon. We hypothesized that evolutionary-tradeoffs would lead to selection for recalcitrant carbon users after warming, because recalcitrant carbon is more easily depolymerized at high temperature.

Results/Conclusions

We found that fungal community composition differed significantly between the warmed and control treatments (P = 0.013). In support of our hypothesis, the degree to which fungal taxa used lignin, a recalcitrant substrate, was positively (albeit weakly) related to the extent to which they responded to warming (n = 256, P = 0.023, R² = 0.020). In contrast, the use of labile or intermediate carbon substrates by taxa was not significantly related to their warming responses. Hence, soil warming induced a proliferation of fungi that decompose recalcitrant carbon. This work reveals a potential positive feedback to changes in climate: warming may lead to decomposition of previously-unavailable resources, likely driven by selected groups of fungi.