Complex interactions between long-term and climate extremes are likely to have strong and unpredictable effects on soil microbial communities and their functioning, although there is potential for plant communities to mitigate these effects. We carried out a microcosm study where soil samples were taken from a long-term (9 year) warming and plant manipulation experiment on peatland and subjected to a severe drought. We tested the hypothesis that long-term warming results in shifts in the composition of peatland microbial communities, which in turn reduces their resilience to further climate extremes, i.e. drought. We also tested the hypothesis that dwarf-shrub presence increases resilience of the microbial community to drought. The microcosm field experiment included plots that have been subject to long term warming, with and without dwarf-shrub removal, and soils were subject to an experimental drought in the laboratory in a full factorial design. After five weeks of severe drought we rewetted the peat, then measured a set of response variables over a time series (immediately before rewetting and days 1, 7, 14, 28 and 56) to assess resilience of the microbial community and its functional capabilities. These included inorganic nitrogen (N), greenhouse gas emissions (GHGs), enzymatic activity and microbial community composition.
Results/Conclusions
We found that long-term warming resulted in a small shift in the microbial community, and that this was associated with an increase in inorganic N availability and GHGs in both well-watered and droughted soil. However, warming was not associated with an increase in enzyme activity. When the peat was droughted, presence of dwarf-shrubs was a more important driver of resilience than warming. We found an interactive effect between warming and shrub presence on resilience of all response variables. In unwarmed peat, shrub presence increased resilience of N, GHGs and enzyme activity to drought. In warmed peat, however, resilience of all functions was highest when shrubs were removed. Our results show that presence of dwarf-shrubs is more important to the resilience of the microbial community, and associated functions, than long-term warming. We suggest that this is because of their role in introducing phenolics into the peat, which slow microbial activity. When shrubs were removed, there was often little effect of drought. This could indicate that activity was mainly impeded by phenolics released by shrubs, and not waterlogging. Taken together, vegetation plays a crucial role in maintaining resilience of a peatland microbial community to long-term and extreme climate changes.