B4WarmED forest warming experiment: Effects on diversity, community structure and function of soil fungi and bacteria

Background/Question/Methods

Global temperatures are rising due to anthropogenic greenhouse gas emissions, and understanding microbial responses to this climatic change is critical given their roles in recycling of organic materials and nutrient transfer to plants.  Identifying trends in diversity and community structure is challenging given the difficulties of accurately characterizing highly-diverse communities.  Furthermore, many microbes (e.g. mycorrhizal fungi) form symbiotic relationships with plants and thus their response to changing abiotic factors are complicated by changes in the distribution and function of their hosts.  Therefore, evolved patterns of microbial diversity and community structure along abiotic gradients and established host ranges may not apply to local communities where temperatures are more rapidly increasing. To investigate these confounding issues, we characterized the taxonomic and functional diversity of microbial communities in one of the B4Warmed Project sites in Minnesota, USA, where above and below ground temperatures at the temperate-boreal ecotone have been experimentally manipulated since 2009.  We sampled roots of planted seedlings of ten native tree species and soil across three heat treatments (ambient, +1.7°C, +3.4°C) and two forest canopy treatments (open and closed) in September 2011, and sequenced fungal-specific (ITS rDNA) and bacterial-archael-specific (V4 rDNA) loci at ultra-high throughput depth using the Illumina MiSeq platform.

Results/Conclusions

Results of 7,002,692 high-quality paired end sequences identified 896 fungal OTUs and 2256 bacterial OTUs across 45 samples.  Fungal taxa were dominated by the phylum Basidiomycota and bacterial taxa were dominated by Acidobacteria and Protobacteria lineages.  Changes in diversity and community structure in response to warming and canopy treatments were not always consistent across the three components of the microbial community.  Chao1 richness estimator values across samples were positively correlated for soil-fungi, root-fungi, and soil-bacteria, and were significantly higher in open than in closed plots though not affected by warming.  There was a significant shift in community structure in response to forest canopy treatment for both fungal and bacterial communities, and soil warming had a significant effect on community similarity and phylogenetic structure of root-fungi and soil-bacteria only when considering abundance sensitive distance metrics.  Soil polyphenol peroxidase activity was significantly lower in open plots, while activities of cellulose-degrading and acid phosphatase enzymes were significantly higher in open plots.  No measured extracellular enzymes were affected by soil warming.  Together, these results suggest that direct soil warming due to climate change might have smaller effects on soil microbial communities than do more abrupt disturbances like clear-cutting and fire in boreal forests.