Fungal communities and their association with nitrogen-fixing bacteria affect early decomposition of Norway spruce deadwood

Abstract

Deadwood decomposition is relevant in nature and wood inhabiting fungi (WIF) are its main decomposers. However, climate influence on WIF community and their interactions with bacteria are poorly understood. Therefore, we set up an in-field mesocosm experiment in the Italian Alps and monitored the effect of slope exposure (north- vs. south-facing slope) on the decomposition of Picea abies wood blocks and their microbiome over two years. Unlike fungal richness and diversity, we observed compositional and functional differences in the WIF communities as a function of exposure. Wood-degrading operational taxonomic units (OTUs) such as Mycena, and mycorrhizal and endophytic OTUs were characteristic of the south-facing slope. On the north-facing one, Mucoromycota, primarily Mucor, were abundant and mixotrophic basidiomycetes with limited lignin-degrading capacities had a higher prevalence compared to the southern slope. The colder, more humid conditions and prolonged snow-coverage at north exposure likely influenced the development of the wood-degrading microbial communities. Networks between WIF and N₂-fixing bacteria were composed of higher numbers of interacting microbial units and showed denser connections at the south-facing slope. The association of WIF to N₂-fixing Burkholderiales and Rhizobiales could have provided additional competitive advantages, especially for early wood colonization.

Figure 1

Overview of the (A) destructive sampling procedure of the P. abies experimental wood blocks and the underlying soil (0–5 cm); and (B) wood blocks at the different time points (12, 25, 52 and 104 weeks) over the 2-year observational period at the north- and the south-facing site.

Figure 2

Relationship between fungal richness and Shannon diversity of the P. abies experimental wood blocks and the underlying soil samples (0–5 cm) at the different time points (12, 25, 52 and 104 weeks) over the 2-year observational period at the north- and the south-facing site.

Figure 3

Taxonomic and functional composition of the P. abies experimental wood blocks and the underlying soil samples. (A) Distribution of taxa across wood blocks and soil samples. Operational taxonomic units (OTUs) detected by Illumina Miseq sequencing of the fungal ITS2 region were summarized based on their phylum annotation. (B) Non-metric multidimensional scaling (NMDS) based on Bray Curtis dissimilarities between the OTU compositions of the samples. Lowest stress was 0.139. The iteration converged after 20 tries. (C) The guild annotation of OTUs detected in all samples was predicted by their taxonomic annotation using FUNGuild. The functional composition of the samples was illustrated as the abundance of OTUs assigned to the guilds. FUNGuild annotations were manually summarized into fewer categories in order to simplify visualization and data accession (Table S1).

Figure 4

Associations between fungal and N₂-fixing operational taxonomic units (OTUs) in the P. abies experimental wood blocks at the north- and the south-facing site. (A,B) Distribution of fungal (A) and N₂-fixing bacterial (B) OTUs across the samples collected at north- and south exposure, respectively. The barcharts indicate the taxonomic distribution of the OTUs at the level of phylum (A) and family (B), respectively. Association networks calculated for the north- (C) and the south-facing (D) slopes. Every symbol represents a fungal or N₂-fixing bacterial OTU. Lines indicate the association of two OTUs. Unconnected OTUs were not included into the model. Symbol sizes correspond to the abundances of the OTUs. The binary adjacency matrix of the calculated networks was visualized in R using package igraph (http://igraph.org). (E) The barchart illustrates the taxonomic annotations of the association pairs in the north- and south-exposed wood blocks.