Bacterial communities of decaying Norway spruce follow distinct slope exposure and time-dependent trajectories.

Deadwood decay employs a complex metabolism and provides carbon and nutrients for soils. Although being highly diverse, the contribution of the bacterial deadwood colonizing community is underexplored compared with the fungal one. Therefore, we performed an in-field mesocosm study and monitored the bacterial communities in decaying experimental Picea abies wood blocks and their underlying soil on north- and south- exposed slopes in the Italian Alps over a 2-year period. The faster deadwood decay at the south-facing slope was associated with a higher bacterial richness and a higher number of specialist operational taxonomic units (OTUs) which were more strongly correlated to environmental parameters than other bacterial community members. With progressing decay, the wood and soil bacterial communities became more similar in terms of richness, diversity and evenness and especially at the south-facing slope, they also became more similar in terms of community composition. Exposure-specific OTUs suggest wood-soil interaction. However, despite the strong influence of exposure on the soil bacterial communities, the P. abies wood blocks shared a comparably high number of OTUs with the soil irrespective of the slope. At finer taxonomic scale, we identified Pseudomonas, Microbacteria, Sphingomonas, Xanthomonas, Methylovirgula and Burkholderia as decay associated, although their functional role needs further studies.