Soil bacteria and archaea during 560 years of boreal forest development: early changes are the most profound

Using a gradient of changing ecosystem age, caused by glacial isostatic adjustment, we compared soils that spanned ages from 25 to 560-years-old. Using Illumina sequencing we investigated archaeal and bacterial composition, investigating how different phylogenetic groups change with ecosystem age and investigating correlations between community composition and edaphic factors. Bacterial communities dramatically changed during early ecosystem development (PerMANOVA, p<0.001), evidenced by significant compositional shifts between 25 and 115 year-old-soils. Although significant differences did occur in the three later aged sites, they did not change as much. This was consistent with vegetation that shifted from meadow (25 year) to alder dominated forest (115 year), to ecosystems containing spruce. Correlation networks revealed that the microbial communities became more interconnected in older age ecosystems with a two-fold increase in network density. Species richness had the opposite trend with a decreased number of species: 781 OTUs in the youngest ecosystem to 366 OTUs in the oldest ecosystem. The observed shifts in community composition are consistent with other reported ecosystem successional gradients, but here we show that not only does composition change, but as ecosystems age the network connectivity increases indicating potentially more microbial interactions or increasingly stringent biotic and edaphic filtering.