Soil bacteria and archaea change rapidly in the first century of Fennoscandian boreal forest development

- Connectivity of microbial communities increased with ecosystem age.
- 25-year-old soil had more diversity due to unique OTUs that disappeared over time.
- Members of Verrucomicrobia and Proteobacteria were common across ages.

Using a gradient of changing ground age, caused by glacial isostatic adjustment, we compared systems that spanned ages from 25 to 560-years-old. Illumina sequencing was applied to determine archaeal and bacterial composition, investigating how different phylogenetic groups change as ecosystems develop. Bacterial communities dramatically changed during early ecosystem development (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 operational taxonomic units (OTUs) in the youngest ecosystem to 366 OTUs in the oldest ecosystem. The observed shifts in community composition are consistent with other reported ecosystem gradients, but here we show that not only does composition change, but as ecosystems age the network connectivity increases indicating potentially more social interactions among microbes or increasingly stringent plant-microbe-soil interactions.

Ecosystem ages, here ground age, and associated vegetation across the land-uplift chronosequence at Bjuren, Sweden. Study sites are 25-year-old with meadow (arrow), 115-year-old with alder, 150-year-old with spruce-alder, 215-year-old with spruce, and 560-year-old with coniferous forest. Illustration made by Gun Lövdahl.440