Cryptogams signify key transition of bacteria and fungi in Arctic sand dune succession

Microbes are the first colonizers of barren soils and an important factor in vegetation establishment. Primary succession models focus on vascular plants, although most soil successions include mosses and lichens. Here, we aim to clarify the role of these cryptogam-dominated stages in plant-microbe interactions by covering the successional trajectory of bacterial and fungal communities in eroded Arctic inland dunes. The earliest successional stages with sparse or no plant cover (bare sand, grass) had low microbial biomass per organic matter. In the cryptogam stages with the moss Polytrichum piliferum and lichens, microbial biomass and community structure shifted towards the vascular vegetated stages (heath, mountain birch forest). Soil chemistry explained microbial community variation better than vegetation, but the transitions in chemistry and in plant and microbial community structure did not align. We detected potential autotrophic and trace gas oxidizing bacteria, such as WD272 (WPS-2), Chloroflexi and Cyanobacteria, which could form the basis of biomass accumulation from bare sand to cryptogam stages. The results indicate that when considering also belowground microbes, the ecosystem transition from vegetation loss to vegetation recovery starts at the moss stage. Polytrichum piliferum and the associated microbes may help stabilize the Arctic dune sand and facilitate vascular vegetation development.