Green alder (Alnus viridis) encroachment shapes microbial communities in subalpine soils and impacts its bacterial or fungal symbionts differently.

Since the mid-twentieth century, subalpine grasslands undergo a progressive encroachment by Alnus viridis shrubs. Thanks to its rapid vegetative reproduction, its nitrogen fixing symbiosis with Frankia and its ectomycorrhizal cohorts, green alders are vigorous colonizers that quickly form mosaic of alder patches that evolves into a close canopy shrub community. To better understand how alder encroachment might influence microbial communities in this successional sequence, symbiont distribution, microbial richness and community structure in both soils and nodules were analyzed at three successional stages: grassland, mosaic and forest. Soil analyses were performed in association with measures of nitrification and denitrification, as well as DNA metabarcoding of three bacterial genes (16S rDNA, nifH and amoA) and one fungal gene (ITS1). Our results show that (i) A. viridis encroachment is associated with soil microbial community changes that are in turn, linked to certain soil properties (i.e., pH, C/N ratio and organic matter content), (ii) both taxonomic and N related functional gene structures of bacteria are modified by alder encroachment and (iii) the distribution in soils of its bacterial symbionts (Frankia) is apparently weakly influenced by alder establishment while Alnus-specific ectomyccorrhizae increase with the increase in alder shrub density.