Impact of long-term fertilizer and summer warming treatments on bulk soil and birch rhizosphere microbial communities in mesic Arctic tundra

Low Arctic tundra soil communities were examined 12 years after the initial establishment of yearly maintained fertilizer and warming treatments near Daring Lake in the Northwest Territories, Canada. Bacterial and fungal soil communities in bulk soil, and in the rhizosphere of Arctic birch, Betula glandulosa, were determined in 5 different treatment plots (control, nitrogen, phosphate, nitrogen plus phosphate, and greenhouse summer warming). Bacterial and fungal communities amplified using 16S rRNA and internal transcribed spacer templates respectively, indicated considerable bulk soil and rhizosphere microbial community differences among the fertilizer treatments, especially for the combined nitrogen plus phosphate amendments. In particular, the N + P treatment significantly increased the relative proportion of Xanthomonadaceae, a bacterial family associated with fertile Arctic soils. Correspondingly, the observed relative increases in the ectomycorrhizal fungi Thelephora and Russula, in both the soil and rhizosphere communities, may provide enhanced nutrient acquisition abilities for their principal host B. glandulosa, thereby making this shrub species more competitive, and ultimately contributing to the recent vegetation 'greening' trends across Arctic tundra. Strikingly however, the bulk soil and rhizosphere microbiota for the control and greenhouse summer warming treatments were very similar. Together, these results underscore the contrast between different experimental simulations of ongoing Arctic climate warming and their impacts on soil and rhizosphere microbial communities in mesic birch tundra ecosystems.