Microbial community dynamics and relationship with biogeochemical cycling across seasons in Arctic tundra
The Arctic is uniquely sensitive to global environmental change. A warmer climate has resulted in longer summers, and regions of sedge-dominated tussock tundra have shifted to woody deciduous shrubs. The above-ground shifts can be dramatic, however, the effect of altered seasons and vegetation are less clear for below-ground soil communities. Our aim was to describe shifts in soil microbial community composition and their relationship with biogeochemical variables across seasons (late winter, spring, fall, early winter) under three arctic vegetation types (shrub tundra, tussock tundra, wet sedge). We asked: 1. are bacterial and fungal community diversity and composition more strongly controlled by vegetation or season? 2. how dynamic are diversity and composition across seasons within each vegetation type and, 3. which biogeochemical variables are correlated with community shifts? Microbial community metrics were assessed using sequence-based analysis. Measured biogeochemical variables included extractable organic carbon and nitrogen (EOC and EON), microbial biomass carbon and nitrogen (MBC and MBN), inorganic N, and gravimetric water content.
Both the bacterial and fungal communities were more strongly shaped by vegetation type than by season, although composition also shifted seasonally across the landscape. The bacterial community was more taxonomically and phylogeneticially diverse in shrub and wet sedge tundra than in tussock tundra from late winter through fall. Tussock tundra microbial community composition was the most dynamic seasonally, with a large community shift at freeze, primarily driven by decreases in Acidobacteria in the bacteria and Ascomycota in the fungi, with the net result of overall increases in microbial diversity leading into winter. Across all, and within each vegetation type, bacterial community composition was strongly associated with several biogeochemical variables (e.g. MBC and MBN, EOC:EON ratio). The fungal community, however, did not correlate with any of the biogeochemical variables that were measured. We explored the hypothetical functional meaning of taxonomic shifts in the bacterial community using an in silico tool for predicting function from 16S rRNA marker genes (PICRUSt), which indicated genes for energy metabolism were most correlated with late summer function in shrub and wet sedge tundra, and genes for glycan biosynthesis were most correlated with late summer metabolism in tussock tundra. A missing link for predicting Arctic ecosystem response to climate change is the potential for soil microbial communities to shift compositionally and functionally. Here we show that the soil microbial community structure, diversity and functional potential are sensitive to both seasonal and vegetation change.