Seasonal dynamics of grassland soil microbial communities under experimental warming
Microorganism are among the most sensitive biota to surrounding environmental changes, such as seasonal biogeochemical variations of natural ecosystems due to their small-scale, fast propagation and ease of dispersal. Yet the seasonal dynamics of microbial communities are rarely analyzed in most microbial ecology studies. To understand the seasonal dynamics of microbial communities in response to warming, A total of 96 surface (0-15cm) soil samples were collected monthly during 2012 from both warmed (for 4 years) and control plots in a field experiment in central Oklahoma, and analyzed by MiSeq sequencing of 16S rRNA gene amplicons and a high-throughput functional gene array (i.e., GeoChip 5.0). Soil respiration and geochemical properties (i.e., temperature, moisture, and nitrogen and carbon content) were also measured to link the soil microbial community structure with environmental factors.
A total of 1.66 million high quality 16S rRNA amplicon sequences were obtained and clustered into 70,022 OTUs (97% identity cutoff) and 78,431 functional gene probes were detected using GeoChip. Both warming and sampling month significantly (P<0.005) affected the soil microbial community structure for taxonomic groups and functional genes. Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Verrucomicrobia and α- and δ-Proteobacteria had significant seasonal variation under warming, while only Gemmatimonadetes, δ-Proteobacteria and Chloroflexi showed seasonal changes in the control. This higher temporal divergence of warmed communities was confirmed by a higher taxonomic β-diversity (Sørensen index, P=0.02) of warmed samples compared with the control. The abundance of δ-Proteobacteria and Chlamydiae decreased, while that of Actinobacteria, Gemmatimonadetes and Firmicutes increased with warming. The α-diversity of functional genes peaked concurrently with spring and fall plant biomass and a month earlier in the warmed plots compared with control plots. Warming tended to increase carbon degradation genes but decrease carbon fixation and nitrogen cycling genes during peak plant biomass months. Both microbial taxonomic and functional structures significantly correlated with soil temperature and moisture, which also varied greatly across seasons and by warming. Together, these results reveal different seasonal succession patterns of soil microbial communities under warming.