Surface soil fungal and bacterial communities in aspen stands are resilient to eleven years of elevated CO2 and O3

Soil microbial community changes associated with altered carbon cycling dynamics were investigated in aspen (Populus tremuloides) stands exposed to elevated atmospheric CO2 (eCO2) and/or elevated ozone (eO3) for eleven years, at the Rhinelander Free Air CO2 Enrichment (FACE) experiment (Wisconsin, USA). The relative abundance and composition of fungal and bacterial communities in the 0-5 cm soil depth was evaluated using qPCR of fungal or bacterial rRNA genes, cloned 16S (bacterial) and LSU (fungal) rRNA gene sequencing, small subunit rRNA pyrosequencing, and soil shotgun metagenomes. Fungal or bacterial biomass, measured by qPCR, did not differ with treatment. Under eO3, the ratio of Basidiomycota to Ascomycota significantly increased by two-fold (P < 0.04) relative to the ambient control, but no robust significant differences (P > 0.1) in the ß diversity of fungal or bacterial communities were detected under eCO2. The overall composition of fungal and bacterial communities was resilient despite substantial changes in plant productivity, plant genotype, root density, and rates of soil C and N cycling under eCO2, documented in prior studies. Nonetheless, some candidate taxa (up to 20% of the total at the genus level) shifted in relative abundance in soils from eCO2 plots, suggesting that although community composition was generally stable, it was not static. The eCO2-responsive taxa did not shift under eCO2 plus eO3 conditions, suggesting an interactive effect of eO3 on the soil community. Our results show that the fungal and bacterial communities in the 0-5 cm depth mineral soil changed only in small ways after eleven years of eCO2. This study reinforces the view that soil microbial community changes in ecosystems responding to global climate change drivers may be highly-localized in soil strata.