Relative abundances of the 8 most abundant bacterial phyla, the 4 most abundant classes within Proteobacteria, the fungal phyla and the 5 most abundant classes within Ascomycota and Basidiomycota after a 45-day incubation under varying mixing ratios of isoprene (0, 2, 20, 200 ppbv). Colors range from white (lowest value) to red (highest value) within each phylum, class and soil. Superscripts (A: Populus tremuloides; B: Pinus contorta) indicate a significant correlation between relative abundance and isoprene mixing ratios.