| Summary: | Many studies have found that future predicted CO<sub>2</sub> levels can increase plant mass but dilute N content in leaves, impacting antiherbivore compounds. Nitrogen-fixing plants may balance their leaf C:N ratio under elevated CO<sub>2</sub>, counteracting this dilution effect. However, we know little of how plants respond to herbivores at the higher CO<sub>2</sub> levels that occurred when nitrogen-fixing plants first evolved. We grew <i>Alnus incana</i> ssp. <i>rugosa</i> was grown at 400, 800, or 1600 ppm CO<sub>2</sub> in soil collected from the field, inoculated with <i>Frankia</i> and exposed to herbivores (<i>Orgyia leucostigma</i>). Elevated CO<sub>2</sub> increased nodulated plant biomass and stimulated the nitrogen fixation rate in the early growth stage. However, nitrogen-fixing plants were not able to balance their C:N ratio under elevated CO<sub>2</sub> after growing for 19 weeks. When plants were grown at 400 and 1600 ppm CO<sub>2,</sub> herbivores preferred to feed on leaves of nodulated plants. At 800 ppm CO<sub>2</sub>, nodulated plants accumulated more total phenolic compounds in response to herbivore damage than plants in the non-<i>Frankia</i> and non-herbivore treatments. Our results suggest that plant leaf defence, not leaf nutritional content, is the dominant driver of herbivory and nitrogen-fixing plants have limited ability to balance C:N ratios at elevated CO<sub>2</sub> in natural soil.
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