Elevated CO2 differentially alters belowground plant and soil microbial community structure in reed canary grass-invaded experimental wetlands

Several recent studies have indicated that an enriched atmosphere of carbon dioxide (CO2) could exacerbate the intensity of plant invasions within natural ecosystems, but little is known of how rising CO2 impacts the belowground characteristics of these invaded systems. In this study, we examined the effects of elevated CO2 and nitrogen (N) inputs on plant and soil microbial community characteristics of plant communities invaded by reed canary grass, Phalaris arundinacea L. We grew the invasive grass under two levels of invasion: the invader was either dominant (high invasion) at >90% plant cover or sub-dominant (low invasion) at <50% plant cover. Experimental wetland communities were grown for four months in greenhouses that received either 600 or 365 μl l−1 (ambient) CO2. Within each of three replicate rooms per CO2 treatment, the plant communities were grown under high (30 mg l−1) or low (5 mg l−1) N. In contrast to what is often predicted under N limitation, we found that elevated CO2 increased native graminoid biomass at low N, but not at high N. The aboveground biomass of reed canary grass did not respond to elevated CO2, despite it being a fast-growing C3 species. Although elevated CO2 had no impact on the plant biomass of heavily invaded communities, the relative abundance of several soil microbial indicators increased. In contrast, the moderately invaded plant communities displayed increased total root biomass under elevated CO2, while little impact occurred on the relative abundance of soil microbial indicators. Principal components analysis indicated that overall soil microbial community structure was distinct by CO2 level for the varying N and invasion treatments. This study demonstrates that even when elevated CO2 does not have visible effects on aboveground plant biomass, it can have large impacts belowground.