We examined the effects of elevated atmospheric CO2 on soil carbon decomposition in an experimental anaerobic wetland system. Pots containing either bare C4-derived soil or the C3 sedge Scirpus olneyi planted in C4-derived soil were incubated in greenhouse chambers at either ambient or twice-ambient atmospheric CO2. We measured CO2 flux from each pot, quantified soil organic matter (SOM) mineralization using δ13C, and determined root and shoot biomass. SOM mineralization increased in response to elevated CO2 by 83–218% (P<0.0001). In addition, soil redox potential was significantly and positively correlated with root biomass (P= 0.003). Our results (1) show that there is a positive feedback between elevated atmospheric CO2 concentrations and wetland SOM decomposition and (2) suggest that this process is mediated by the release of oxygen from the roots of wetland plants. Because this feedback may occur in any wetland system, including peatlands, these results suggest a limitation on the size of the carbon sink presented by anaerobic wetland soils in a future elevated-CO2 atmosphere.