Enhanced CH4 emissions from a wetland soil exposed to elevated CO2

Methane emissions from wetland soils are generally a positive function of plant size and primary productivity, and may be expected to increase due to enhanced rates of plant growth in a future atmosphere of elevated CO2. We performed two experiments with Orontium aquaticum, a common emergent aquatic macrophyte in temperate and sub-tropical wetlands, to determine if enhanced rates of photosynthesis in elevated CO2 atmospheres would increase CH4 emissions from wetland soils. O. aquaticum was grown from seed in soil cores under ambient and elevated (ca. 2-times ambient) concentrations of CO2 in an initial glasshouse study lasting 3 months and then a growth chamber study lasting 6 months. Photosynthetic rates were 54 to 71% higher under elevated CO2 than ambient CO2, but plant biomass was not significantly different at the end of the experiment. In each case, CH4 emissions were higher under elevated than ambient CO2 levels after 2 to 4 months of treatment, suggesting a close coupling between photosynthesis and methanogenesis in our plant-soil system. Methane emissions in the growth chamber study increased by 136%. We observed a significant decrease in transpiration rates under elevated CO2 in the growth chamber study, and speculate that elevated CO2 may also stimulate CH4 emissions by increasing the extent and duration of flooding in some wetland ecosystems. Elevated CO2 may dramatically increase CH4 emissions from wetlands, a source that currently accounts for 40% of global emissions.