Enhanced CH4 emissions from a wetland soil exposed to elevated CO2

Abstract

Methane emissions from wetland soils are generally a positive function ofplant size and primary productivity, and may be expected to increase dueto enhanced rates of plant growth in a future atmosphere of elevatedCO2. We performed two experiments with Orontium aquaticum, acommon emergent aquatic macrophyte in temperate and sub-tropical wetlands, todetermine if enhanced rates of photosynthesis in elevated CO2atmospheres 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 initialglasshouse study lasting 3 months and then a growth chamber study lasting 6months. Photosynthetic rates were 54 to 71% higher underelevated CO2 than ambient CO2, but plantbiomass was not significantly different at the end of the experiment. Ineach case, CH4 emissions were higher under elevated thanambient CO2 levels after 2 to 4 months of treatment, suggestinga close coupling between photosynthesis and methanogenesis in our plant-soilsystem. Methane emissions in the growth chamber study increased by 136%. We observed a significant decrease in transpirationrates under elevated CO2 in the growth chamber study, andspeculate that elevated CO2 may also stimulate CH4 emissions by increasing the extent and duration offlooding in some wetland ecosystems. Elevated CO2 maydramatically increase CH4 emissions from wetlands, a sourcethat currently accounts for 40% of global emissions.