Will Wetlands Amplify or Attenuate Climate Change In Response to Elevated Carbon Dioxide?.

Wetlands influence climate because they hold a large pool of soil organic carbon and are a strong source of methane gas. Likewise, changes in climate and other global factors have important implications for the delivery of wetland ecosystem services. I will review research on wetland responses to rising CO₂, with particular attention to effects on the balance between soil carbon sequestration and methane emissions. Elevated CO₂ experiments in peatlands, non-tidal and tidal marshes, and rice paddies demonstrate perturbations of the greenhouse gas balance that vary strongly across ecosystem types. Elevated CO₂ generally stimulates photosynthesis and growth in temperate wetlands, but results are equivocal for peatlands. In wetland forests and tidal wetlands, a portion of the elevated CO₂-enhanced or ‘excess’ plant production is sequestered in long-term pools such as wood or soil organic matter. The implications of increased soil carbon sequestration for tidal wetlands are profound because the process affects their ability to gain elevation as sea level rises and avoid conversion to open water. The positive effects of elevated CO₂ on tidal marsh elevation gain are diminished by soil nitrogen fertilization, indicating that elevated CO₂ will interact with other global changes such as eutrophication. The largest portion of the additional plant matter produced at elevated CO₂ is rapidly recycled in plant and microbial respiration, including methanogenesis. Elevated CO₂ studies report an increase in CH₄ emissions, potentially offsetting the decrease in radiative forcing expected from increased plant growth. More elevated CO₂ research is needed in peatlands, freshwater marshes, and forested wetlands. Progressive experimental designs should seek to understand how elevated CO₂ interacts with other global change factors such as sea level rise, eutrophication, elevated temperature, and invasive plant species.