Divergent responses of wetland methane emissions to elevated atmospheric CO₂ dependent on water table

Elevated atmospheric CO₂ may have consequences for methane (CH₄) emissions from wetlands, yet the magnitude and direction remain unpredictable, because the associated mechanisms have not been fully investigated. Here, we established an in situ macrocosm experiment to compare the effects of elevated CO₂ (700 ppm) on the CH₄ emissions from two wetlands: an intermittently inundated Calamagrostis angustifolia marsh and a permanently inundated Carex lasiocarpa marsh. The elevated CO₂ increased CH₄ emissions by 27.6-57.6% in the C. angustifolia marsh, compared to a reduction of 18.7-23.5% in the C. lasiocarpa marsh. The CO₂-induced increase in CH₄ emissions from the C. angustifolia marsh was paralleled with (1) increased dissolved organic carbon (DOC) released from plant photosynthesis and (2) reduced (rate of) CH₄ oxidation due to a putative shift in methanotrophic community composition. In contrast, the CO₂-induced decrease in CH₄ emissions from the C. lasiocarpa marsh was associated with the increases in soil redox potential and pmoA gene abundance. We synthesized data from worldwide wetland ecosystems, and found that the responses of CH₄ emissions to elevated CO₂ was determined by the wetland water table levels and associated plant oxygen secretion capacity. In conditions with elevated CO₂, plants with a high oxygen secretion capacity suppress CH₄ emissions while plants with low oxygen secretion capacity stimulate CH₄ emissions; both effects are mediated via a feedback loop involving shifts in activities of methanogens and methanotrophs.