Contrasting wetland CH₄ emission responses to simulated glacial atmospheric CO₂ in temperate bogs and fens

Wetlands were the largest source of atmospheric methane (CH₄) during the Last Glacial Maximum (LGM), but the sensitivity of this source to exceptionally low atmospheric CO₂ concentration ([CO₂]) at the time has not been examined experimentally. We tested the hypothesis that LGM atmospheric [CO₂] reduced CH₄ emissions as a consequence of decreased photosynthate allocation to the rhizosphere. We exposed minerotrophic fen and ombrotrophic bog peatland mesocosms to simulated LGM (c. 200ppm) or ambient (c. 400ppm) [CO₂] over 21months (n=8 per treatment) and measured gaseous CH₄ flux, pore water dissolved CH₄ and volatile fatty acid (VFA; an indicator of plant carbon supply to the rhizosphere) concentrations. Cumulative CH₄ flux from fen mesocosms was suppressed by 29% (P<0.05) and rhizosphere pore water [CH₄] by c. 50% (P<0.01) in the LGM [CO₂], variables that remained unaffected in bog mesocosms. VFA analysis indicated that changes in plant root exudates were not the driving mechanism behind these results. Our data suggest that the LGM [CO₂] suppression of wetland CH₄ emissions is contingent on trophic status. The heterogeneous response may be attributable to differences in species assemblage that influence the dominant CH₄ production pathway, rhizosphere supplemented photosynthesis and CH₄ oxidation.