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. 200 ppm) or ambient (c. 400 ppm) [CO2] over 21 months (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.