Wetland methane emission response to last glacial maximum atmospheric carbon dioxide concentration

Ice core records show that the atmospheric concentration of methane (CH4) during the Last Glacial Maximum (LGM) (~21,000 years ago) was 40% lower than the preindustrial Holocene. The contribution of natural wetlands to the global CH4 budget during the LGM is determined by modelling their spatial extent and productivity. Although models provide an estimated flux of ~75-180 Tg yr-1, they adopt present day physiological relationships to reconstruct past wetland emissions. Here we show that the LGM (180 ppm) carbon dioxide (CO2) concentration lowers CH4 emissions from peat cores incubated in controlled environments compared to cores maintained under a modern atmospheric CO2 concentration (380 ppm). Peat cores (110 x 400 mm) collected from a UK minerotrophic fen and upland ombrotrophic bog were maintained either in a [CO2] of 180 ppm or 380 ppm over 21 months. CH4 fluxes were measured on a monthly/weekly basis using static chambers with [CH4] measured via an LGR Fast CH4 Analyser and GC-FID. Results show that total CH4 flux from the minerotrophic fen was suppressed by 17 and 31% in season 1 and 2 respectively under LGM CO2 starvation. The ombrotrophic bog cores were suppressed by 20% in year 1 and 10% in year 2. Both peat types exhibited a rapid initial response to the sub-ambient [CO2] treatment with a change in CH4 flux recorded 5 days into the experiment. We also measured the influence of an LGM [CO2] atmosphere on CH4 flux temperature response during years 1 and 2. These results suggest that both wetland plants, and the underlying biogeochemistry of the rhizosphere, are sensitive to a reduction in [CO2] in the atmosphere and this has yet to be incorporated into global wetland CH4 models