Ice age wetland biogeochemistry: The influence of carbon dioxide starvation on wetland methane emissions

Boardman, Carl Philip (2010). Ice age wetland biogeochemistry: The influence of carbon dioxide starvation on wetland methane emissions. PhD thesis The Open University.



Ice core records show that the atmospheric concentration of methane (CH4) during the Last Glacial Maximum (LGM) was 40-50% lower than during the preindustrial Holocene. To understand this natural variation it is important to know how the sources and sinks of CH4 change over time. Natural wetlands were the single largest contributor of CH4 to the atmosphere in glacial times, yet models used to estimate their behaviour and CH4 flux are largely based around relationships derived under modem day conditions. This thesis responds to this issue by exposing wetland mesocosms with contrasting nutrient availability, to the atmospheric concentration of carbon dioxide (CO2) present at the LGM for 2 years.

At the end of this experiment, total CH4 flux was suppressed by an average of 29% in the nutrient rich fen (P < 0.05). In contrast, the nutrient poor bog showed no response to the treatment (P > 0.05). Further exploring the effects of CO2 starvation showed that the fen ecosystem exhibited notable reductions in dissolved organic carbon, dissolved CH4 and a change in the response of CH4 flux to changing temperature, variables and relationships which all remained unchanged in the bog. The contrasting response of the two ecosystems to CO2 starvation could be largely explained by differences in nutrient status, species composition and dominant CH4 production pathways. In particular, it is hypothesised that bog plants under LGM CO2 concentrations supplemented photosynthesis through the use of subsurface derived CO2, thus counteracting the treatment effect.

The results from this thesis suggest that the CH4 source strength of late-glacial and early Holocene wetlands may currently be over-estimated because fen ecosystems are a far smaller CH4 source under low atmospheric [CO2] than they are today. Furthermore, the results provide new insights into the role of glacial atmospheric CO2 concentrations in influencing CH4 emissions from terrestrial ecosystems and provide empirical evidence for a connection between glacial-interglacial changes in atmospheric CH4 and CO2 concentrations observed in ice cores.

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