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.

DOI: https://doi.org/10.21954/ou.ro.0000d3c9

Abstract

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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