Impacts of Anaerobic Methane Oxidation, Electron Acceptors, and Physical Controls on Net Methane Emissions from Northern Peatlands in Alaska and Finland PDF Download

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ISBN: 9781321363418
Category : Electronic books
Languages : en
Pages : 141

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Book Description
In this dissertation I focused on some of the least-understood aspects of the carbon cycle in northern peatlands - the biological controls on production, the presence and importance of the anaerobic CH4 consumption pathway, and the physical controls on emission. A more clear understanding of the controls on CH4 emissions from critical northern peatland systems will help constraint predictive models of carbon-climate feedbacks. In chapter 1, I evaluated the linkages between porewater CH4, CO2, and iron concentrations within the upper active layer of a chronosequence of wetland basins in Barrow, Alaska. Iron concentrations varied amongst basin ages, with younger basins containing more iron in the upper soil profiles. Basin age also correlated with the thickness of the organic layer. Basinspecific seasonal mean porewater CH4 concentrations had a negative relationship with total Fe and Fe(III) concentrations; CH4 concentrations were positively related to organic layer thickness. Thus, the highest seasonal mean concentrations of CH4 were found in older basins with thick organic layers and low Fe loads. A manipulated experiment confirmed a direct suppression effect on net CH4 fluxes following Fe(III) and humic acids soil amendments, thus connecting in situ CH4 production and release with soil electron acceptor availability. Chapters 2 and 3 present the findings of a pair of anoxic soil incubations that use stable isotope tracers to simultaneously determine methanogenesis and anaerobic oxidation of methane (AOM) rates. In both experiments, I used treatments to determine the effect of different electron acceptors on CH4 cycling rates. The in vitro incubations of Alaskan soil showed a significant positive correlation between methanogenesis and AOM rates, and an increase in methanogenesis rates with increasing depth within the active layer. There was also an interaction between soil depth and the kinetic rate constant for AOM, suggesting that AOM increased with Fe(III) presence in shallow soil depths. Genetic surveys of Barrow soils show 16S rRNA and mcrA gene evidence for microbes closely related to known methanotrophs in the ANME groups 2 and 3. In Barrow soil incubations, AOM rates were greater than methanogenesis rates, causing negative net CH4 fluxes; net fluxes were lowest in shallow, Fe(III)-treated soils. Using soils from Finland, in vitro incubations revealed no relationship between rates of methanogenesis and AOM. Nitratetreated soils showed a significant suppression of methanogenesis, and a significant delay in the onset of AOM. While methanogenesis was greater than AOM, leading to a net positive soil CH4 flux, AOM consumed a considerable percentage of CH4 produced (6-39%), constituting a formidable constraint on CH4 emissions. Chapter 4 presents the results of a year-round field campaign in Finland. The annual flux data show strong seasonality in the CH4 fluxes. Interseasonal variations in carbon fluxes were not significantly related to either air or soil temperatures, although summer fluxes were positively related to air temperatures. There is also evidence for a substantial autumnal CH4 burst, and a lesser but still distinguishable burst during spring soil thaw, which combined accounted for a significant portion of the annual landscape CH4 flux. Summer CH4 fluxes were measured in situ, which allowed for the collection of data on the frequency and magnitude of CH4 ebullition events. Growing season CH4 ebullition events contributed an additional 50% of the diffusive CH4 atmospheric flux, and showed strong fine-scale heterogeneity within the wetland landscape.