Browsing by Subject "redox potential"
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(2024)Peatlands are a significant carbon and nitrogen reservoirs, making them potential sources of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions. Variations in water table level change the oxygen content of peat, affecting the oxidation-reduction or redox state of the peat, which is known to influence the biochemical processes and thus greenhouse gas (GHG) emissions. The aim of this study was to assess the effect of controlled anoxic redox conditions and inorganic electron acceptors (TEAs) on redox potential (Eh), and N2O, CH4, and CO2 emissions. In this study during an anaerobic incubation experiment, the rates of formation of these GHGs and Eh values as a function of time were measured from drained (D) and undrained (UD) peat of three nutrient levels: mesotrophic (ME), oligotrophic (OL), and ombrotrophic (OM). Redox conditions were controlled to three levels by nitrate (NO3-), ferric iron (Fe3+), and sulphate (SO42-). In addition, measurements were performed on untreated (Ctrl) peat. The peat was in an anoxic state throughout the incubation (Eh < 300 mV) and the values were in the order of TEA reduction, even though they were mainly in the iron and manganese reduction zones, probably due to the naturally high iron content of the peat. As expected, N2O formation was highest in flasks with added NO3-, and N2O formation was weak and ceased without addition. CH4 formation was reduced in flasks with added NO3- or SO42-, and SO42- addition also inhibited CO2 formation on which NO3- addition had no effect. In contrast, the addition of Fe3+ increased both CO2 and CH4 formation compared to Ctrl treatment, and it is possible that methanogens were involved in the reduction of Fe3+. In Ctrl flask, the redox state did not decrease to the lowest level compared to the other treatments as expected, but the Ctrl treated UD ME peat had the highest CH4 formation at the end of incubation. For all treatments, GHG emissions were higher from nutrient-rich peat in the descending order ME > OL > OM. In general, UD peat also had higher gas formation than D peat. All GHGs were formed the most while Eh values were around 0 mV and the value was especially high for CH4 formation, probably due to the linkage between methanogens and iron. The poor ability of the Pt electrode to detect NO3- or oxygen was the most likely reason for the variable and low Eh values of the flasks with NO3- addition. For the same reason, oxygen leakage of the anaerobic chamber was most likely responsible for the varying Eh values measured from Ctrl treated OM peat. This study suggests that Eh measurement is a useful predictor of the redox state and reactions, but it must be considered together with other measurements and analyses such as microbial analysis, nutrient analysis, and GHG measurements to predict redox processes and GHG emissions in anaerobic peatland. In particular, the role of iron on CH4 emissions requires further research.
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(2020)Boreal mires are natural sources of methane and contribute considerably to the global methane budget. Therefore, in order to comprehend the overall impact that these ecosystems have on climate change, it is essential to understand the factors that influence processes involved in methane production and consumption. Factors affecting methane flux vary between different mires, but there is also great spatial and temporal variation in flux within mires. In previous studies, temperature and water table position have been shown to influence methane flux, but vegetation could aid in explaining the small-scale variation. Vegetation can indicate spatial variation in water table position, but also affect methane flux directly by the transportation of methane through plant tissues, and by providing substrate for microorganisms through primary production. Furthermore, redox potential is a poorly studied factor that can reflect if chemical conditions in peat are suitable for methane production or consumption, making it a useful tool in predicting methane flux. In this thesis, I seek to identify if small-scale spatial variation in the methane flux occurs within the studied mire area. In addition, I strive to identify important controllers of the observed spatiotemporal variation in methane flux, with a specific focus on the effect of vegetation properties and redox potential. Methane and carbon dioxide fluxes were measured with the closed chamber technique at a boreal fen in Sodankylä (67°22'06.6"N 26°39'16.0"E) during the growing season in 2019. Flux measurements were carried out at nine measurement plots belonging to three different vegetation types: flark, lawn and string. Coverage and height of plant functional groups were followed during the summer and continuous redox potential was measured for each plot. CH4 fluxes of different plots and vegetation types were compared to study the spatial variation in methane flux. Generalized additive models (GAM) were used to determine which variables are best to explain spatiotemporal variation in methane flux over the growing season. Mean methane flux during the summer was 0.94 ug CH4 m-2 s-1 which is in the same magnitude as observed in a previous study at the fen. Some small-scale spatial variation in the methane fluxes was observed at the study site, with strings having lower flux than flaks and lawns. However, overall the spatial variation was small, while temporal variation in methane flux over the growing season was considerable. The best model, that was a combination of vegetation, redox potential and environmental variables, and it explained 72 % of the observed variation in methane flux. Vascular plant variables were the most important variables in the model, whereas moss functional groups were of lesser importance. Redox potential in deeper peat layers was also important in the model, but redox potential closer to the surface was not found to be significant. Vegetation is an important controller of methane flux, and this information could potentially be used when predicting methane flux over larger areas by using remote sensing to map vegetation characteristics. Redox potential, on the other hand, is relatively easy to measure, and the result suggests that it could provide a useful tool for improving the predictions of methane flux.
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