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Soil methane uptake and release around Lutjewad

Laagland, P.J.M. (2012) Soil methane uptake and release around Lutjewad. Bachelor's Thesis, Physics.

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The enhanced greenhouse effect is changing the earth’s climate. Methane is one of the long-lived greenhouse gasses contributing to this enhanced greenhouse effect. It has, next to carbon dioxide, the second highest contribution to the total global mean radiative forcing of the longlived greenhouse gasses (0.48 W/m2 vs. 1.66 W/m2). That is why it is important to know the (sizes of the) sources and sinks of methane, releasing or taking up respectively methane from the atmosphere. One of the natural sources and sinks is the soil. In aerated soils, it is likely that aerobic organisms decompose methane into water and carbon dioxide. However, under anaerobic conditions, organic material will be decomposed to methane. The Centre for Isotope Research (CIO) has an atmospheric monitoring and sampling station called Lutjewad. It is located in the north of the Netherlands on the coastal side near the Wadden Sea. Therefore it is an interesting location to measure the methane fluxes in and out of the soil. The inverted cup method was used to determine the methane fluxes from the soil on a number of spots in the area around Lutjewad. Sampling sessions typical took an hour. An air sample from the start and end of each sampling session was analyzed with a gas chromatograph in the CIO laboratory for its methane content. Since the concentration does not increase linearly, it is not suitable to make a linear fit to those two data points. Therefore semi-continuous measurements on the carbon dioxide concentrations were performed in situ. These carbon dioxide records are used to determine the methane flux. This could be done since the relative diffusivity of methane with respect to carbon dioxide in air is known. In total 19 measurements have been performed to give an insight in the methane fluxes out of the soil at different locations around Lutjewad. As expected almost all measurements taken on the aerated agricultural areas on the main land showed a methane uptake. The negative fluxes on these locations ranges from -0.7±0.1 up to 0.000 g/m2/yr, with an average of -0.15 g/m2/yr. On the mudflat and wetlands outside the sea dyke the water table is near the soil surface, creating anaerobic conditions. As expected a net release of methane from these areas was recorded. The three measurement on the wetland resulted in a flux of 0.015±0.01, 0.20±0.04 and 0.42±0.04 g/m2/yr. The mudflat looked like a major source of methane after the first measurement of 10±3 g/m2/yr, but the following two measurements of 0.2±5 and 0.03±0.4 g/m2/yr couldn’t confirm this. Soil conditions like porosity and moisture content, and environmental conditions like temperature, atmospheric pressure and the water table depth influence the methane flux. No clear relationships between the methane flux and atmospheric temperature, soil temperature, soil moisture content, water table depth and atmospheric pressure have been found in this research. To find such relationships a number of measurements need to be performed at the same conditions of which the one under investigation should be varied. This was not the case and the origin of this research. The limiting number of measured fluxes was the main reason that clear relations were not visible. But the main goal was to give an indication of the methane fluxes from the soil in the area surrounding Lutjewad and that has succeeded.

Item Type: Thesis (Bachelor's Thesis)
Degree programme: Physics
Thesis type: Bachelor's Thesis
Language: English
Date Deposited: 15 Feb 2018 07:49
Last Modified: 15 Feb 2018 07:49

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