Production and emission of peat gas has attracted great interest because substantial amounts of methane (CH4) are emitted to the atmosphere from peat soils. Many studies indicate supersaturation of CH4 in peat water, implying a high potential for gas bubble formation. However, observations of bubbles in peat are often only qualitatively described, and in most cases the presence of entrapped gas has been largely ignored in peatland studies. On the basis of a review of literature, a conceptual model of entrapped gas dynamics was developed and investigated using field and laboratory measurements at a poor fen in central Québec. We investigated variations in production and volume of gas and the effect of this gas on trace gas emissions, peat buoyancy, and pore water chemistry during 2002 and 2003. Measurements made with moisture probes and subsurface gas collectors revealed that gas volume varied throughout the growing season in relation to hydrostatic and barometric pressure. Shifts in entrapped gas volume were also coincident with changes in dissolved pore water CH4. The presence of these bubbles has important biogeochemical effects, including the development of localized CH4 diffusion gradients, alteration of local flow paths affecting substrate delivery, peat buoyancy, and the potential episodic release of CH4 via ebullition events. These interactions must be included in peatland models to describe accurately the hydrology and greenhouse gas emissions from these ecosystems and to make predictions about their response to environmental change.
[1] Recent studies suggest that ebullition of biogenic gas bubbles is an important process of CH 4 transfer from northern peatlands into the atmosphere and, as such, needs to be better described by models of peat carbon dynamics. We develop and test a simple ebullition model in which a threshold gas volume in the peat has to be exceeded before ebullition occurs. The model assumes that the gas volume varies because of gas production and variations in pressure and temperature. We incubated peat cores in the laboratory for 190 days and measured their volumetric gas contents and the ebullition flux. The laboratory results support the threshold concept and, considering the simplicity of the model, the calculated ebullition compared well with measured fluxes during the final 120 days with an r 2 of 0.66. An improved, more realistic description would also include temporal and spatial variations in gas production and bubble retention terms.
[1] Dynamics of biogenic bubbles in peat soils were studied at a field site in southern Québec, Canada. The maximum gas content measured in this study varied spatially with a maximum seasonal increase in volumetric gas content of 0.15. The size of changes in total gas content of a 1 m deep profile was comparable to the seasonal water storage change. Changes in bubble volume in the saturated zone alter the water table level and, consequently, the water content in the unsaturated zone and the apparent water budget. In highly compressible soils (and floating root mats), buoyancy forces from bubbles also cause relations between the surface and the water table to change. These effects cannot be omitted in modeling the hydrology of peatlands. Our results indicate a great spatial variability of trapped bubbles. Using pressure transducers sealed to the surface, we found pressure deviations indicating small areas closed off by bubbles clogging the pores. The hydrological influence of these areas may be considerable as they may restrict or deflect water flows. Open pipe piezometers did not show these pressure deviations, possibly because the closed zones were too small to influence the head in pipes or because of less amount of gas close to the pipe screen.Citation: Kellner, E., J. M. Waddington, and J. S. Price (2005), Dynamics of biogenic gas bubbles in peat: Potential effects on water storage and peat deformation, Water Resour. Res., 41, W08417,
Since peat soil differs from mineral soil in several respects, mineral-soil calibration functions for time domain reflectometry (TDR) are not necessarily applicable. This paper evaluates a number of calibration functions, both empirical polynomial and theoretical mixing models, on the basis of laboratory measurements on undisturbed Sphagnum peat samples. Deviations between different samples within this study indicate dissimilarities in dielectric properties between peats with different degrees of humification. Connections to physical properties such as amount of bound water and structural orientation are likely to exist. There is, however, a lack of methods to measure and quantify parameters expressing these properties. Therefore, until further studies on physical properties are accomplished, empirical or semi-empirical calibration curves are preferred. The best fit was obtained by an empirical, third order polynomial model. This model also gave a better fit than the mixing models when data were grouped into humification classes. However, all models reproduced pooled data with an r2 better than 0.93.
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