Gas Diffusion Coefficient in Variably Saturated Peat Soil: Development and Tests of Predictive Models

NASA Astrophysics Data System (ADS)

The soil-gas diffusion coefficient (Dp) and its dependency on air-filled porosity (?) govern gas diffusion and reaction processes in soil. Accurate Dp(?) prediction models for variably saturated peat soils are needed to evaluate vadose zone transport and fate of greenhouse gases such as methane in peaty wetlands. In this study, we measured Dp on undisturbed peat soil samples at different soil-water matric potentials, and developed new, linear and nonlinear expressions for describing and predicting Dp(?). The new Dp(?) models together with existing Dp(?) models were tested against both measured data and independent data sets from literature. Twelve undisturbed 100cm3 peat soil cores were taken between the soil surface and down to 30-cm depth at Bibai wetland, Hokkaido, Japan. The soil cores were initially saturated with water, and drained at given matric potentials, pF=1.0, 1.5, 1.8, 2.0, 3.0, and 4.1 (where pF equals to log | ? | , ?: the soil-water matric potential in cm H2O), using the hanging water and pressure plate methods. At each matric potential, simultaneous measurements of volume shrinkage, soil-water retention, and Dp were conducted. Literature datasets of Dp(?) for peat soil cores taken from different areas within the same wetland, specifically 12 samples from Iiyama and Hasegawa (2005) and 12 samples from Iiduka et al. (2008), were also used. A total of 191 measurements of Dp(?) at pF ? 2.0 were applied for developing new Dp(?) models for pF ? 2.0 where effects of shrinkage on Dp were assumed negligible. By modifying 3 existing Dp(?) models, the Buckingham (1904) model, the Macroporosity-Dependent Model (MPD; Moldrup et al., 2000), and the Penman-Call model (Moldrup et al., 2005), we suggested 3 new Dp(?) expressions for peat soil. In the Buckingham-based Dp(?) model, a variable X(?"w relationship (where X is the pore connectivity factor) derived from measurements was introduced in the Dp(?) expression. In the Penman-Call-based Dp(?) model, new expressions for the model parameters defining the linear Dp(?) relationship, the slope of Dp(?)/D0 and the threshold air-filled porosity where gas diffusion ceases due to complete water blockage, were derived from measured data. In the MPD-based Dp(?) model, a new Dp,100(?100) relationship for peat soil (where Dp,100 and ?100 are the gas diffusion coefficient and the air-filled porosity at pF2.0) was introduced in the model. To validate the new Dp(?) models, we tested the models against independent data for peat soil samples from Freijer (1994). The new Dp(?) models, except for the Penman-Call-based model, predicted well the independent data, and the Buckingham-based Dp(?) model performed the best among the existing and newly-developed models.

Unno, M.; Kawamoto, K.; Moldrup, P.; Komatsu, T.

2008-12-01