Gas diffusivity in undisturbed volcanic ash soils: Test of soil-water-characteristic-based prediction models

Soil-water-characteristic-dependent (SWC-dependent) models to predict the gas diffusion coefficient, DP, in undisturbed soil have only been tested within limited ranges of pore-size distribution and total porosity. Andisols (volcanic ash soils) exhibit unusually high porosities and water retention properties. The Campbell SWC model and two Campbell SWC-based models for predicting DP in undisturbed soil were tested against SWC and DP data for 18 Andisols and four Gray-lowland (paddy field) soils from Japan. The Campbell model accurately described SWC data for all 22 soils within the matric potential range from -10 to -15000 cm H2O. The SWC-dependent Buckingham-Burdine-Campbell (BBC) gas diffusivity model predicted DP data well within the same matric potential range for the 18 Andisols. The BBC model showed a minor but systematic underprediction of DP for three out of the four Gray-lowland soils, likely due to a blocky soil structure with internal fissures. A recent DP model that also takes into account macroporosity performed nearly as well as the BBC model. However, DP in the macropore region (air-filled pores greater than 30 mu m) was consistently underpredicted, likely due to high continuity of the macropore system in both Andisols and Gray-lowland soils. In agreement with previous model tests for 21 European soils (representing lower porosities and water retention properties), both SWC-dependent DP models gave better predictions for the 22 Japanese soils than soil-type independent models. Combining DP and SWC data, a so-called gas diffusion fingerprint (GDF) plot to describe soil aeration potential is proposed.