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DIVISION S-1—SOIL PHYSICS

Three-Porosity Model for Predicting the Gas Diffusion Coefficient in Undisturbed Soil

^a Environmental Engineering Section, Dep. of Life Sciences, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^b City and Environment Section, Aalborg Municipality, Vesterbro 14, DK-9000 Aalborg, Denmark
^c Dep. of Hilly Land Agriculture, National Agricultural Research Center for Western Region, Ikano 2575, Zentsuji, Kagawa, 765-0053 Japan
^d Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Saitama, 338-8570 Japan
^e Soils and Biogeochemistry, Dep. of Land, Air and Water Resources, University of California, Davis, CA 95616

^* Corresponding author (pm{at}bio.auc.dk).

The soil gas diffusion coefficient (D_P) and its dependency on air-filled porosity () govern most gas diffusion-reaction processes in soil. Accurate D_P() prediction models for undisturbed soils are needed in vadose zone transport and fate models. The objective of this paper was to develop a D_P() model with lower input parameter requirement and similar prediction accuracy as recent soil-type dependent models. Combining three gas diffusivity models: (i) a general power-law D_P() model, (ii) the classical Buckingham (1904) model for D_P at air saturation, and (iii) a recent macroporosity dependent model for D_P at –100 cm H₂O of soil–water matric potential (), yielded a single equation to predict D_P as a function of the actual , the total porosity (), and the macroporosity (₁₀₀; defined as the air-filled porosity at = –100 cm H₂O). The new model, termed the three-porosity model (TPM), requires only one point (at –100 cm H₂O) on the soil–water characteristic curve (SWC), compared with recent D_P() models that require knowledge of the entire SWC. The D_P() was measured at different on undisturbed soil samples from dark-red Latosols (Brazil) and Yellow soils (Japan), representing different tillage intensities. The TPM and five other D_P() models were tested against the new data (17 soils) and data from the literature for additional 43 undisturbed soils. The new TPM performed equally well (root mean square error [RMSE] in relative gas diffusivity <0.027) as recent SWC-dependent D_P() models and better than typically used soil type independent models.

Abbreviations: AIC, Akaike's information criterion • BBC, Buckingham–Burdine–Campbell • D_p, soil gas diffusion coefficient • ODR, oxygen diffusion rate • RMSE, root mean square error (of prediction) • SWC, soil water characteristic curve • TPM, three-porosity model • , soil air-filled porosity

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