Modeling Air Permeability in Variably Saturated Soil from Two Natural Clay Gradients

Understanding soil–gas phase properties and processes is important for finding solutions to critical environmental problems such as greenhouse gas emissions and transport of gaseous-phase contaminants in soils. Soil–air permeability, ka (μm2), is the key parameter governing advective gas movement in soil and is controlled by soil physical characteristics representing soil texture and structure. Models predicting ka as a function of air-filled porosity (ɛ) often use a reference-point measurement, for example, ka,1000 at ɛ1000 (where the measurement is done at a suction of –1000 cm H2O). Using ka measurements from two Danish arable fields, each located on natural clay gradients, this study presents a pore tortuosity–disconnectivity analysis to characterize the soil–gas phase. The main objective of this study is to investigate the effect of soil–moisture condition, clay content, and other potential drivers of soil texture and structure on soil-gas phase characteristics based on a ka–based pore tortuosity parameter, Xa [= log(ka/ka,1000)/log(ɛ/ɛ,1000)]. Results showed that Xa did not vary significantly with soil matric potential (in the range of –10 to –1000 cm H2O), but the average Xa across moisture conditions showed a strong linear relation (R2 = 0.74) to clay content. The Xa, further showed promising relations to specific surface area, Rosin–Rammler particle size distribution indices, α and β (representing characteristic particle size and degree of sorting, respectively), and the Campbell water retention parameter, b. Considering clay as a main driver of soil–gas phase characteristics, we developed expressions linking clay content and ka,1000 at ɛ1000 and discussed the effect of clay content on general ka–ɛ behavior.