Effect of Particle Size and Soil Compaction on Gas Transport Parameters in Variably Saturated, Sandy Soils

The soil gas diffusion coefficient ( D p ) and air permeability ( k a ) and their dependency on soil air content (ε) control gas diffusion and advection in soils. This study investigated the effects of average particle size ( D 50 ) and dry bulk density (ρ b ) on D p and k a for six sandy soils under variably saturated conditions. Data showed that particle size markedly affects the effective diameter of the drained pores active in leading gas through the sample at −100 cm H 2 O of soil water matric potential (calculated from D p and k a ) as well as the average pore diameter at half saturation (calculated from the water retention curve), both exhibiting similar and exponential relationships with D 50 Under variably saturated conditions, higher D p and k a in coarser sand (larger D 50 ) were observed due to rapid gas diffusion and advection through the less tortuous large‐pore networks. In addition, soil compaction (larger ρ b ) simultaneously caused reduced water blockage effects and a reduction of large‐pore space, resulting in higher D p (ε) but lower k a (ε). Two recent models for D p (ε) and k a (ε) were evaluated: the water‐induced linear reduction (WLR) model for D p , and the reference‐point power law (RPL) model for k a , with reference point k a set at −100 cm H 2 O. The performance of both models for the sandy soils (particle size range 0.02–0.9 mm) was improved if the pore connectivity–tortuosity factor and water blockage factors were assumed to be functions of D 50 and ρ b Water blockage factors, N for the WLR D p (ε) model and M for the RPL k a (ε) model, showed a strong nonlinear relationship ( R 2 = 0.95) that seems promising for predicting D p (ε) from the more easily measureable k a (ε).