Relationships among water and gas transport properties in soils from natural and induced clay gradients

Clay content in soil potentially has strong influence on soil structure and thereby controls soil physical, hydraulic, and gas transport properties. Only few studies have investigated the effect of clay content on saturated hydraulic conductivity (K-s) and gas transport parameters such relative gas diffusivity (D-p/D-0) and air permeability (K-a) under varying conditions of clay contents. In this study, two clay gradients were investigated: a natural clay gradient and an induced (clay mineral attapulgite-controlled) clay gradient. We determined the effect of the two clay gradients on soil air and water parameters and identified the potential relationships between these parameters and other physical properties. The results indicated that all D-p/D-0, K-a, and K-s decreased with increasing clay content. The differences among the various treatments were dependent on the soil pore characteristics (air-filled porosity, epsilon; pore continuity, POA; pore tortuosity, tau; and pore size distribution). Under the natural clay gradient, the increased clay content resulted in 52.8% more micropores and 35.2% fewer macropores. The slope of tau vs. clay content under the induced clay gradient was found to be higher than that under the natural gradient. Statistically meaningful correlations could be found between K-s and K-a (|r| = .904) and also between K-s and D-p/D-0 (|r| = .810), whereas D-p/D-0 was evidenced to be better correlated with K-s regardless of the type of clay gradient (P < .01). The results thus provide a valuable experimental and numerical insight to develop useful correlations among K-s, K-a, and D-p/D-0 under differing clay gradients.