Evaluating Soil Aeration Dynamics in a Non‐Rigid Vertisol Through Shrink and Swelling Curves During Wetting‐Drying Cycle

ABSTRACT Soil air‐filled porosity (AFP) in non‐rigid soil is closely associated with the coupled changes in soil moisture and total porosity due to the high shrinkage‐swelling capacity. Although the dynamic of AFP during soil shrinkage (AFP sh ) has been well quantified by using a soil shrinkage model, the changes in AFP during soil swelling (AFP sw ) remain poorly understood. The objectives were to (1) characterize the dynamic of AFP sw during soil swelling and (2) evaluate the effect of initial bulk density (ρ b ) and wetting/drying (WD) cycles on the dynamic of AFP sw for a Vertisol. Repacked soil samples with different initial ρ b of 1.15, 1.30, and 1.45 g cm −3 were subjected to two WD cycles. Soil shrinkage/swelling curves, water/air‐filled porosity, and relative gas diffusion coefficient (D s /D 0 ) were determined during the two WD cycles. Our results showed that the dynamic of AFP sw was well described by the established AFP sw equation. Soil swelling curves intersected across ρ b treatments, whereas the shrinkage curves remained parallel between the 1.30 and 1.45 g cm −3 treatments. Soils with higher initial ρ b exhibited greater changes in porosity and AFP sw during swelling. Although neglecting soil shrinkage led to an overestimation of AFP (0.12–0.18 cm 3 ), ignoring the swelling‐derived pore deformation resulted in a greater underestimation of AFP (0.12–0.33 cm 3 cm −3 ). Over the two WD cycles, the soil exhibited the greatest structural change and the largest ΔAFP between AFP c and AFP sw in the first wetting process. Although soil swelling increased the AFP sw at θ FC and expanded the θ v range for AFP > 0.10 cm 3 cm −3 , the D s /D 0 at AFP sw = 0.10 cm 3 cm −3 did not reach the non‐limiting value of 0.02. Our result demonstrated that quantifying the dynamic of AFP sw/sh improved the assessment of soil aeration condition WD cycles and enhanced the accuracy of soil gas transport predictions in non‐rigid soils.