Linking Soil Shrinkage Behavior and Cracking in Two Paddy Soils as Affected by Wetting and Drying Cycles

Soil cracks develop in paddy soils during wetting and drying (WD) cycles. Although changes in soil structure related to cracking have often been described as soil shrinkage, the relationship between soil shrinkage and cracking remains unclear. The objectives of this study were to investigate how WD cycles affect soil shrinkage and cracking and to establish a link between soil shrinkage in the laboratory and cracking in the field. Two paddy fields (one young paddy field and one old paddy field) were subjected to multiple WD cycles during rice growth. Soil cracks were photographed in the field, and the area density (D-c), compactness of cracks (CP), and equivalent width (EW) of the cracks were analyzed. The shrinkage curves of soil cores that were sampled in the two paddy fields were determined after they were subjected to various laboratory-based simulated WD cycles (including variations in intensity, frequency, and sequence). To estimate soil cracking, three parameterizations based on the shrinkage curves of the soil cores treated with non-predrying (Parameterization A), slight drying (Parameterization B), and intense drying (Parameterization C) were established. The YPF soil presented greater D-c and CP than the OPF soil (P < 0.05), whereas the OPF soil exhibited greater EW and coefficient of linear extensibility (P < 0.05). Area density was not only affected by the current soil water content or by the previous drying history. As the number of WD cycles increased, equivalent width generally increased, whereas CP decreased. The intensity of the WD cycles affected shrinkage more than the frequency or sequence. The fitted geometry factor, r(s), decreased from Parameterization A to Parameterization C for both soils. The D-c values were more accurately estimated by using soil shrinkage curves that included the intensity of the WD cycles. Our results demonstrate that the soil shrinkage curve associated with r(s) can better predict cracking when the effects of WD cycles are considered.