Soil properties respond to crust forming under variable simulated rainfall events

Soil surface crusting is a common phenomenon on agricultural soils susceptible to rainfall drop impact. Crust affects soil hydrological properties, erosion, crop quality and yield, which implicates both agriculture and the environment. Whereas methods for determining hydraulic or basic properties of soil layers (such as thicker than 2 cm) are well established, measuring the soil characteristics of a thin crust (< 5 mm) remains a challenge. Therefore, in this study, we combine traditional lab methods and advanced techniques to test the variation in soil properties during the crust forming process. Composite samples from two soil textures were collected, dry-sieved at 8 mm, packed in soil pans and exposed to a range of rainfall amounts and two rainfall intensities, using a laboratory nozzle-type rainulator. Intact soil ring samples were collected after each rainfall event and scanned using X-ray Computed Tomography (CT) to gain more insight into rainfall-induced crust formation. Soil porosity, bulk density and the thickness of crust were derived from CT scans. Meanwhile, a scanning electron microscope (SEM) was employed to verify the variation of the crust layer thickness and soil properties. In addition, the water retention and infiltration dynamics of the developing seals were investigated with a minidisk infiltrometer placed on the crusts developed in the pans and a falling head permeameter (KSAT®) and evaporation method (HYPROP®) on soil cores taken. Shear strength was evaluated by hand vane. Disturbed soil was collected to explore variation in organic matter content and texture with rainfall. During the simulated rain events, soil loss, splash and runoff were followed as well. Overall, the purpose of this study was to reveal temporal variations of seal micro-morphology and their effect on soil properties with increasing rainfall amount. Our results showed the runoff volume and sediment mass increased, while splash and infiltration volume decreased with the increase in rainfall amount. Shear strength increased until 200 mm of rainfall. Additionally, (crust forming) rainfall amount had a rapid and strong effect on the hydraulic properties, with the unsaturated hydraulic conductivity being reduced as rainfall duration increased and the high rainfall intensity having a greater impact. These results were associated with rainfall-induced aggregate breakdown processes, which was confirmed by SEM images. It also demonstrated that crust development occurred up to at least 200 mm rainfall after cultivation. In summary, it was possible to illustrate the structural seal formation process and the temporal interrelated dominance and significance of the associated sub-processes which contribute to overcoming the challenge of testing the thin crust (< 5 mm).