Soil surface properties and infiltration response to crust forming of a sandy loam and silt loam

Soil surface crusting is a common phenomenon on agricultural soils susceptible to raindrop impact. Crusts affect soil hydrological properties, erosion, crop quality and yield, which implicates both agriculture and the environment. While methods for determining hydraulic or basic properties of thick soil layers are well established, measuring the soil characteristics of a thin crust still remains a challenge. In this study, we combined traditional lab methods and advanced techniques to reveal temporal variations of crust micro-morphology and their effect on soil properties with cumulative rainfall. Composite samples from two soil textures, a sandy loam and a silt loam, were collected and 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. They were scanned using X-ray micro-computed Tomography (CT) to determine the evolution of soil porosity, bulk density and crust thickness during the crust formation process. The water permeability and infiltration dynamics of the developing crusts were investigated with minidisk infiltrometers placed on the crusts developed in the pans. Shear strength was evaluated by a hand vane. Disturbed soil was collected to explore variation in organic matter content and texture with cumulative rainfall. During the simulated rainfall events, soil loss, splash and runoff were recorded as well. We found that runoff volume and sediment mass increased, while splash and infiltration volume decreased with increasing rainfall amount. Shear strength increased until 200 mm of rainfall. Rainfall that resulted in crust formation had a rapid and strong effect on the hydraulic properties, with the unsaturated hydraulic conductivity being reduced as rainfall duration increased, and with high rainfall intensity having a greater impact than the low intensity. This was associated with rainfall-induced aggregate breakdown processes, which was confirmed by micro-CT. From the micro-CT images, we found that porosity reached a minimum value after 50 mm rainfall, while bulk density reached a maximum value. The dense crust was then partially removed/dissolved by further rainfall events. Crust thicknesses were about 3.19 and 4.85 mm, and the mean porosity of the crust layers was about 24 % and 27 % smaller than that of the underlying layer, at relatively high and low rainfall intensity, respectively. In conclusion, rainfall events significantly affect crust formation, on which the early-stage has the greatest influence. The crusts are rapidly formed under high rainfall intensity, but a thicker crust is formed under a longer duration of low rainfall intensity. The thickness of the crust increases with increasing rainfall, but its porosity does not decrease correspondingly.