Development of Soil Crusts Under Simulated Rainfall and Crust Formation on a Loess Soil as Influenced by Polyacrylamide

Abstract

This study evaluated the morphological characteristics and dynamic variation in characteristics of soil crust and identified the relationships between soil crust and splash erosion under simulated rainfall. The effect of polyacrylamide (PAM) on soil aggregate stabilization and crust formation was also investigated. A laboratory rainfall simulation experiment was carried out using soil sample slices. The slices were examined under a polarized light microscopy and a scanning electron microscope (SEM). The results revealed that the soil crusts were thin and were characterized by a greater density, higher shear strength, finer porosity, and lower saturated hydraulic conductivity than the underlying soil. Two types of crusts, i.e., structural and depositional crusts, were observed. Soil texture was determined to be the most important soil variable influencing surface crust formation; depositional crust formation was primarily related to the skeleton characteristics of the soil and happened when the soil contained a high level of medium and large aggregates. The crust formation processes observed were as follows: 1) The fine particles on the soil surface became spattered, leached, and then rough in response to raindrop impact and 2) the fine particles were washed into the subsoil pores while a compact dense layer concurrently formed at soil surface due to the continual compaction by the raindrops. Therefore, the factors that influenced structural crust formation were a large amount of fine particles in the soil surface, continual impact of raindrops, dispersion of aggregates into fine particles, and the formation of a compact dense layer concurrently at the soil surface. It was concluded that the most important factor in the formation of soil crusts was raindrop impact. When polyacrylamide (PAM) was applied, it restored the soil structure and greatly increased soil aggregate stabilization. This effectively prevented crust formation. However, this function of PAM was not continuously effective and the crust reformed with long-term rainfall. In conclusion, this study showed that soil micromorphological studies were a useful method for evaluating soil crust formation.