Effects of lithology and soil horizons on gully morphology in the Mollisol region of China

Gully erosion is the most destructive form of land degradation in NE China and many other regions around the globe. It is generally accepted that lithology and topography have an important impact on gully morphology. However, the effects of lithology and slope gradient on gully cross-sectional morphology remain unclear. This study aims to elucidate how the sequence of soil horizons and rock strata influence gully cross-sectional morphology in the Mollisol region of NE China. We measured the gully cross-sectional morphology by a combination of differential GPS, laser range finder, and ruler. Profile samples were collected by core drilling with depths from 1 m to 4 m at 39 sites along the gully thalweg and cross-sections. The physical and geo-chemical properties of 5 cm sub-samples were measured. The sequence of soil horizon and lithology in our study area was as follows (with increasing depth): (1) mollic epipedon (black soil horizon) and (2) subsoil developed in loess, (3) fine to coarse sands, (4) alternating layers of sand and shale debris, and (5) homogeneous shale. We found that the combined effect of soil horizons, lithology, and longitudinal slope gradient of the gully bottom determines gully cross-sectional morphology, which contrasts to simplified generalizations for homogeneous underground material. U-shaped gullies were related to the mollic epipedon, or cohesive sandy loess and fine sands, with an average gentle gully bottom gradient of < 10°. V-shaped gullies were either developed with cohesionless shale, and unconsolidated sediments even at a relatively gentle gradient of < 10°, or cohesive materials at steep gradients ranging from 16° to 27°. Overall, a detailed understanding of underground materials and their proper management are extremely urgent on steep gully slopes, especially in arable land that experienced severe gully erosion. Generalized models of the sequence of gully forms need to be differentiated prior to taking effective countermeasures.