Prewinter soil water regime affects the post-winter cracking position on gully sidewall and slumping soil dynamics in Northeast China

Abstract

Gully erosion is one of the most destructive land degradation processes. Soil slumping along cracks on a gully sidewall is a crucial process of gully development, yet the factors controlling the dynamics and amount of slumped soil remain unclear. It was hypothesized that soil slumping along the cracks in summer might be affected by the prewinter soil water regime, as it determines the depth of freezing-induced fissures within the soil profile, which affects the depth and dynamics of the cracks on the gully sidewall. To test this hypothesis, the soil water regime was manipulated by infiltrating 250 mm of water into the plots placed with one edge 10, 30 and 50 cm away from a gully sidewall before winter. This plot layout eventually resulted in four treatments with different amounts of infiltrated water. The isolines of near-saturation soil water contents appeared closer to the ground and the sidewall under the higher infiltration treatments before and after winter. Soil freezing started from the topsoil on 14 Nov. 2017 and penetrated the 1-m soil profile within 24 days. Soil thawing started from both the topsoil and subsoils (1 m depth) on 13 Apr. 2018 and ended on 24 Apr. 2018. The cracks that appeared on the sidewalls in Apr. had similar lengths and widths at the depths with a near-saturation soil water content (between 25 and 45 g kg−1). The cracks appeared at deeper position on gully sidewall under lower infiltration treatments. The soil mass slumped from the sidewall was greater in Apr. and less from Jul. to Aug. under higher infiltration treatments. The annual total of slumped soil mass was the lowest in the low infiltration treatment, where the near-saturation isolines were the deepest. These findings suggested that the prewinter soil water regime along the slope controlled the depth of freezing within the soil profile. The freezing depth determined the depth and time of soil cracking and slumping on the gully sidewalls the following year. Our study implies that gully development through soil slumping depends on pre-winter slope hydrology, which should be considered in modeling and controlling gully development.