Quantitative studies of gully slope erosion and soil physiochemical properties during freeze-thaw cycling in a Mollisol region

Gully erosion has been widely studied during the rainy season due to soil loss that seriously reduces arable area and decreases soil quality. However, very few publications have focused on gully slope erosion (GSE) during freeze thaw cycle (FTC). In this study, GSE on both active and stable gullies in Mollisol fields was investigated by 3D-photogrammetry. Soil bulk density (BD), soil moisture (SM), soil temperature (ST), daily maximum difference in soil temperature (MDT), saturated water (SW), field capacity (FC), soil organic carbon (SOC), soil total nitrogen (TN), water-stable soil aggregate (WA), vegetation cover rate (VC), root dry weight (RW), root length (RL), slope length (SL) and slope steepness (SS) were compared before- and after FTCs. The main results are as follows: (1) combined with both front and profile views, 3D photogrammetry can be used to monitor GSE; (2) GSE mainly occurred at the early stage of FTCs (approximately 80%) and was mainly determined by snowmelt of both the gully slope and farmland and was driven by the solar radiation in activity gully; (3) the high ST in surface soil layers (0–5 cm) of active gullies accelerated the GSE; (4) GSE on the active gully slope was 7.3–9.8 times greater than that on the stable gully slopes; (5) the plough pan as the important layer can effectively reduce GSE at upper slope positions in an active gully; (6) low values of VC, BD, SOC, RW, RL and macro-WA and high values of SL, SW and MDT in the middle of the gully slope typically accelerate the GSE; (7) the index SS*SL/VC can be used to predict GSE on Mollisol gully slopes. Generally, GSE was greatest after FTCs compare to the soil loss tolerance in the Mollisol region, especially in the middle slope position of the active gully, and should urgently be controlled.