Contribution of bedrock dip angle impact to nitrogen and phosphorus leakage loss under artificial rainfall simulations on slopes parallel to and perpendicular to the bedrock dip in a karst trough valley

•Surface nutrient loss was higher on dip-parallel slope than dip-perpendicular slope.•Surface runoff-associated TN and TP were highest under gentle bedrock dip angle.•Critical bedrock angle was 40° for underground nutrient on dip-perpendicular slope.•Underground sediment-associated nutrient loss was highest under steep bedrock angle. The bedrock dip angle is a critical factor for nutrient leakage loss on karst trough valley slopes. This study aimed to a) reveal the effects of the bedrock dip angle on the characteristics of nutrient leakage loss on dip-parallel/dip-perpendicular slopes and b) determine the bedrock dip angle threshold for nutrient leakage loss in karst trough valleys. To achieve this objective, the geographical structure of a karst trough valley slope was simulated using a newly designed steel experimental flume and a monoclinal bedrock tilt, respectively. The results revealed that i) for a flat bedrock dip angle of 10°, the surface runoff-associated TN and TP pollution moduli were maximal for dip-parallel slopes, while the underground sediment-associated nutrient leakage loss is minimum on dip-perpendicular slope. ii) Under dip-perpendicular slopes, 40° was the critical bedrock dip angle for underground runoff-associated nutrient leakage loss. iii) Surface TN and TP losses on dip-parallel slopes were always higher than those of dip-perpendicular slopes, and underground nutrient leakage showed the opposite trend. The underground sediment-associated nutrient loss rate was dominant under steep rock dip angle, and the opposite was found for flat and gentle rock dip angle conditions. These results could provide a theoretical foundation for controlling agricultural non-point source pollution and modelling in karst trough valleys under similar conditions.