Abstract: The Anning River Valley is characterized by dramatic topographic relief, extensive distribution of slo-ping farmland, and intensive tillage practices, which together lead to frequent soil preferential flow. We investigated the spatial heterogeneity of preferential flow and its dominant controlling factors under different tillage practices. We compared the spatial morphological characteristics and driving mechanisms of preferential flow under the long-term downslope tillage and contour tillage practices in two typical sloping farmlands, by using field dye tracer experiment and image analysis method. The results showed that downslope tillage led to the perferential flow extending longitudinally in a tree-like pattern, with obvious path differentiation and strong connectivity, whereas contour tillage results in a clod-like, laterally diffused preferential flow with uniform distribution and weak fracture development. Under the condition of long-term downslope tillage, the preferential flow fraction, length index, and macroporosity followed the order of lower slope > middle slope > upper slope, while the order under contour tillage was upper slope > middle slope > lower slope. At the same slope position, all these indicators were higher under downslope tillage than under contour tillage. Under both tillage practices, the dye coverage ratio generally decreased with increasing soil depth, and downslope tillage showed higher dye coverage than contour tillage at all soil depths. The average staining area ratio under downslope tillage practice was higher than that under contour tillage practice. Under the condition of downslope tillage practice, the staining area ratio along the path increased rapidly in the top layer (0-5 cm) and then gradually decreased. In contrast, the vertical distribution of the staining area ratio under contour tillage practice tended to be stable. Slope position was the key factor governing the development of preferential flow pathways under downslope tillage, whereas soil layer depth was the dominant one influencing the spatial distribution of preferential flow under contour tillage. In conclusion, downslope tillage significantly promoted the development of preferential flow, while contour tillage effectively inhibited excessive preferential flow by altering water flow paths and spatial distribution patterns. These findings would provide a scientific basis for the integrated management of soil erosion on sloping farmlands in the Anning River Valley area.

Key words: Anning Valley area, tillage practice, soil preferential flow, soil erosion