Ecological drainage ditches function not only as conduits for surface runoff but also as effective measures for reducing nutrient loads, particularly nitrogen and phosphorus, and are therefore widely applied in mitigating agricultural non-point source pollution. Slope is a critical factor influencing nitrogen removal, as it regulates flow velocity and hydraulic retention time, both of which govern nutrient transport and biogeochemical reaction. Despite this importance, the effects of slope on nitrogen removal performance in drainage ditches remain insufficiently investigated. In this study, three types of ecological drainage ditches—concrete-pier porous-brick (ED1), vegetated porous-brick (ED2), and sod-lined gravel–sand–soil ditches (ED3)—were evaluated, with a conventional concrete ditch (CD) serving as a control. Field scouring experiments were conducted under three slope gradients (1°, 5°, and 9°), representing gentle, moderate, and steep conditions, with two flow rates (70 and 140 L·min−1) applied to assess nitrogen removal efficiency under different hydraulic regimes. Results showed that slope markedly affected TN removal efficiency, with distinct responses across ditch types. ED2 was the most sensitive to slope changes, with its TN removal efficiency decreasing by 15.8 % as slope increased from 1° to 9°. In comparison, TN removal efficiency declined by 14.0 % in ED3 and by only 3.7 % in ED1. Notably, ED3 maintained relatively stable TN removal efficiency across slope gradients while sustaining a high overall level of removal. Functional priorities of ditches varied with slope conditions: gentle slopes favored nutrient removal, moderate slopes necessitated a balance between water conveyance and nutrient removal, and steep slopes emphasized water conveyance supplemented by localized ecological interventions. These findings underscore the importance of aligning ditch types with slope conditions, thereby optimizing ecological engineering strategies for agricultural non-point source pollution control and improving regional water quality to support agricultural sustainability.
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