Abstract: Alpine meadow degradation in the Yellow River source area aggravates slope hydraulic erosion and induces severe loss of topsoil nitrogen and phosphorus via runoff and sediment transport, leading to prominent non-point source pollution risks. Previous studies have failed to quantify the independent soil and water conservation effects of above- and below-ground components of native hedgerows in fragile alpine environments. The synergistic mechanisms whereby canopy and root systems stabilize soil, retain nutrients and mitigate erosion under interactive slope and rainfall conditions remain unclear, limiting targeted ecological restoration of alpine meadows. This study conducted field simulated rainfall experiments in Henan Mongolian Autonomous County of the Yellow River source area. Three slope treatments were established: bare slope control (CK), root-only hedgerow (R), and intact Poa crymophila keng hedgerow (P). Experiments were implemented with a 45 min rainfall duration, two rainfall intensities (30 and 60 mm/h), and three slope gradients (10°, 20°, and 30°). Real-time monitoring of runoff yield, sediment generation, and nitrogen and phosphorus loss was performed to clarify the regulatory effects of hedgerow structures on slope erosion and nutrient loss. The results indicated that: 1) Complete hedgerows demonstrated the best soil and water conservation capacity; compared to bare slopes, they reduced total runoff volume by 55.77%–71.29% and total sediment yield by 70.99%–88.69%, and significantly delayed the onset of initial runoff on the slope (P<0.05); 2) The underground root system is the core functional unit of the hedgerows in preventing soil erosion, with average contribution rates to runoff and sediment reduction of 57.80% and 64.05%, respectively; its erosion control effectiveness was significantly superior to that of the above-ground stems and leaves (P<0.05); 3) Each functional component of hedgerows can significantly reduce nitrogen and phosphorus loss from slopes (P<0.05). The root system’s contribution to retaining nitrogen and phosphorus loss both exceeded 60%, making it a key component in maintaining surface soil nutrients; Nutrient loss from degraded alpine meadows is primarily in the form of particle-adsorbed compounds; the total losses of nitrogen and phosphorus bound to sediment were 5.56–15.17 times and 6.90–16.47 times, respectively, the amount of dissolved nutrients lost via runoff; 4) Three-Way ANOVA revealed that slope, rainfall intensity, slope treatment, and their interactions had highly significant effects (P<0.01) on both soil erosion and nutrient loss, with slope treatment being the primary dominant factor; Structural equation modeling further indicated that environmental factors such as slope, rainfall intensity, and slope treatment can directly influence, or indirectly affect through the regulation of soil erosion processes, the characteristics of nitrogen and phosphorus loss in runoff and sediment (P<0.01). In summary, as a near-natural ecological engineering measure, the Poa crymophila Keng hedgerows, relying on a three-dimensional synergistic protection system of “canopy retention of runoff and root consolidation of soil,” can effectively alleviate soil degradation in the high-altitude cold meadows of the Yellow River headwaters, providing a practical technical solution for the control of slope soil erosion and the prevention and control of non-point source pollution in plateau watersheds.