Effects of hedgerows on soil erosion and nutrient loss in degraded alpine meadows in the Yellow River source area

Alpine meadow degradation has induced severe loss of topsoil nitrogen and phosphorus via runoff and sediment transport, leading to non-point source pollution risks. Particularly, there is slope hydraulic erosion in the Yellow River source area. It is often required to quantify the independent soil and water conservation for above- and below-ground components of native hedgerows in fragile alpine environments. Ecological restoration of alpine meadows is also urgent to clarify the synergistic mechanisms, where canopy and root systems stabilize soil and nutrients to reduce erosion under the interaction of slope and rainfall. This study aims to explore the effects of hedgerows on soil erosion and nutrient loss in degraded alpine meadows. Experiments were conducted to simulate field rainfall 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). Experimental simulations were implemented with a 45 min rainfall duration, two rainfall intensities (30 and 60 mm/h), and three slope gradients (10°, 20°, and 30°). Parameters were then monitored in real time, including runoff yield, sediment generation, and nitrogen and phosphorus loss. A specific mechanism was finally determined to clarify the regulatory effects of hedgerow structures on slope erosion and nutrient loss. The results indicated that: 1) Complete hedgerows demonstrated that the best performance was achieved for soil and water conservation; The total runoff volume and the sediment yield were reduced by 55.77%-71.29% and 70.99%-88.69%, respectively, compared with bare slopes. The initial runoff was significantly delayed on the slope (P<0.05); 2) Underground root system was contributed most to the effects of the hedgerows on soil erosion; and 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 was significantly reduced nitrogen and phosphorus loss from slopes (P<0.05). The contribution rates of the root system exceeded 60% to maintain surface soil nutrients, thereby reducing nitrogen and phosphorus loss. Nutrient loss from degraded alpine meadows was represented by 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, indicating the amount of dissolved nutrients lost via runoff; 4) Three-Way ANOVA revealed that the slope, rainfall intensity, slope treatment, and their interactions shared significant effects (P<0.01) on both soil erosion and nutrient loss, where slope treatment was the primary dominant factor; Structural equation modeling further indicated that environmental factors, such as slope, rainfall intensity, and slope treatment, also shared the significant effects on soil erosion, as well as the nitrogen and phosphorus loss in runoff and sediment (P<0.01). In summary, the Poa crymophila Keng hedgerows can serve as near-natural ecological engineering. A three-dimensional synergistic protection 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. The findings can also provide a practical technical solution to prevent slope soil erosion and non-point source pollution in plateau watersheds.