Effects of roots and canopy of Medicago sativa L. on soil and water conservation for steep spoil heaps

Spoil heaps are vulnerable to intense rainfall, and the understanding of the impact of vegetation on runoff and sediment production is limited. Models of spoil heaps with dimensions of 3.46 m × 2.00 m × 0.60 m (with a 30° slope and 10% gravel content in the entire soil profile) were constructed. Three planting densities of grass ( Medicago sativa L. 20 cm × 20 cm, 15 cm × 15 cm, and 10 cm× 10 cm) were established, with a bare slope (BS) as the control. A rainfall simulator was used to investigate how planting densities of two treatments of intact grass (IG) with an average height of the canopy of 38.07±15.52 cm and only roots (OR) influenced runoff and sediment production under rainfall intensities of 0.8, 1.2, and 1.8 mm/min. Compared with the BS plots, the runoff generation times (RGTs) of the IG and OR treatments decreased by 18.34%–79.73%. Compared with BS, the IG canopy can reduce the runoff rates (RRs) by 6.02%–82.71% (except for 1.8 mm/min rainfall intensity and 20 cm × 20 cm density conditions) under different rainfall intensities, and the reduction benefit increased with increasing vegetation density. However, the RRs of the OR plots increased by 6.02%–70.68% compared to those of the BS plots (except for MS10 with a rainfall intensity of 0.8 mm/min). The grass canopy under different planting densities can reduce the soil loss rate (SLR) by 24.74%–98.53%. The SLRs of the OR treatment decreased by 64.28%–87.51% at a rainfall intensity of 0.8 mm/min. However, the SLRs increased by 2.03%–84.72% under heavy rainfall conditions (1.2 and 1.8 mm/min). Compared with the IG treatment, the sediment production from the OR treatment under the same runoff conditions increased, ranging from 81.10% to 364.91%. The contribution to soil and water conservation of different density grass canopies was generally positive, which is contrary to the root systems except at the rainfall intensity of 0.8 mm/min. Rainfall intensity was the primary factor contributing to soil and water conservation effectiveness (53% and 65%), followed by vegetation components (30% and 25%). • Effects of grass component on runoff and soil production of steep spoil heaps were revealed. • Grass canopy can effectively reduce the slope runoff rate while contrary to root system. • Soil conservation benefit of canopy was positive while root was negative under heavy rainfall. • Rainfall intensity was the primary factor influencing erosion followed by grass components.