Rapid economic development in southwestern China has resulted in numerous high and steep slopes, prone to soil erosion and landslides. These slopes negatively impact the environment through landscape fragmentation and biodiversity loss. Vegetation plays a crucial role in slope stabilization through hydrologic and mechanical effects. Hydrologically, vegetation intercepts rainfall, reducing soil erosion. Mechanically, roots reinforce soil, increasing shear strength. Concrete Biotechnical Slope (CBS) technology combines slope protection with vegetation restoration. However, optimal planting density for maximizing these effects is crucial, as high densities can lead to competition and reduced plant growth. Elymus nutans, a perennial herbaceous plant, is a suitable candidate for ecological restoration in the region due to its extensive root system and resistance to harsh conditions. This study aimed to investigate the effects of E. nutans sowing density on aboveground and belowground growth, water interception, erosion resistance, shear strength, and determine the optimal sowing density for use in CBS for slope protection in southwestern China.

The literature extensively highlights the importance of vegetation in slope stabilization, emphasizing both hydrologic and mechanical effects. Studies have shown that vegetation reduces rainfall impact on soil surfaces and enhances soil strength through root systems. Concrete Biotechnical Slope (CBS) is established as an effective ecological slope protection technology. However, research on optimal planting density and its effects on soil reinforcement and slope stabilization is limited, especially for E. nutans. Existing studies reveal the importance of appropriate density for maximizing soil reinforcement and slope stabilization properties while acknowledging the potential for competition at high densities to affect plant growth. Understanding the interplay between plant density and these soil properties is vital for effective ecological slope restoration projects.

E. nutans seeds were sown at six densities (control, 1100, 2200, 3300, 4400, and 5500 seeds/m²) in concrete substrates mimicking slope conditions. Six replicates were used for each treatment. After six months, aboveground biomass, plant height, tiller number, and water interception were measured. The maximum water interception level and rate were calculated using equations 1 and 2. For belowground parameters, soil columns were sampled, and erosion resistance was measured using a method described by Burylo et al. Shear strength was measured using a ZJ Strain Controlled Direct Shear Test Apparatus. Root diameter, length, and surface area were measured using a WinRHIZO root analysis system. One-way ANOVA and Duncan's multiple range test were used to analyze the significance of differences between treatments. Pearson correlation analysis assessed the relationships between sowing density and the measured parameters.

Sowing density significantly affected both aboveground and belowground plant growth (p<0.05). Aboveground biomass increased continuously with density, while plant height, root surface area, root length, and belowground biomass initially increased then decreased. Tiller numbers and average root diameter gradually decreased with increasing density. The highest aboveground biomass was observed at the highest density (V, 5500 seeds/m²). Water interception by stems and leaves significantly increased with density (maximum interception, F=5.156, p=0.002; maximum interception rate, F=6.055, p=0.001). Erosion resistance and soil shear strength increased initially with density, reaching maximum values at the medium-high density treatment (IV, 4400 seeds/m²) before decreasing. Sowing density was highly correlated with aboveground biomass, plant height, tiller number, and maximum water interception. However, density was not significantly correlated with belowground biomass, root length, root surface area, erosion resistance enhancement, and soil shear strength. Belowground biomass was highly correlated with erosion resistance and shear strength (p<0.01 and p<0.05, respectively). The optimal sowing density was determined to be around 4400 plants/m².

Increased density led to increased competition for resources, resulting in decreased plant height and tiller number to reduce intraspecific competition. Aboveground biomass increased, but at a slower rate beyond a medium density due to competition. Maximum water interception increased with density, consistent with the observed increase in aboveground biomass. The non-significant correlation between density and belowground parameters can be explained by the plants prioritizing aboveground growth under density stress, diverting resources away from root development. However, belowground biomass strongly influenced erosion resistance and shear strength. These results highlight the interplay between aboveground and belowground processes in determining the effectiveness of E. nutans in slope stabilization. The optimal density of 4400 plants/m² balances the benefits of increased biomass and water interception with the need to avoid excessive competition and maintain sufficient root development for soil reinforcement.

This study demonstrates that sowing density significantly influences the performance of E. nutans in vegetation–concrete structures for slope stabilization. The optimal sowing density of approximately 4400 plants/m² maximizes soil reinforcement and slope stabilization properties. Future research could investigate the long-term effects of this optimal density, explore the effects of different soil types, and examine other plant species suitable for similar applications.

The study was conducted under controlled conditions in laboratory settings. The results may not fully reflect the complexities of natural slopes. The experimental duration of six months may not fully capture the long-term effects of E. nutans on soil reinforcement and slope stability. Further studies are needed to validate these findings in field conditions.