The effect of Elymus nutans sowing density on soil reinforcement and slope stabilization properties of vegetation–concrete structures

Rapid infrastructure development in southwestern China has produced numerous high and steep anthropogenic slopes in alpine and subalpine regions, characterized by severe soil erosion, low stability, and susceptibility to landslides. Vegetation-based slope protection can mitigate soil erosion and enhance shallow slope stability through hydrologic effects (rainfall interception and storage by stems and leaves) and mechanical effects (root reinforcement and anchorage). Concrete Biotechnical Slope (CBS) technology integrates structural slope protection with vegetation restoration and is widely used in China. Appropriate sowing density is critical to quickly establish vegetation cover and maximize soil reinforcement and slope stabilization, but higher densities can intensify competition and affect plant growth, thereby influencing protective functions. Elymus nutans, a perennial grass with extensive roots and tolerance to drought, cold, and salinity, is a promising pioneer species for alpine/subalpine restoration; however, its specific effects on soil reinforcement and slope stabilization within CBS and the role of sowing density remain unclear. This study established vegetation–concrete structures with varying E. nutans sowing densities to: (1) evaluate aboveground growth and canopy water interception relevant to soil reinforcement, (2) evaluate belowground traits, erosion resistance, and shear resistance relevant to slope stabilization, (3) analyze relationships among sowing density, plant traits, and reinforcement/stabilization properties to clarify direct and indirect effects, and (4) determine the optimal sowing density for slope ecological restoration.

Prior work shows vegetation enhances slope stability via hydrologic interception and mechanical root reinforcement, reducing soil erosion and shallow landslide occurrence. The CBS approach effectively couples engineering protection with ecological restoration on steep slopes. Plant density influences growth through competition for light and space, often reducing individual biomass but increasing biomass per unit area, with potential effects on height and tillering. Canopy interception studies, largely in forests, demonstrate that greater vegetation cover generally increases rainfall interception; dense meadows can intercept substantial rainfall due to low canopy height and high cover. Root traits and architecture (length, surface area, distribution) are key to resisting concentrated flow erosion and improving shear strength, with the strongest reinforcement typically occurring in near-surface soil layers where herbaceous roots concentrate. Planting density affects belowground development and thus erosion resistance and shear strength, but optimal densities for maximizing these protective functions can be species- and context-specific. For E. nutans specifically, previous studies documented its cold/drought tolerance and root morphology advantages for alpine restoration, but quantitative links between sowing density, plant traits, and slope protection performance in vegetation–concrete systems had not been established.

Seeds of Elymus nutans (1000-grain weight 3.40 g) were sown into vegetation–concrete substrates constructed following NB/T 35082-2016. Substrate composition (dry weight ratio) was sandy loam:cement:greening additive:organic matter (dry cow manure):micro silicon powder = 100:8:4:7:4; micro silicon powder was included to enhance concrete durability under freeze–thaw. Substrate was evenly mixed, layered into plastic test chambers (34 cm × 26 cm × 12 cm) with bottom drainage, wetted and compacted to 10 cm thickness. Chambers were inclined at 45° to simulate slope conditions and watered daily to maintain moisture. Six treatments were established: control (no seed) and five sowing densities— I: 1100, II: 2200, III: 3300, IV: 4400, V: 5500 seeds/m²—each with six replicates (36 units total). Seeds were weighed, mixed with substrate, and evenly distributed within each chamber. After six months, soils in each chamber were saturated and sampled using a cutting ring (8 cm diameter, 10 cm depth). Six samples per treatment were collected (36 total). Aboveground measurements: stems and leaves were clipped at the soil surface; plant height, tiller number, and fresh weight per unit area were recorded. Canopy water interception was quantified using a simplified water absorption method: stem–leaf samples were immersed in water for 5 min, drained by gravity, and reweighed. Maximum interception rate Rmax (%) = (W2 − W1)/W1 × 100, where W1 and W2 are fresh weights before and after soaking. Maximum interception Wmax (mm) was calculated from Rmax and fresh mass per unit area (M1; t/hm²). Aboveground biomass was obtained by oven-drying at 105 °C for 20 min to deactivate enzymes, then at 80 °C to constant weight. Belowground performance: Erosion resistance was assessed by static water collapse tests on root–soil cores. Cores were weighed, placed vertically on mesh, immersed in static water to disintegrate for up to 30 min; residual mass and disintegration time were recorded to compute disintegration rate V = (M1 − M2)/t (g/min). The coefficient of enhancement in erosion resistance (Ce) reflects reduction in disintegration rate due to roots, computed from control and rooted samples. Shear resistance was measured using a ZJ strain-controlled direct shear apparatus. Root–soil cores were stratified into 0–3, 3–6, and 6–9 cm layers; trimmed samples (shear box 6 cm diameter, 2 cm depth) were sheared under 100 kPa vertical pressure. Shear strength τ (kPa) was derived from dial readings via τ = K·R (K = 2.50 kPa per 0.01 mm). Three replicates per layer were tested; layer strengths were averaged for each treatment. Root traits (average diameter, total length, surface area) were obtained after carefully washing roots and scanning with WinRHIZO; roots were then dried at 80 °C to constant weight to determine belowground biomass. Statistics: One-way ANOVA tested sowing density effects on plant traits and protection metrics; Duncan’s multiple range test assessed pairwise differences. Pearson correlations quantified associations among sowing density, plant traits, water interception, erosion resistance, and shear strength. Analyses were conducted in SPSS 19.0.

- Growth responses to density: Aboveground biomass per unit area increased monotonically with sowing density. Plant height, root surface area, root length, and belowground biomass exhibited hump-shaped responses—first increasing then decreasing with density. Tiller numbers and average root diameter decreased with increasing density. In Table 1, maximum belowground biomass, root surface area, and root length occurred at density IV (4400 seeds/m²), showing increases over density I (1100 seeds/m²) of 26.39%, 217.41%, and 135.80%, respectively. - Canopy interception: Water interception by stems and leaves varied significantly with density (maximum interception: F=5.156, p=0.002; maximum interception rate: F=6.055, p=0.001). Both maximum interception and interception rate increased with density. Treatment V (5500 seeds/m²) had the highest values (2.50±0.26 mm; 87.81±9.72%), 42.05% and 24.75% higher than low density, respectively. - Erosion resistance: Root–soil composites enhanced erosion resistance, with Ce ranging 0.31–0.72. Sowing density significantly affected enhancement (F=192.211, p<0.001), following a hump-shaped pattern peaking at density IV. Enhancement at IV exceeded I and V by 136.05% and 65.17%, respectively. - Shear strength: All planted treatments, except I, significantly increased shear strength relative to the control (F=4.044, p=0.022). Shear strength increased then decreased with density, maximized at IV (15.05% above control). Enhancement was greatest in the surface layer (0–3 cm), and inter-layer differences grew with increasing density. - Correlations: Sowing density was significantly correlated with plant height (negative), tiller number (negative), and aboveground biomass (positive), and highly correlated with water interception. Density was not significantly correlated with belowground biomass, root length, root surface area, erosion resistance enhancement, or shear strength. Aboveground biomass correlated significantly with root length, root surface area, and root diameter, indicating indirect density effects on belowground growth. Erosion resistance enhancement correlated strongly with belowground biomass (p<0.01), and shear strength correlated with belowground biomass, root length, and root surface area (p<0.05), identifying belowground biomass and root system development as primary drivers of protection performance. - Optimal density: Medium–high sowing density (≈4400 seeds/m²; treatment IV) maximized combined erosion resistance and shear strength while maintaining high canopy interception.

Increasing sowing density intensifies competition for light and space, shifting biomass allocation and altering morphology. In E. nutans, per-unit-area aboveground biomass rose with density, while plant height and tillering declined due to crowding; beyond medium densities, aboveground biomass gains slowed, reflecting resource limitation. Greater canopy cover and biomass at higher densities directly enhanced rainfall interception by stems and leaves, improving hydrologic protection. Belowground, density constraints favored allocation to shoots over roots, leading to thinner average roots and a hump-shaped response of belowground biomass, root length, and surface area. Because erosion resistance and shear strength were tightly linked to belowground biomass and root system development, these protection metrics also peaked at medium–high density and declined at the highest density as root development was curtailed by competition. Root reinforcement effects were strongest in the upper soil layer (0–3 cm), consistent with herbaceous root distribution. Overall, sowing density directly affected aboveground-mediated hydrologic protection (interception) and indirectly affected mechanical protection (erosion resistance and shear strength) through its influence on root growth.

This study quantifies how sowing density of Elymus nutans in vegetation–concrete systems influences soil reinforcement and slope stabilization via coupled above- and belowground pathways. Increasing density directly enhanced canopy water interception, while belowground protection (erosion resistance and shear strength) followed a hump-shaped response driven by root system development. A medium–high sowing density of approximately 4400 plants/m² provided the best overall slope protection performance, maximizing erosion resistance and shear strength while maintaining strong interception capacity. Future research directions were not specified.