Root distribution features of typical herb plants for slope protection and their effects on soil shear strength

Abstract: Plant root systems play an important role in soil erosion control. To investigate characteristics of root systems of herb vegetation for erosion control in the Three Gorges Reservoir region and their influences on soil shear strength, we used bare land as a control. Four typical herb plants of erosion control, Vetiveria zizanioides (Lin.) Nash, Paspalum notatum Flugge, Cynodon dactylon (L) Pers. and Medicago sativa L., were selected and investigated in the experiments. Circular knives (base area: 30 cm2, high: 10 cm) were used to take samples from the test area for analysis. Direct shear apparatus was applied to analyze soil shear strength. A root analysis system (WinRHIZOPro.2004c) was used to characterize the root systems. The results indicated that: 1) Vetiveria zizanioides (Lin.) Nash had the largest root length density and root surface area density. 2) Root length density, root surface area density, and root weight density of all typical herb plants were decreased with the increasing of soil depth, but the difference on root length density, root surface area density, and root weight density among different herb plants was not significant with the increasing soil depth. The root length density, root surface area density, and root weight density were 0.92 cm/cm3, 0.29 cm2/cm3, and 2.10 mg/cm3 for Cynodon dactylon (L) Pers., 1.01 cm/cm3, 0.77 cm2/cm3 and 34.72 mg/cm3 for Medicago sativa L., 8.12 cm/cm3, 3.12 cm2/cm3 and 27.70 mg/cm3 for Vetiveria zizanioides (Lin.) Nash and 1.66 cm/cm3, 0.71 cm2/cm3 and 9.26 mg/cm3 for Paspalum notatum Flugge, respectively. 3) Soil shear strength was significantly different in the 4 herb plants. Even in the same herb plant, it had different patterns with the difference of soil layers. The internal friction angle φ and cohesion c declined with the increasing of soil depth. We found that the internal friction angle φ and cohesion c of all herb plants were higher than that of bare land. The sequence from high to low were: Vetiveria zizanioides (Lin.) Nash＞Paspalum notatum Flugg＞Medicago sativa L. ＞Cynodon dactylon (L) Pers. 4) The internal friction angle φ, cohesion c, and shear displacement were 14.79o, 15.96 kPa, and 3.43 mm for Cynodon dactylon (L) Pers., 17.44o, 20.93 kPa and 4.89 mm for Medicago sativa L., 24.04o, 31.36 kPa and 2.88 mm for Vetiveria zizanioides(Lin.) Nash and 18.71o, 30.34 kPa and 3.63 mm for Paspalum notatum Flugge and 9.45o, 14.18 kPa and 5.51 mm for bare land, respectively. 5) The internal friction angle φ was logarithmic growth and cohesion c was linear growth with the increase of root length density and root surface area density, and soil shear strength had an obvious correlation relationship with root systems when the diameter was less than 5 mm. It is concluded that root length density and root surface area density are the best indexes to characterize soil shear strength, especially root length density and root surface area density of roots with D≤5 mm. The results will be useful for hillslope vegetation protection and soil erosion control.