| In terms of the soil properties, vegetation type and climate, Southern Red areas are different from the north. And in order to explore the South herb root-soil composite resist mechanism, the paper selected the red soil developed from shale, Vetiveria zizanioides, Paspalum notatum and Cynodon dactylon as the subjects in this study. Meantime, this research adopted remolded root-soil composite to set a control group of bare ground and three test groups of grass root-soil composite, and it used an experiment to test the remolded soil anti-erosion index(ANS). This research measured soil shear strength index(internal friction angle and cohesion) by direct shear apparatus and measured the initial infiltration rate(K1), the steady infiltration rate(K2), the average penetration rate(K3) and the total penetration(K4) by bicyclic method. However, it used conventional methods to test the herbal root morphology characteristics, tensile strength, the main ingredient content and soil physical and chemical properties. All these research above reveal the biomechanical anti-erosion mechanism of herbal root-soil composite and can provide some proofs for forecasting the anti-erosion performance of the herbal root-soil composite and the implementation of soil conservation measures in the area. The main conclusions are as follows:(1) There were differences in root shape of Vetiveria zizanioides, Paspalum notatum and Cynodon dactylon which had planted in the shale red soil nearly one year. The maximum root diameter of Vetiveria zizanioides was about 1.5 mm, which was larger than the Paspalum notatum and Cynodon dactylon; but the difference of the root diameter between Paspalum notatum and Cynodon dactylon was very small, which was generally less than 1.0 mm. With the difference of the soil layers, the root length density(RLD), the root surface area density(RSAD) and the root weight density(RWD) of the Paspalum notatum and Cynodon dactylon was different; which means, in 0~20 cm soil layer, Paspalum notatum>Cynodon dactylon, but it was reverse in 20~30 cm soil layer. The RLD, RSAD and RWD of Vetiveria zizanioides were significantly higher than the other two herbs in each soil, and the difference tended to diminish with the deepening of soil.(2) The permeability of root-soil composite and bare soil decreased as the soil deepening, in which the change of Vetiveria zizanioides between 0~10 cm soil layer and 10~20 cm soil layer was the most obvious. The permeability size showed Vetiveria zizanioides root-soil composite > Paspalum notatum root-soil composite > Cynodon dactylon root-soil composite>bare land in each soil layer, and the size difference was gradually shrinking as the soil deepening. The 4 kinds of permeability parameters were positively correlated with the RLD, RSAD, RWD, and the role of permeability improvement for Vetiveria zizanioides roots was more important than the other two grass. The soil bulk density(T1), non-capillary porosity(T3) and organic matter content(T5) were the main soil physical and chemical factors in affecting the permeability parameter.(3) The ultimate tensile force(F) of the 3 herb roots increased as the diameter increasing, but their tensile strength(P) showed an reverse trend, in which the Vetiveria zizanioides had the biggest change. The ultimate tensile force and tensile strength of Vetiveria zizanioides root of the same root diameter were significantly higher than the other two herbs, and the ultimate tensile force of Paspalum notatum was slightly larger than Cynodon dactylo. Compared with the fresh root, the ultimate tensile force and tensile strength of the 3 kinds of re-flooding grass root improved significantly, in which the ultimate tensile force of Vetiveria zizanioides increased sharply(89.16%), and the tensile strength of Cynodon dactylon increased sharply(46.82%). The ultimate tensile force and tensile strength of 3 herbaceous roots increased in some degrees as moisture content decreasing, in which the ultimate tensile force of Paspalum notatum increased the sharpest, and the tensile strength of Vetiveria zizanioides increased the most. The main ingredients of the 3 kinds of grass roots increased as the diameter increasing, while the lignocellulosic ratio(L/C) showd different changing trends. The ultimate tensile force had the better correlation with all major components(the correlation coefficient r≥0.931), their relationship could be expressed as F=-0.984C-1.131He+7.655L-31.499(wherein C represents cellulose, He represents hemicellulose, L represents lignin); while the correlation between tensile strength and the ingredients was poor.(4) The value of cohesion(C) and internal friction angle(υ) of Vetiveria zizanioides root-soil composite were significantly larger than the other three modes of soil, especially in the 10~20 cm soil layer. The cohesion and internal friction angle of 3 kinds of herbaceous root-soil composite decreased after the first increased as soil deepening, while the bare land showed a sustained increasing trend. The added value of cohesion of herbal root-soil composite was mainly affected by the added value of soil bulk density and root surface area density, and the added value of internal friction angle was mainly affected by the added value of soil bulk density, both of which decreased as the added value of soil bulk density increasing. There was a good contact between the tensile strength of single root and its enhancement value of the shear strength, but it could not use the root area ratio to measure the shear strength of root-soil composite in practice.(5) The soil anti-erosion index(ANS) of Vetiveria zizanioides root-soil composite was significantly higher than the other three modes of soil in different flow or soil layer. The ANS of herb root-soil composite first increased and then showed a decreasing trend as soil deepening, while the variation of the bare land was opposite. The Vetiveria zizanioides root performed better than the other two grass on enhancing the soil anti-erosion performance. The added value of anti-erosion performance(△ANS) of each herb root-soil composite decreased as scour flow increasing or soil layer deepening. It had a good logarithmic relationship between ANS and K1, K2 or K3, and ANS increased as they increasing. The ANS increased as the C or υ growing within a certain range, and ANS increased rapidly when C≤20 k Pa. L/C was negatively correlated with △ ANS, while the other main component of root, the ultimate tensile force or tensile strength of root was positively correlated with △ ANS, of which the correlation was not significant. The flow factor was better than the soil layer factor in terms of the correlation with △ ANS. △ ANS had a high association with RLD, RSAD or RWD in a small flow, and RLD was the best correlation factor, and △ ANS had a quadratic function with each of the three factors. The comprehensive parameters(β) significantly affecting ANS of herbal root-soil composite could be expressed as: β=0.113T3+0.127T5+0.126K1+0.128K2+0.130K3+0.069C+0.128RLD+0.129RSAD+0.124RWD(each of the dependent variables in the formula is the standardized data), and the relationship between β and ANS could be expressed as: ANS = 3.234β+9.599. |
