Background  It is the typical fragile ecological environment in China, with dry and hot climate, serious soil erosion, and difficulty for vegetation restoration in the Dry-hot Valley of Jinsha River. Due to the rapid growth and adaptation to the local natural environment, indigenous herbaceous plants have unique advantage in restoration of ecological environment. The aim of this study is to provide the basis for plant selection and calculating their abilities of soil fixation in ecological restoration, slope protection, soil erosion control and other ecological projects in Dry-hot Valley of Jinsha River.

Methods  The location of the test was in Wumao Township (E101°45' and N 25°43', with an elevation of 1280 m), Yuanmou County, Chuxiong Autonomous Prefecture of Yi Nationality, Yunnan Province, on January 2016 on hot-red soil. Heteropogon contortus, Eulaliopsis binata, Cymbopogon distans and Schizachyrium delavayi, widely distributed in Dry-hot Valley of Jinsha River, were selected to measure the shear strength of root-soil composites by in-situ direct shear test, and compared with bare soil. Five treatments with six replicates were set up. No significant difference about soil moisture and density of roots-contained soil were found in the treatments, except that density between the root-soil composites and bare soil. Shear strength and displacement of composite of the root-soil composites and bare soil with 200 mm length×200 mm width×100 mm height centering on the plant was test by the Pull Test Instrument for Bolt (ML-20, Qingdao Yifeng Instrument Manufacturing Co., Ltd.). Root tensile strength was test by digital pull tester (SN100, Shadu Instrument Manufacturing Co., Ltd.). The data such as root diameters, quantities and tensile strength was put to Wu & Waldron vertical root model (WWM) and Fiber bundle root enhancement model (FBM) respectively to predict the enhanced shear ability by roots.

Results  1) Root diameters of four herbs were distributed mainly in the range of 0.1-1 mm. The relationship between root diameter and tensile strength of H. contortus, E. binate and S. delavayi was a negative power function, while C. distans' had no correlation. 2) Compared with bare soil, the soil shear strength was improved 20.7%, 85.3%, 84.4% and 16.1%, respectively by the roots of H. contortus, E. binate, S. delavayi and C. distans. 3) The predicted value was 3.59-10.83 times of the measured value, by using WWM model to predict the shear capacity of the soil. The predicted value of the FBM model was 1.24-2.62 times of the measured value.

Conclusions  The ability of E. binate to improve the shear strength of soil is stronger than S. delavayi, thus it could be selected as the priority of ecological construction in Dry-hot Valley of Jinsha River. FBM model was better than WWM model, in terms of prediction of shear strength of soil enhanced by roots.