Abstract: This study investigated the effect of planting a grass-legume mixture and adding polyacrylamide (PAM) on the shear performance and erodibility-resistance of purple soil in a pot trial. Cynodon dactylon and Trifolium repens were chosen as the plant species. The experiment had four treatments: CK, bare soil; GL, planted in C. dactylon and T. repens; PAM, a single application of PAM; and GL+PAM. Root parameters, basic physical and chemical properties, shear strength (including its constituent components), and erodibility-resistance parameters were measured. The roots of C. dactylon and T. repens significantly reduced soil bulk density, and increased soil porosity and soil organic matter (P<0.05), and improved the soil shear performance and erodibility-resistance. PAM effectively promoted the growth of the root system, significantly improved the soil microstructure (P<0.05), especially the non-capillary porosity, which after the single application of PAM was 1.15-1.31 times that of CK. PAM also increased the shear strength and internal friction angle φ, the erodibility-resistance parameters of water stable aggregates >0.25 mm (WSA_0.25), water stable aggregates >0.5 mm (WSA_0.5), mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates. The improvement in shear performance and erodibility-resistance obtained using optimal concentration of PAM would be expected to stabilize purple soil in barren hillsides better. The main effect of roots in the GL pots was the improvement of soil shear performance (P<0.05), especially cohesion. PAM had a stronger effect on erodibility-resistance because of the formation of larger aggregates, thus significantly improving the degree of aggregation and soil aggregate structure (P<0.05). Compared with GL or PAM alone, GL+PAM produced the most obvious improvement in the shear performance and erodibility-resistance of purple soil in barren hillsides. The best performance was obtained with C. dactylon planting and addition of polyacrylamide at 30 g·m^-3 soil. The critical root diameter conferring shear performance and erodibility-resistance of soil was 0<d≤0.2 mm. Root length density and root surface area density of roots for which 0<d≤0.2 mm (RLD_0.2, RSAD_0.2), root length density (RLD) and root volume density (RVD) are key indexes of the contribution of roots to soil shear performance.

Key words: root, root length density, polyacrylamide, shear strength, water stable aggregate