Soil erosion can be effectively controlled through vegetation restoration. Specifically, roots combine with soil to form a root-soil complex, which can effectively enhance soil shear strength and play a crucial role in soil reinforcement. However, the relationship between root mechanical traits and chemical compositions and shear performance and reinforcing capacity of soil is still inadequate. In this study, we determined the root chemical properties, performed root tensile tests and root-soil composite triaxial tests using two plants-one with a fibrous root system (ryegrass, Lolium perenne L.) and the other with a tap root system (alfalfa, Medicago sativa L.)-and calculated the factor of safety (FOS). The results revealed that the relationship between root diameter and tensile strength differed among different root characters. Holocellulose content and cellulose content were the main factors controlling the root tensile strength of ryegrass and alfalfa, respectively. The shear properties of the root-soil complex (cohesion (c) and internal friction angle (phi)) are correlated with soil water content (SWC) and root mass density (RMD). Root traits had a more substantial effect on c than phi, with significant differences in c between ryegrass and alfalfa at 7 % and 11 % SWC. The root-soil complex had an optimum RMD, and the maximum increase rates of c were 80.57 % and 34.4 %, respectively. Along slopes, sliding first occurs at the foot of the slope, thus demanding emphasis on protection and reinforcement. On steep gradients with low SWC, ryegrass strongly contributes to soil reinforcement, whereas alfalfa is more effective on gentle gradients with high SWC. The results provide scientific references for species selection for vegetation restoration in the Loess Plateau and a deeper understanding of the mechanical mechanism of soil reinforcement by roots.

In the loess tableland, gully slope instability induces severe soil erosion and land degradation, yet the synergistic effects of dominant vegetation under varying restoration modes combined with dynamic rainfall regimes and topographic variations on gully slope stabilization mechanisms remain inadequately quantified. Therefore, the dominant vegetation species under natural (NR) and artificial restoration (AR) was chosen as the object. Through field sampling, root-soil complex mechanical experiments, and numerical simulations, the protection effect of dominant vegetation under different restoration modes combination with rainfall and topographic variations was investigated. The result revealed significant differences in basic soil physical properties, root morphological characteristics, root and root-soil complex mechanical properties among five dominant vegetated plots under the different restoration modes (P < 0.05). The soil properties in the Scop plot under AR were slightly better than those in the other plots. The roots in the Spp plot developed better under NR. The shear strength of Lespedeza bicolor Turcz. was the highest under NR. The tensile strength of Digitaria sanguinalis (L.) Scop. was greatest under AR. The tensile force and tensile strength of single roots exhibited a significant positive linear correlation and a significant negative exponential correlation, with root diameter, respectively (P < 0.01). For the unstable gully slopes (F-s < 1.0), maximum displacement occurred at the slope foot, where tensile shear failure dominated, while the interior experienced compressive yielding. The grey relational analysis identified rainfall intensity as the primary destabilizing factor, followed by dominant vegetation species, slope height, and slope gradient. Notably, when rainfall intensity reaches or exceeds 0.06 m/h, or when slope height exceeds 20 m combined with long-duration rainfall, the regulatory impacts of dominant vegetation under different restoration modes on the gully slope stability are substantially diminished and become negligible. This study provides a theoretical basis for gully slope protection and ecological environmental construction in loess tableland.

Using the discrete element method (DEM) to simulate the rice machine transplanting operation is important for assessing the plant injury and optimizing the rice transplanter performance, while the DEM flexible model establishment that can accurately reflect the mechanical properties of the rice blanket seedling root blanket is an important foundation. Based on the root blanket's stratification and the root system structure's measurement and statistics, a new method for root blanket flexible modeling was proposed in this study. Firstly, the Hertz-Mindlin with bonding V2 contact model was used to establish substrate I (SI), substrate II (SII), substrate III (SIII), stemroot combination (SRC), and netted layer (NL) flexible models, respectively, and the model parameters were calibrated and determined by angle of repose (AOR), direct shear, and mechanical tests. The calibration results showed that the deviations of AOR simulated values for SI and SII were both less than 1.5 %, and the deviations of shear strength simulated values were both less than 4 %. Secondly, the shear characteristics of SI and SII were determined by direct shear test. The results showed that the physical and simulated shear stress-displacement relationship curves of SI and SII were basically the same; the hair roots mainly relied on the cohesive between them and the substrate to improve the substrate strength; the fitted lines of simulated shear strength and normal stress of SI and SII were in high agreement with these of the measured values; the deviations of the simulated cohesion and internal friction angle were both less than 5 %. After that, the Hertz Mindlin with JKR V2 contact model was used between SRC and substrate. The interfacial surface energy of the root blanket and the bonding parameters of SIII were calibrated by stem, half-SRC, and SRC pulling-out tests layer by layer. The calibration results showed that the deviation of the maximum pulling-out force of SRC was 5.83 %, verifying that the model could accurately simulate the intertwining effect of the crown roots. Finally, the flexible model of the root blanket was verified by cutting, curling, and tensile tests. The simulated test results were consistent with the trends of the physical test results; the deviations of the maximum cutting resistance of front cutting and side cutting were both within 8 %, the error percentage range of the marked points height was 0.35 % to 17.16 %, and the deviation of the maximum tensile force was 9.22 %, indicating the good feasibility of the modeling method and accuracy of the flexible model. The results of this study lay a foundation for the DEM simulation of the rice machine transplanting operation. They can also provide a reference for the numerical simulation of other multiplant root-soil complexes.

Repeated wet swelling and dry shrinkage of soil leads to the gradual occurrence of cracks and the formation of a complex fracture network. In order to study the development characteristics and quantitative analysis of cracks in root-soil complex in different growth periods under dry-wet cycles, the alfalfa root-loess complex was investigated during different growth periods under different dry-wet cycles, and a dry-wet cycle experiment was conducted. The crack rate, relative area, average width, total length, and the cracks fractal dimension in the rootsoil complex were extracted; the crack development characteristics of plain soil were analyzed under the PGDWC (dry-wet cycle caused by plant water management during plant growth period), as well as the crack development characteristics of root-soil complex under PG-DWC and EC-DWC (the dry-wet cycles caused by extreme natural conditions such as continuous rain); the effects of plant roots and dry-wet cycles on soil cracks were discussed. The results showed that the average crack width, crack rate, relative crack area, and total crack length of the alfalfa root-loess complex were higher than those of the plain soil during PG-DWC. The result indicated that compared with plain soil during PG-DWC, the presence of plant roots in alfalfa root-soil complex in the same growth period promoted the cracks development to some extent. The alfalfa root-soil complex crack parameters during different growth periods were relatively stable during PG-DWC (0 dry-wet cycle). During ECDWC (1, 3, and 5 dry-wet cycles), the alfalfa root-loess complex crack parameters increased with the number of dry-wet cycles during different growth periods. Unlike PG-DWC, the EC-DWC accelerated crack development, and the degree of crack development increased with the number of dry-wet cycles. The existence of plant roots promoted crack development and expansion in the root-soil complex to a certain extent, and the dry-wet cycle certainly promoted crack development and expansion in the root-soil complex. This result contradicts the improvement in the root-soil complex's macro-mechanical properties during plant growth, due to differences in the mechanical properties of roots and soil. The research results will provide reference for the root soil complex crack development law and the design of slope protection by vegetation.

Triaxial compression tests were conducted on the alfalfa root-loess complex at different growthperiods obtained through artificial planting. The research focused on analyzing the time variation law of the shear strength index and deformation index of the alfalfa root-loess complex under dry-wet cycles. Additionally, the time effect of the shear strength index of the alfalfa root-loess complex under dry-wet cycles was analyzed and its prediction model was proposed. The results show that the PG-DWC (dry-wet cycle caused by plant water management during plant growth period) causes the peak strength of plain soil to change in a V shape with the increase of growth period, and the peak strength of alfalfa root-loess complex is higher than that of plain soil at the same growth period. The deterioration of the peak strength of alfalfa root-loess complex in the same growth period is aggravated with the increase of drying and wetting cycles. Compared with the 0 days growth period, the effective cohesion of alfalfa root-loess complex under different dry-wet cycles maximum increase rate is at the 180 days, which are 33.88%, 46.05%, 30.12% and 216.02%, respectively. When the number of dry-wet cycles is constant, the effective cohesion of the alfalfa root-loess complex overall increases with the growth period. However, it gradually decreases comparedwith the previous growth period, and the minimum increase rate are all at the 180 days. For the same growth period, the effective cohesion of the alfalfa root-loess complex decreases with the increase of the number of dry-wet cycles. This indicates that EC-DWC (the dry-wet cycles caused by extreme natural conditions such as continuous rain) have a detrimental effect on the time effect of the shear strength of the alfalfa root-loess complex. Finally, based on the formula of total deterioration, a prediction model for the shear strength of the alfalfa root-loess complex under dry-wet cycles was proposed, which exhibits high prediction accuracy. The research results provide useful guidance for the understanding of mechanical behavior and structural damage evolution of root-soil composite.