Surface soil cracking in alpine meadows signifies the transition of degradation from quantitative accumulation to qualitative deterioration. Quantitative research remains insufficient regarding changes in the mechanical properties of degraded meadow soils and the mechanical thresholds for cracking initiation. This study explored the relationships between surface cracking and the physical properties, tensile strength, and matrix suction of root-soil composites in alpine meadow sites with different stages of degradation (undegraded (UD), lightly degraded (LD), moderately degraded (MD), and heavily degraded (HD)) under different water gradients (high water content (HWC), medium water content (MWC), and low water content (LWC)) corresponding to different drying durations at a constant temperature of 40.0 degrees C. The Huangcheng Mongolian Township in Menyuan Hui Autonomous County, Qinghai Province, China was chosen as the study area. The results indicated that as the degradation degree of alpine meadow intensified, both water content of root-soil composite and the fine grain content of soil decreased. In contrast, the root-soil mass ratio and root area ratio initially increased and then decreased with progressive degradation. Under a consistent water content, the tensile strength of root-soil composite followed a pattern of MD>HD>LD>UD. The peak displacement of tensile strength also decreased as the degradation degree of alpine meadow increased. Both the tensile strength and matrix suction of root-soil composite increased as root-soil water content decreased. A root-soil water content of 30.00%-40.00% was found to be the critical threshold for soil cracking in alpine meadows. Within this range, the matrix suction of root-soil composite ranged from 50.00 to 100.00 kPa, resulting in the formation of linear cracks in the surface soil. As the root-soil water content continued to decrease, liner cracks evolved into branch-like and polygonal patterns. The findings of this study provide essential data for improving the mechanical understanding of grassland cracking and its development process.

Quantifying surface cracks in alpine meadows is a prerequisite and a key aspect in the study of grassland crack development. Crack characterization indices are crucial for the quantitative characterization of complex cracks, serving as vital factors in assessing the degree of cracking and the development morphology. So far, research on evaluating the degree of grassland degradation through crack characterization indices is rare, especially the quantitative analysis of the development of surface cracks in alpine meadows is relatively scarce. Therefore, based on the phenomenon of surface cracking during the degradation of alpine meadows in some regions of the Qinghai-Tibet Plateau, we selected the alpine meadow in the Huangcheng Mongolian Township, Menyuan Hui Autonomous County, Qinghai Province, China as the study area, used unmanned aerial vehicle (UAV) sensing technology to acquire low-altitude images of alpine meadow surface cracks at different degrees of degradation (light, medium, and heavy degradation), and analyzed the representative metrics characterizing the degree of crack development by interpreting the crack length, length density, branch angle, and burrow (rat hole) distribution density and combining them with in situ crack width and depth measurements. Finally, the correlations between the crack characterization indices and the soil and root parameters of sample plots at different degrees of degradation in the study area were analyzed using the grey relation analysis. The results revealed that with the increase of degradation, the physical and chemical properties of soil and the mechanical properties of root-soil composite changed significantly, the vegetation coverage reduced, and the root system aggregated in the surface layer of alpine meadow. As the degree of degradation increased, the fracture morphology developed from linear to dendritic, and eventually to a complex and irregular polygonal pattern. The crack length, width, depth, and length density were identified as the crack characterization indices via analysis of variance. The results of grey relation analysis also revealed that the crack length, width, depth, and length density were all highly correlated with root length density, and as the degradation of alpine meadows intensified, the underground biomass increased dramatically, forming a dense layer of grass felt, which has a significant impact on the formation and expansion of cracks.