Soybean roots architecture and the mechanical properties of the root-soil complex in mountain red soil on sloping farmland

The sloping farmland area in Yunnan accounts for 70% of the total arable land area, and its sustainability is affected by severe soil erosion. Approximately 89.4% of the sloping farmland in the province is utilized for planting crops, and soybeans are one of the main crops in summer. Previous studies have shown that the soil fixing capacity of vegetation roots plays a significant role in soil and water conservation. This study was conducted to explore the soil-fixing capacity of soybean roots and to provide a basis for the calculation of the soil-fixing ability of crop roots. Thus, a field experiment was designed to have two treatments with three replications for a total of six plots: bare land (CK) and mono-soybean field (SS). Unconfined compression tests were used to determine the shear strength and stress-strain characteristics of rootless soil and root-soil complexes during the three main growth stages of soybeans (blooming stage, initial grain forming stage, and seed filling stage). The WinRHIZO (Pro.2019) system was employed to analyze the geometric characteristics, fractal characteristics, and topological structure of the roots. The structural characteristics of soybean roots at different growth stages and their effects on soil mechanical properties were analyzed. The results indicated that:1) the fractal dimension of soybean roots was the smallest at the seed filling stage, and the fractal abundance was the smallest at the blooming stage, while both were the largest at the initial grain forming stage. 2) The topological index of soybean roots was the largest at the seed filling stage, followed by the blooming stage, and was the smallest at the initial grain forming stage, when it was approximately 0.5, with the smallest average link length, suggesting that the soybean roots tended to have a dichotomous topology pattern and reached the most complicated branching status at the initial grain forming stage. 3) The fractal characteristics of soybean roots were significantly and positively correlated with the main morphological parameters, such as root length (P<0.01), while the topological characteristics were significantly and negatively correlated with them (P<0.01). 4) Compared with rootless soil, soybean roots could significantly enhance the strength of root-soil complexes. The unconfined compressive strength of the soybean root-soil complexes was the highest at the initial grain forming stage, followed by the seed filling stage, and was the lowest at the blooming stage, at 41.44 kPa, 37.95 kPa, and 29.29 kPa, respectively. The fractal dimension and fractal abundance were significantly and positively correlated with the mechanical properties of the root-soil complexes (P<0.01). In conclusion, the greater the fractal dimension and fractal abundance of soybean roots are, and the smaller the topological index is, the greater the unconfined compressive strength of the root-soil complex is, and the more significant the soil-fixing capacity of soybean roots is. The fractal and topological characteristics of crop roots can not only be used to express the branching status, spatial distribution, and expansion mode of roots in soil, but can also be used as the main parameters to evaluate the soil-fixing capacity of crop roots. This study provides a reference for crop configuration on sloping farmlands in mountainous areas. Soil erosion can be prevented by cultivating soybean varieties with complex branches and well-developed roots.