| Soil detachment is the initial stage of soil erosion. It is a dynamic and complicated process as influenced by many factors. Accurate simulation on soil detachment is the base of process-based soil erosion model development. The current researches mainly focused on the hydrodynamic mechanism of soil detachment and its static quantitative relations with soil properties, plant roots et al. However, both root growth and soil properties showed obvious temporal variations, whether these variations could in turn affect soil detachment capacity and its mechanism was still unclear. Thus, in this research, the region of Ziwuling was selected as the test area, which was located in Gansu Province of the Loess Plateau, and five different abandonment years of grassland vegetation were investigated in the survey region in the summer of 2014. The soil detachment rates were measured by the improved method of scouring their undisturbed soil samples that had been exposed to a concentrated flow at the rate of 4 L/min on a two-meter-long washing flume slope of 15°for 15 minutes, and soil root parameters and soil physicochemical properties were also measured in the soil sampling plots. The dynamics of soil detachment rate, grass roots and soil characteristics would be thoroughly analyzed, aiming at exploring the central driving factor that inducing the dynamic variations of soil detachment capacity and further identifying its driving mechanism, based on the theories and methods in disciplines of soil erosion, vegetation ecology and mathematical physics. The relation between soil detachment and grass roots, soil physicochemical properties in the process of vegetation restoration would be revealed. The contribution of plant roots making to enhance the resistance of soil detachment capacity by the direct impact of combining soil and the indirect effect of improving soil physical and chemical properties would be distinguished. The researched results showed:1) With the abandoned years prolonging, the soil bulk density significantly increased and stabilized in the surface soil layers(0-20 cm), had a small variation in the sub-surface soil layers (20-40 cm) and prominently changed in the bottom soil layers (40-60 cm); the soil organic matter contents decreased first, then increased, significantly changed with a wave mode in another two soil layers; water stable soil aggregates presented a wave mode variation in three soil layers, and had significant differences in the surface and sub-surface soil layers, but changed a little in the bottom soil layers. The soil bulk density significantly increased, however, the soil organic matter and water stable soil aggregates prominently decreased with the soil layers deepening. The soil detachment rate decreased and tended to be stable in the same soil layer against increasing abandonment years. Nevertheless, the soil detachment rate significantly rose with the increase of soil depth at the same abandonment age.2) Root length density(RLD), root surface area density(RSAD) and root weight density(RWD) gradually increased in the surface (0～20 cm) soil layers, sub-surface soil layers(20～40 cm)and bottom(40～60 cm)soil layers with grassland vegetation restoring. The RSAD gradually decreased, however, the RLD and RWD significantly diminished with the increase of soil depth at the same abandonment age.3) Soil detachment rates were closely related to plant roots in soil, and presented a significant negative correlation (P< 0.01) with RLD, RSAD and RWD since grass root could abate soil erosion evidently. The RLD could preferably reflect the network series, the RSAD could better represent the adhesion of roots and soil, and the RWD could mirror physiological function of roots, these three root parameters could reasonably and effectively reveal the reinforcement effect of roots on the resistance of soil detachment capacity. The soil bulk density positively affected soil detachment rate, and that soil organic matters and water stable soil aggregates showed negative relation with soil detachment rate. Changes of soil properties caused by grassland vegetation restoration can prominently lead to the variation of the resistance to soil erosion dynamics, the improvement of soil characteristics played an important role in reducing soil detachment rate and reinforcing the resistance of soil detachment capacity.4) As root length density, root surface area density and root weight density increased, soil organic matters and water stable soil aggregates accreted, while the soil bulk density constantly decreased, and a significant correlation existed with each other. The RLD, RSAD and RWD significantly affected soil properties, and soil tended to be benign by means of plant roots exerted influences on improving soil characteristics. The soil detachment rates were closely related to plant roots and soil properties, the linear equation to express the effects of root length density, root surface area density and root weight density, as well as soil bulk density, water stable soil aggregates and soil organic matters on the soil detachment rate was obtained by stepwise regression, and the results showed that all variables were important influencing factors except soil organic matters. What’s more, grass roots would make more contribution to enhancing the resistance of soil detachment capacity by the direct impact than the indirect effect.The above researched results will provide scientific basis for establishing a deep understanding of runoff detachment process and developing or improving process-based erosion models. It also has very important theoretical significance and practical value for soil and water conservation, ecological environment restoration and construction on the Loess Plateau.
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