Biological soil crust (BSC), as a groundcover, is widely intergrown with grass. The combined effects of BSC and grass during rainfall on runoff and sediment yield are still unclear. In this study, simulated rainfall experiments were applied to a soil flume with four different slope cover treatments, namely, bare (CK) and Stipa bungeana Train. (STBU), BSC, and STBU + BSC to observe the processes of runoff and sediment yield. Additionally, the soil moisture at different depths during infiltration was observed. The results showed that the runoff generated by rainfall for all treatments was in the following order: BSC>STBU+BSC>CK>STBU. Compared with CK, the STBU promoted infiltration, and BSC and STBU+BSC inhibited infiltration. The BSCs obviously inhibited infiltration at a depth of 8 cm. As the soil depth increased to 16 cm and 24 cm, the effects of STBU on promoting infiltration were stronger than those of BSC on inhibiting infiltration. Compared with the CK, the flow velocity for the BSC, STBU and STBU + BSC treatments was reduced by 62.79%, 32.27% and 68.29%, respectively. The BSC and STBU increased the critical shear stress by increasing the resistance. Additionally, the BSC, STBU and STBU+BSC treatments reduced the sediment yield rate by 80.8%, 99.8% and 99.9%, respectively. The soil erosion process was dominated by the soil detachment capacity in the CK, BSC and STBU+BSC treatments, while the STBU treatment showed a transport-limited process. This study provided a scientific basis for the reasonable spatial allocation of vegetation in arid and semiarid areas and provided a scientific basis for the correction of vegetation cover factors in soil erosion prediction models.

Aggregate disintegration is a critical process in soil splash erosion. However, the effect of soil organic carbon (SOC) and its fractions on soil aggregates disintegration is still not clear. In this study, five soils with similar physical and chemical properties and different contents of SOC have been used. The effects of slaking and mechanical striking on splash erosion were distinguished by using deionized water and 95% ethanol as raindrops. The simulated rainfall experiments were carried out in four heights (0.5, 1.0, 1.5, and 2.0 m). The result indicated that the soil aggregate stability increased with the increases of SOC and light fraction organic carbon (LFOC). The relative slaking and the mechanical striking index increased with the decreases of SOC and LFOC. The reduction of macroaggregates in eroded soil gradually decreased with the increase of SOC and LFOC, especially in alcohol test. The amount of macroaggregates (>0.25mm) in deionized water tests were significantly less than that in alcohol tests under the same rainfall heights. The contribution of slaking to splash erosion increased with the decrease of heavy fractions organic carbon (HFOC). The contribution of mechanical striking was dominant when the rainfall kinetic energy increased to a range of threshold between 9 J m-2 mm-1 and 12 m-2 mm-1. This study could provide the scientific basis for deeply understanding the mechanism of soil aggregates disintegration and splash erosion.

Vegetation restoration can input amounts of organic matter into soils to improve soil aggregate stability; for this process, the interaction between organic matter and charged soil particles is a key. However, the molecular-scale knowledge about organic matter-mineral interactions remains largely qualitative. Here, we analyzed soil particles surface electrochemical properties and soil internal forces (electrostatic, hydration , and van der Waals forces) along vegetation restoration to quantitatively evaluate how SOM increase the stability of soil aggregate. Our results revealed that the enrichment of SOM after revegetation increased the cation exchange capacity (CEC), specific surface area (SSA), and soil surface charge density (σ0), thereby strengthening the electrostatic repulsive pressure. Besides, the increasing SOM led to the increase in Hamaker constant at the molecular level and thus enhanced the van der Waals attractive force. As a result, the net pressure of soil internal forces was repulsive and decreased with increasing SOM during vegetation restoration. Meanwhile, the net pressure increased first and then leveled off with the decrease of electrolyte concentration in the bulk solution. The determined soil aggregate breaking strength showed similar trends to that of net pressure. Soil aggregate stability under different succession stages followed the order of farmland < grassland < shrub < forest. Overall, the experimental results of soil aggregate stability were in excellent consistent with the theoretical predictions of soil internal forces. Consequently, we conclude that organic matter input during vegetation restoration increased aggregate stability mainly due to the decrease of the repulsive net pressure of soil internal forces.