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Soil particle interactions are strongly influenced by the concentration, valence and ion species and the pH of the bulk solution, which will also affect aggregate stability and particle transport. In this study, we investigated clay aggregate stability in the presence of different alkali ions (Li+, Na+, K+, and Cs+) at concentrations from10−5 to 10−1 mol L−1. Strong specific ion effects on clay aggregate stability were observed, and showed the order Cs+>K+>Na+>Li+. We found that it was not the effects of ion size, hydration, and dispersion forces in the cation–surface interactions but strong non-classic polarization of adsorbed cations that resulted in these specific effects. In this study, the non-classic dipole moments of each cation species resulting from the non-classic polarization were estimated. By comparing non-classic dipole moments with classic values, the observed dipole moments of adsorbed cations were up to 104 times larger than the classic values for the same cation. The observed non-classic dipole moments sharply increased with decreasing electrolyte concentration. We conclude that strong non-classic polarization could significantly suppress the thickness of the diffuse layer, thereby weakening the electric field near the clay surface and resulting in improved clay aggregate stability. Even though we only demonstrated specific ion effects on aggregate stability with several alkali ions, our results indicate that these effects could be universally important in soil aggregate stability. [ABSTRACT FROM AUTHOR]

• The interaction forces between soil particles at different restoration stages were quantitatively evaluated. • The charge properties of soil particle surface were altered during revegetation. • Electrostatic repulsive and van der Waals attractive forces were enhanced during revegetation. • The net repulsive force declined and thus soil erosion resistance increased during revegetation. Vegetation restoration is a valid way to improve soil erosion resistance by enhancing the stability of soil aggregates. Past studies have shown that the net force (the sum of electrostatic, hydration, and van der Waals forces) between soil particles had crucial effects on the stability of soil aggregates. However, little is known about the influence of vegetation restoration on the net force and soil erosion resistance, which hinders our quantitative understanding of this process at the particle to particle level. In this study, to quantitatively analyze the influence of vegetation restoration on soil net force and their relationship with aggregate stability and erosion resistance, we collected soil samples from four different restoration stages (including farmland, grassland, shrubland, and forest), then used Na+-saturated soil aggregates (1 to 5 mm model aggregates) to conduct our experiments. The stability of soil aggregates and erosion resistance were tested through the wet sieving method and rainfall simulation experiments. Our results indicated that the soil net force was repulsive and decreased with vegetation restoration. The stability of soil aggregates and erosion resistance increased with vegetation restoration. With the decrease of electrolyte concentration, the net force and splash erosion rate both first increased (from 1 to 10−2 mol L−1) then remained stable (from 10−2 to 10−4 mol L−1), while the soil aggregate stability first decreased (from 1 to 10−2 mol L−1) then remained stable (from 10−2 to 10−4 mol L−1). The experimental results of soil aggregate stability and splash erosion rate changing with electrolyte concentration could be well explained by soil net force. Moreover, the rate of splash erosion and the stability of soil aggregates showed a significantly negative linear relationship (P < 0.01). Overall, the results indicated that the vegetation restoration process could reduce the net repulsive force of soil particles and thus increase the stability of soil aggregates and erosion resistance. These findings provided quantitative information about the mechanistic understanding of vegetation restoration improving soil erosion resistance. [ABSTRACT FROM AUTHOR]

• The soil aggregate stability increased and splash erosion rates decreased with revegetation. • The decrease of repulsive soil internal forces led to the improvement of the stability of aggregates. • The contribution rates of soil internal forces on splash erosion decreased with revegetation. Vegetation restoration may effectively improve aggregate stability and reduce soil erosion by increasing soil organic matter (SOM). Soil internal and raindrop impact forces are the main breakdown mechanisms of soil aggregates, and the main erosive factors that generate splash erosion. However, it remains unclear about how these forces affect the aggregate stability and the splash erosion during natural vegetation restoration. In this study, four soils at different succession stages (farmland, grassland, shrub, and forest) were selected. Deionized water and ethanol were employed to evaluate the effect of soil internal and raindrop impact forces on aggregate stability and splash erosion during vegetation restoration, by performing fast-wetting test and simulated rainfall experiment. Deionized water was used to simulate the combined effects of soil internal and raindrop impact forces, whereas ethanol was used to simulate the sole effect of raindrop impact force. The results indicated that soil organic matter (SOM) content, cation exchange capacity (CEC), specific surface area (SSA), and soil aggregate stability for the four soils increased with revegetation. The soil aggregate stability in deionized water test (MWD W) was lower than these in ethanol test (MWD E), and the relative difference of the MWD (R MWD) decreased with revegetation. Besides, the splash erosion rate in deionized water test (S W) was much higher than those in ethanol test (S E). S W and S E decreased with the order of grassland < shrub < forest. With the increasing rainfall kinetic energy, splash erosion rates increased in both deionized water and ethanol tests; but the contribution rates of soil internal forces to rainfall splash erosion decreased. The overall results suggested that the process of vegetation restoration altered soil particle surface properties mainly due to the increase SOM, and decreased the net repulsive internal forces, which thereby improved aggregate stability and erosion resistance with revegetation. [ABSTRACT FROM AUTHOR]

Surface charges can produce strong electric field around soil particles, and profoundly influence soil water movement. The influence of soil electric field on water transport in soil column with a permanently charged soil was studied. It was found that soil electric field influences soil water transport by affecting: (1) the electrostatic forces between the electric field and water molecule dipole; and (2) soil water conductivity through controlling stability of soil aggregates. When soil electric field was high, soil aggregates were broken and soil electric field influenced soil water transport through its effect on the interaction forces between water dipole and the electric field. Conversely, when soil electric field was low, the effect of soil electric field on soil water transport was mainly through its influence on soil aggregate stability. The surface potential of 153 mV for the soil was the critical potential, below this potential, the influence of soil electric field on soil aggregate stability was the main mechanism affecting soil water transport; and above this potential, the effect of soil electric field on the interaction forces between water dipole and the electric field was the main influence on soil water transport. This study suggests that it might be possible to regulate soil water transport by adjusting soil electric field using electrolytes. [ABSTRACT FROM AUTHOR]

• Soil particle surface properties were investigated after vegetation restoration. • Grassland revegetation significantly increased soil CEC and SSA. • SOM and silt contents were the main contributor to CEC and SSA in silty soils. • Vegetation restoration can pose important effects on soil surface charge properties. Vegetation restoration is an effective way to rebuild degraded ecosystems and restore soil function. Soil surface electrochemical properties, including specific surface area, cation exchange capacity, surface charge density, surface electric field strength, and surface potential, are often used as indicators to evaluate agricultural soil quality and fertility. However, little attention has been payed to the effect of natural revegetation on soil surface electrochemical properties. In this work, we selected a grassland restoration chronosequence (slope cropland, 0 years; natural grasslands restored for 3, 8, 22 and 24 years, i.e. NR3, NR8, NR22 and NR24, respectively) to investigate the changes of soil surface electrochemical properties along with vegetative restoration. Our results showed that surface charge density and surface potential for restored grassland ranged from 0.16 to 0.20 C m−2 and from −94 to −109 mV, with an average of 0.18 C m−2 and −101 mV, respectively. The value of surface electric field strength could reach to the order of magnitude of −107 V m−1 and showed a decreasing trend with increasing period of natural vegetation recovery. Cation exchange capacity and specific surface area significantly increased with the extension of natural restoration period except for NR3 (P < 0.05). It was found that soil organic matter (SOM) and silt content were the main factors to determine cation exchange capacity and specific surface area. Both cation exchange capacity and specific surface area were closely related to silt content but weakly related to clay content, possibly due to the high silt content in our studied soils. Moreover, cation exchange capacity and specific surface area showed a significant linear relationship with SOM, which may be ascribed to the diversity in cation substitution sites and high specific surface area of SOM. Our results indicated that the long-term natural restoration of grassland contributed to the improvement of cation exchange capacity and specific surface area. [ABSTRACT FROM AUTHOR]