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Soil aggregate breakdown is the key step of splash erosion. Recent studies have shown that it is the soil internal forces that induce aggregate breakdown and subsequent splash erosion. However, little is known about the size characteristics of soil aggregate breakdown and corresponding effects on splash erosion. In this study, Cinnamon soil and Heilu soil were adopted to investigate the effects of soil internal forces on the characteristics of aggregate fragments and splash erosion through wet sieving and rainfall simulation. The results showed the following. (1) With a decrease in electrolyte concentration in soil solution, soil net repulsive force increased; meanwhile, aggregate stability decreased and splash erosion rate increased. Additionally, when the electrolyte concentration was less than 10−2 mol L−1, soil net repulsive force hardly changed, and aggregate stability and splash erosion rate accordingly varied very little. (2) With the increase in the soil net repulsive force, the content of fragments <0.053 mm released from aggregates increased, whereas the content of fragments >0.25 mm decreased, thus suggesting that the aggregates tended to break into fine fractions as soil net repulsive forces increased. (3) Regression analyses showed that soil splash erosion rate decreased linearly with increases in the aggregate stability; moreover, correlation analyses showed that splash erosion rate was significantly and positively correlated with the fragments with a diameter of <0.053 mm, and negatively correlated with the fragments with a diameter of >0.25 mm. This study suggests that soil internal forces induce the aggregate crush and determine the size distribution of fragments available for mobilization by the raindrop impact force. Also, the raindrop impact force is the driving mechanism leading to the movement of fragments. This study provides new insights into the mechanism responsible for splash erosion and could be valuable for controlling soil erosion. Highlights: Soil aggregates breakdown dynamics and splash erosion were quantitatively investigated.Soil internal forces determined the size distribution of fragments after aggregates breakdown.The movement of soil fragments largely depended on raindrop impact force.Splash erosion was driven by the soil internal forces and raindrop impact force. [ABSTRACT FROM AUTHOR]

Soil aggregates profoundly influence soil fertility and environmental problems, and usually improving soil aggregation is the central issue in soil management. Compared with external forces, the internal forces of soil, i.e., surface hydration force, electrostatic force and van der Waals force, may play a crucial role in aggregate formation and stability. However, there are few quantitative investigations on those fundamental issues. In the present work we aim to calculate surface hydration force, electrostatic force and van der Waals force of soil/clay particles in aqueous solution, and then quantitatively evaluate the effects of the three forces on soil/clay aggregates breakdown. There was critical surface potential in particles interaction pressure and aggregates breakdown, and if the surface potential exceeded this critical point, a further increase of the surface potential could not significantly increase particles interaction pressure and aggregate breakdown. The critical surface potentials for particle interaction pressure were 207.0 and 179.7 mV for the soil and montmorillonite, respectively. Our study suggested two steps in aggregate breakdown when dried aggregates were re-wetted: (1) separating soil particles in aggregates to a distance of 1.2–1.4 nm between two adjacent particle surfaces by the surface hydration forces (swelling process); (2) breaking soil aggregates in a way of explosion or dispersion under strong or weak electric field conditions. Surface hydration force played a crucial role in aggregate swelling, and without this repulsive pressure, a dried aggregate could not be dispersed again after re-wetting. [ABSTRACT FROM AUTHOR]

• The effects of biochar on aggregate stability were quantitatively evaluated in simplified soil systems. • Soil surface charge properties increased with biochar application rate. • Electrostatic repulsion and van der Waals attraction strengthened with biochar addition. • Biochar reduced net repulsive soil internal forces and thus stabilized soil aggregates. Biochar amendments are effective for stabilizing soil aggregates and improving the quality and fertility of soils. Soil internal forces (SIFs), including electrostatic, hydration, and van der Waals forces, can substantially affect aggregate stability; however, there has been relatively little focus on the effects of biochar addition on SIFs and their relation to aggregate stability. In this work, in order to quantitatively investigate the influence of biochar on aggregate stability, we collected soil samples with different biochar application rates (0, 2.5%, 5.5%, and 7.0%; w/w) after a two-year field experiment, and then used Na+-saturated soil aggregates (1–5 mm) to conduct our experiments. Our results demonstrated that after 2-year, specific surface area (SSA), soil organic carbon (SOM), cation exchange capacity (CEC), and surface charge density (σ 0) increased while soil pH slightly decreased with biochar application from 0% to 7%. Soil aggregate stability increased with the biochar application rate. Theoretical calculations indicated that forces of electrostatic repulsion and van der Waals attraction both increased in response to biochar incorporation, however the net pressure of SIFs decreased. Soil aggregate stability was well explained by the theoretical calculations. Biochar addition reduced the net pressure of SIFs and stabilized soil aggregates. Overall, the investigation by using the mono-cationic model system showed that SIFs played important roles in the aggregate stability and explained well the aggregate stability of amended soils. The findings of this work initiate the process of elucidating and quantifying the complex biochar-mineral interaction mechanism in soil systems. [ABSTRACT FROM AUTHOR]

Soil erosion is a severe ecological and environmental problem and the main cause of land degradation in many places worldwide. Soil aggregate breakdown is the first key step of splash erosion and is strongly influenced by soil internal forces, including electrostatic, hydration, and van der Waals forces. However, little is known about the influence of soil internal forces on splash erosion. In this study, we demonstrated that both splash erosion rate (SER) and soil aggregate breaking strength (ABS) were significantly affected by soil internal forces. SER and ABS increased first (from 1 to 10 −2  mol L −1 ) then became stable (from 10 −2 to 10 −4  mol L −1 ) with decreasing electrolyte concentration in bulk solution. The electrolyte concentration of 10 −2  mol L −1 in bulk solution was the critical point for both soils in splash erosion and soil aggregate stability. The experimental results can be well interpreted by the theoretical analysis of soil internal forces. The surface potential and electric field around soil particles increased with decreasing electrolyte concentration, thereby increasing the electrostatic repulsive force among soil particles. This phenomenon led to soil aggregate breakdown and release of fine soil particles. Soil splash erosion rate and aggregate stability showed a linear relationship ( R 2  = 0.83). Our results suggest that soil internal forces induce soil aggregate breakdown and then release of fine soil particles when the soil was wetted, supplying the original material for splash erosion. Furthermore, the raindrop impact force is the driving mechanism causing soil particle movement. In summary, splash erosion could be due to the coupling effects of soil internal forces and the raindrop impact force. Our study provides a possible internal controlling method for reducing splash erosion by adjusting soil internal forces between soil particles. [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]

• Three methods were used to evaluate soil aggregate stability. • There were significant correlations among pipette method, wetting sieving and rainfall simulation experiment. • Soil interparticle forces were the main factors inducing aggregate breakdown. • There was no one method that can be used for any soil types. Soil interparticle forces, involving in van der Waals attractive force, and surface hydration and electrostatic repulsive forces, greatly influence the soil aggregate stability. However, current studies on methods for evaluating the impact of soil interparticle forces in aggregate stability are scarce. This research was aimed to examine the impact of soil interparticle forces on aggregate stability using diverse methods for different soil types. Soil aggregate stability was tested through the pipette method, wet sieving, and rainfall simulation, respectively characterized by aggregate stability index (ASI), mean weight diameter (MWD), and splash erosion mass (SE). Soil interparticle forces were adjusted by the changing concentrations of NaCl solution. The results showed that all three approaches can be applied to study the impact of soil interparticle forces on aggregate stability. The ASI, MWD, and SE showed little change below 10−2 mol L−1 NaCl concentration and then ASI and MWD increased at a high rate above 10−2 mol L−1 concentration of NaCl, while the SE showed the opposite trend. These results were as expected for soil interparticle forces. Moreover, for a single soil, a substantial correlation existed between the aggregate stability indicators obtained from three methods. However, the order of soil aggregate stability, measured by three methods were varied among soil types. Our results suggest that a single method cannot be applied to determine the aggregate stability of all soil types even if the breakdown mechanism was identical, because organic matter content and particle size distribution of soil are important factors influencing aggregate stability. Hence, in order to compare the difference in aggregate stability between various soils, multiple methods should be considered to investigate the impact of interparticle forces of soil on its aggregate stability. If a single method was to be chosen, wet sieving may be a good choice as it was not only relatively simple and time-saving but also reflected more comprehensive information about sizes and amount of fragments released from soil aggregates. [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]