Effects of soil surface electric field on the breakdown and splash erosion of soil aggregate

Abstract: Soil aggregate stability is the most important indicator for assessing erosion resistance and soil fertility. In contrast, aggregates breakdown is the key step of rainfall splash erosion, where most particles transport and soils loss in water erosion. Surface electric field of soil particles can significantly affect soil aggregate stability, which can inevitably affect the rainfall splash erosion process. The purpose of this study was to clarify the effect of surface electric field on the size distribution of fragments resulting from aggregate breakdown, then to determine the relationship between fragments size distribution and splash erosion, and finally to identify which size fractions of aggregates fragment largely affect splash erosion during rainfall simulation. The Loessal and Lou soils were used from loess parent materials on Loess Plateau. According to the electric double layer theory, the electric field strength can be adjusted by the electrolyte concentration in the soil solution, which can affect the thickness of the diffuse double layer. Therefore, five electrolyte solutions (0.000 1, 0.001, 0.01, 0.1 and 1 mol·L?1) were selected to quantitatively adjust the electric field strength of particles. The aggregate stability was evaluated by the wetting sieving method, where the aggregates were immersed in different electrolyte concentrations of NaCl solution, and then sieved in the medium of alcohol with a set of sieves of 1, 0.5, 0.25 and 0.15 mm. The splash erosion was measured through the rainfall simulation experiment using the NaCl solution with different concentrations as rainfall materials. The results showed that: 1) The surface potential and electric field of the Loessal and Lou soils increased with the decrease of electrolyte concentration in bulk solution. Meanwhile, the aggregate stability decreased, whereas the amount of splash erosion increased; 2) When the electrolyte concentration was lower than 0.01 mol/L, or the surface potentials of Loessal and Lou soil were higher than 202.0 mV and 231.6 mV, respectively, the aggregate stability and splash erosion amount nearly kept constant, indicating that the surface potentials of 202.0 mV and 231.6 mV were the critical potentials for the aggregate breakdown and splash erosion of Loessal soil and Lou soil, respectively; 3) With the decrease of the surface electric field strength of soil particles, the content of particles in the diameter less than 0.15 mm that released from aggregates decreased, whereas the content of the large aggregates (>0.25 mm) increased, suggesting that the aggregates tended to break into the larger fractions as electric field strength decreased; 4) The amount of the splash erosion was positively correlated with the content of particles in the diameter less than 0.15mm that released from aggregates, while negatively correlated with the content of the large aggregates (>0.25 mm). This revealed that the fragmentation characteristics of the aggregate breakdown under the electric field have important effects on the rainfall splash erosion. The findings indicated that as the rainfall enters into the soil, the soil bulk solution is diluted; at the same time, the strong electric field immediately generated from the soil particles surface, which will essentially affect the degree of aggregate breakdown, thereby influence the process of splash erosion. This study can enrich our understanding of the rainfall splash erosion, and also provide the theoretical basis for regulating the soil aggregate stability and rainfall erosion.