The Effects Of Soil Internal Forces On Aggregate Stability And Splash Erosion Of Main Soils In Loess Region

Soil erosion is still one of the main problems threatening global ecological environment and food security.As the initial stage of rainfall erosion,As the initial stage of rainfall erosion,splash erosion originates from the soil aggregates breaking aggregates breaking and have an important effect on the whole erosion process.At present,it is widely accepted that the breakdown of soilaggregates during rainfall is caused by raindrop impact,water shear,differential swelling,slaking effect,and osmotic stress.However,recent studies have indicated that soil internal forces,including electrostatic repulsion,van der Waals attraction and hydration repulsion,may be the primary cause of aggregate breaking.The Loess Plateau is the region with the most serious soil erosion in China.Many studies have been carried out on soil erosion and aggregate stability in this region,but most of them focus on external forces such as raindrop impact force and flowing water shear,while soil internal forces are hardly mentioned.Therefore,in this study,four different soil types which widely distributed in the Loess Plateau,including Loessial soil,Dark loessial soil,Cinnamon soil and Lou soil,were selected.Based on the determination of surface charge properties of four loess soils,the soil internal forces were quantitatively calculated under different internal conditions（ionic concentration and ion types）,and their effect on rainfall splash erosion and soil aggregate stability was clarified.Moreover,by controlling different internal and external conditions,the rainfall splash erosion initiated by the coupling effects of soil internal forces and raindrop forces was revealed.This study can provide reliable theoretical support for the measure establishment of prevention and control of soil erosion on the Loess Plateau.The main results of this study are as following:1)The surface charge properties and the soil internal forces distribution characteristics of Loessial soil,Dark loessial soil,Cinnamon soil and Lou soil were quantitatively obtained.The results showed that the poorly developed loessial soil had the lowest surface charge quantity,specific surface area and surface charge density,which were 7.21 cmol·kg^-1,23.00m²·g^-1 and 0.31 C·m^-2,respectively.The specific surface area of the other three soils were similar,which was about 1.80～2.15 times of that in Loessial soil;and the surface charge quantity and surface charge density are both the largest in Lou soil,followed by Cinnamon soil and Dark loessial soil.Different soil types had the same water repulsion,but the electrostatic repulsion,van der Waals attraction and net force were different.Lou soil had the largest electrostatic repulsion,and the Dark loessial soil with the highest organic matter content had the biggest van der Waals attractive force.Soil organic matter,clay minerals,clay and silt were the main factors influencing surface charge properties and soil internal forces.In addition,for the same soil,the electrostatic repulsion between soil particles was also affected by electrolyte concentration.With the decrease of electrolyte concentration,the electrostatic repulsion and net repulsion increase This indicated that soil internal force can be regulated by electrolyte concentration.2)The relationship between soil internal forces and aggregate stability was clarified.The stability and particle size distribution of four loess soil aggregates at different electrolyte concentrations were determined by wet sieving,rainfall simulation and petite methods.The results showed that when the electrolyte concentration was 1 mol·L^-1,the soil net repulsion was the lowest,correspondingly,the aggregate was the most stable.With the decrease of electrolyte concentration to 10^-2 mol·L^-1,the net repulsive force of soil increased,induced the aggregates stability decreased,and the content of<0.15 mm microaggregates increased,while of>0.25 mm macroaggregate decreased,which indicated that the breaking degree of soil aggregate increased.When the electrolyte concentration was less than 10^-2 mol·L^-1,soil internal forces basically remained unchanged with the decrease of electrolyte concentration,and thus leading to the little change of aggregates stability.The soil internal forces had significant exponential relationships with the aggregate breaking strength（ABS）,mean weight diameter（MWD）,geometric mean diameter（GMD）,fractal dimension（D）and large aggregate content(R_0.25)（P<0.05）,indicating that soil internal forces could strongly affect the stability and particle size distribution characteristics of soil aggregates.3)Revealing that rainfall splash erosion was caused by the combined effect of soil internal forces and raindrop impact force external force（raindrop impact force）.The internal forces were regulated by electrolyte solution with different concentrations,and the impact force of raindrop was regulated by different rainfall height.The results showed that the splash erosion mass increased with increasing rain drop impact force（external force）.At the same raindrop impact force,with the decrease of electrolyte concentration,soil net repulsive force increased,leading to the splash erosion mass increase;while the concentration<10^-2 mol·L^-1,soil internal force nearly unchanged,the splash erosion mass revealed little change.,which indicated that splash erosion was affected by the soil internal force.There was a negative linear relationship between splash erosion mass and aggregate stability.Under the strong repulsive force(<10^-2 mol·L^-1),the total content of splash fraction<0.053 mm and 0.053-0.15 mm was more than 90%at each rainfall height.As concentration≥10^-2 mol·L^-1,the content of splash fraction 0.15-0.25,0.25-0.5 and 0.5-1mm increased with the increase of rain drop impact force.The enrichment rates of splash aggregates increased with the decrease of the aggregate size,and the enrichment rate of<0.053 mm was>1.These results indicated that the splash erosion was caused by the combined effect of soil internal forces and raindrop impact force.Soil internal force4)The relative contribution rate of soil internal force to rainfall splash erosion is quantified.Electrolyte solution and ethanol were applied as rainfall materials to measure the total splash erosion rate SER（i+e）caused by the combined effects of soil internal and raindrop impact force,and the splash erosion rate SER（e）caused by the raindrop impact force,respectively.The results showed that at the same rainfall kinetic energy,SER（i+e）was higher than the SER（e）,and the difference between them,i.e.,the splash erosion rate SER（i）caused by the soil internal forces increased with the decreasing electrolyte concentrations.With the increasing rainfall kinetic energy,SER（i+e）and SER（e）increased exponentially（R²>0.98）.When the concentrations were>10^-1 mol·L^-1,the contribution rate of soil internal force were less than 50%,which meant that the contribution rate of soil internal forces was lower than the raindrop impact force to the splash erosion.This was because at high electrolyte concentration,soil internal force was small,aggregates were broken to release large-size aggregates,thus the splash erosion was mainly controlled by the migration ability of rain impact force.When the concentrations were≤10^-1 mol·L^-1,the contribution rate of soil internal forces to splash erosion of loess soils were higher than 50%,which indicating that soil internal force was contribute more than the raindrop impact force.5)It is revealed that the cationic non-classical polarization was the reason for the specific ion effects on loess soil aggregates stability and rainfall splash erosion.The non-classical polarization of cations in soil strong electric field had an important effect on soil internal forces.When considering the difference in non-classical polarization of Na⁺and K⁺,the electrostatic repulsion and net repulsion at Na⁺and K⁺systems were Na⁺>K⁺,and the differences of soil electrostatic repulsion and net repulsion between Na⁺and K⁺increased with the decreasing concentration.This meant that K⁺system will stronger than Na⁺system in maintaining the stability of aggregates and resisting splash erosion,and these differences will more significant at lower concentration.The experimental results of soil aggregate stability and rainfall splash erosion under different electrolyte concentrations of Na⁺and K⁺systems showed that ionic specificity strongly affected the aggregate stability and splash erosion.The splash erosion rates（SER）and aggregate breaking strength（ABS）of the four loess aggregates were Na⁺>K⁺,and the differences of ABS or SER between Na⁺and K⁺saturated aggregates were amplified firstly and then remained constant with the decrease of electrolyte concentration,indicating that the specific ion effects were more prominent at lower concentrations.These experimental results were consistent with the theoretical prediction of soil internal forces considering cationic non-classical polarization.And the relationship of soil internal forces influenced by the cationic non-classical polarization with ABS or SER indicated that ABS or SER of the two cationic systems changed with soil internal forces in the same way.These revealed that the soil internal forces affected by the cationic non-classical polarization was the essential cause of specific ion effects on soil aggregate stability and splash erosion.Based on the above results,we could conclude that the rainfall splash erosion was resulted from the coupling effects of soil internal forces and raindrop impact force.The soil internal forces initiated the aggregates breakdown and released abundant fine particles,which is the"source"power of the splash erosion,while raindrop impact force caused soil particles to transition,which is the"sink"power of the splash erosion.The concentration and species of ions in the soil solution could affect the soil aggregate stability and splash erosion by affecting soil internal forces.