Effect Of Soil Electric Field On Aggregates Breakdown And Soil Erosion

Soil erosion ranks as one of the most serious environmental problems in the world, and China is one of the countries which are suffering from the most serious soil erosion. Soil erosion can lead to enormous hazards, which poses a seriously threat to ecology and environment, not only directly damages soil and causes fertility decline, but is prone to release and transform soil carbon and nitrogen which are acknowledged as contribution for global warming and ozone hole. Also, soil erosion extends to the water body and induces agricultural nonpoint source pollution which is the culprit of water eutrophi cation.Soil aggregates breakdown was the first step and the most crucial step in the process of soil erosion. Before exploring the root cause of soil erosion and taking corresponding control measures, it should be clearly understand the breakdown mechanism of soil aggregates. So far, the breakdown mechanisms of soil aggregates mainly concentrated in four reasons:the raindrop impact, differential swelling of aggregates, compression of trapped air and physico-chemical dispersion. Indeed, these effects partly explained the breakdown mechanism of soil aggregates, and relevant experiments also showed that the four mechanisms surely played significant roles in soil erosion occurrence and intensity. However, they also had some defects and could not reasonably explain the root reason for soil aggregates breakdown under certain conditions. Recently, Li et al developed a new theory to describe the interaction forces between soil colloid particles in the mesoscopic scale, and provided a new train of thought to reveal that the soil electric field mechanism was the root cause of aggregates breakdown.In this paper, the interaction forces between purple soil particles were calculated theoretically. Rainfall simulation experiments were studied at different soil electric field intensities which were adjusted by different concentration and types of electrolyte solution. Then the interrelation between soil electric field and soil erosion was acquired and showed that:(1) A large number of negative charge carried by soil particles could form a very strong electric field in the several nanometers space near the soil particle surface, and the electric field strength could run up to107～109V.m-1. Soil electric field strength was influenced strongly by the soil bulk electrolyte concentration and types. Two repulsive forces, electrostatic repulsive force and hydration repulsive force, would be produced between two adjacent soil particles by soil electric field. The force range of electrostatic repulsive force was long (up to100nm), but the force was relatively weak. The hydration repulsive force could produce strong repulsive force (up to tens of thousands of atmospheres) in the range of0～1.5nm between two adjacent soil particles, which could overcome the long-range van der Waals attractive force and prevent the colloid particles falling into the infinitely deep potential well. The two repulsive forces pushed away two adjacent soil particles and resulted in aggregates breakdown.(2) Hydration repulsive force determined whether the soil aggregates would swell, and the electrostatic repulsive force determined the breakdown strength of soil aggregates. When soil got wet, regardless of the bulk electrolyte concentration and types, hydration repulsive force, which was gigantic and had similar strength, would produce between two adjacent soil particles, so the soil aggregates would swell as long as water existed. Thereafter, if the soil electric field intensity was enough strong, soil aggregates would disperse violently with the drive of electrostatic repulsive force, however, if the soil electric field intensity was low, soil aggregates would only swell with the resistance of net attractive force.(3) Soil electric field intensity determined the degree of soil aggregates breakdown. According to the different effects of soil electric field, soil electric field intensity could be divided into three grading. First, when the surface potential was greater than200mV, the soil aggregates would explode and release a large number of microaggregates and single grain. Within this potential range, however, even if the potential continued to increase, the amount of released particles had only a small increase, showing more constant breakdown strength. Second, when the surface potential reduced to200-100mV, the soil aggregates still released small particles, but the amount of released parucles ranidly reduced vith the electric field intensity decreases, indicating that aggregates stability was very sensitive to the surface potential, and this potential range was the key area for controlling aggregates breakdown. Third, when the surface potential was less than100mV, the soil aggregates hardly released small particles, indicating that the aggregates almost did not disperse, but only partly swelled.(4) Rainfall simulation experiments showed that the soil electric field had a strong impact on soil erosion. In strong electric field conditions (>200mV), the erosion intensity was very large, and the curves of cumulative soil loss were almost linear. But at the moderate electric field intensity (200～100mV), soil loss decreased with soil electric field strength, the curves of cumulative soil loss gradually leveled off. If soil electric field intensity continued decline (<100mV), soil erosion would almost disappear. The three parameters, soil electric field intensity, soil erosion strength and aggregates breakdown strength, had the same developing trend, indicating that soil electric field had objective effect on soil erosion and aggregates breakdown. When apparent factors (raindrop impact et al.) were removed, it was further proved that soil electric field was the root cause of aggregates breakdown and soil erosion.(5) A suitable concentration of PAM could effectively improve soil aggregates stability and anti-erosion ability. PAM showed its capacity through overcoming the electrostatic repulsive force between the two adjacent soil particles, rather than hydration repulsive force. This would cause that soil aggregates would not disperse violently because electrostatic repulsive force was restrained by PAM in the strong soil electric field, which resulted in lower soil loss. PAM could not stop soil swelling and tiny soil loss because hydration repulsive force was stronger than PAM. In the weak soil electric field, the electrostatic repulsive force was relatively small, so the effect of PAM was also relatively low.