Coupling effects of humus and 2:1 type electrolyte on soil water movement

Abstract

Recent studies have shown that, soil particle interaction forces strongly affect soil water movement. Classically, humus could increase the attractive forces through the complex interactions between humus and clay particles, but in the meanwhile humus could increase soil electric field and thus increase electrostatic repulsive forces due to surface charges of humus. On the other hand, electrolyte concentrations could affect soil particle interactions through the influence on soil electric field and osmotic pressure. Therefore, the effect of humus and electrolyte concentration would be coupled in soil water movement. In this study, the coupling effects were explored with a purple soil under different humus contents and MgCl2 concentrations. The results showed that: (i) the total repulsive energies among soil particles would increase with increasing humus content, and soil aggregate stability as well as the soil water transportation and infiltration would correspondingly decrease; (ii) a critical MgCl2 concentration was observed, and at the critical concentration the total repulsive energy was the lowest, while both the soil aggregate stability and soil water movement were the highest; (iii) humus content did not significantly change the critical concentrations. The analysis of water movement changing with soil particle interaction energy showed that, both of them decreased linearly with increasing total repulsive energies between soil particles, and different humus contents obeyed the same linear relationship for water movement. We concluded that, (i) the total repulsive energies between soil particles control soil water movement through their effect on soil aggregate stability; (ii) humus affected soil water movement mainly through its effect on the total repulsive energy between soil particles; (iii) the total repulsive energy was mainly from the electrostatic repulsive force among soil particles as MgCl2 concentration was lower than the critical concentration, while it was mainly from the osmotic pressure when the MgCl2 concentration was higher than the critical concentration.