Abstract
Phosphorus (P) transportation from agricultural soil to surface water is a major contributor to P pollution in the environment, and particle phosphorus (PP) transportation is the major contributor to the total P transportation. The intensity of soil particle interaction and transportation is strongly influenced by ion-surface reactions; however, quantitative study regarding the influence of soil particle interactions on P transportation during runoff is still lacking. A simulation study on P transportation of an Entisol during runoff was conducted in this study. To quantitatively characterize the non-classic polarizability of cations on P transport in runoff, the soil was first saturated with Li+, Na+, and K+. The saturated soil layer (5 cm × 5 cm area, 3-cm thickness) was packed in a synthetic glass tray for each experiment, and the slope of the soil surface was set as 30°. The runoff water was replaced by electrolyte solutions of KNO3, NaNO3, and LiNO3 with concentrations of 0.0001, 0.001, 0.01, and 0.1 mol/L, respectively, and the solution temperature was set as 298 K. The height of water dropping was set to 3 cm. Each runoff simulation experiment lasted for 90 min. Runoff and sediment were collected by time, and the solids and solution in the collected suspensions were separated by a high-speed centrifuge. The dissolved P (DP) in the supernatant and the PP in the sediment were measured. The amount of PP transportation for the K+ treatment was 45 or 69 times smaller than that for the Na+ and Li+ treatment. The amount of DP transportation for the K+ treatment was 1.7 times higher than that for the Na+ and Li+ treatment. Additionally, increasing soil electrolyte concentration decreased both PP and DP transportation from soil to surface water. Cationic non-classic polarization could quantitatively explain the observed experimental results in PP transportation. Soil could strongly enhance the polarizability of cations; the observed polarizabilities of K+, Na+, and Li+ reached 507, 124, and 45.8 A3, respectively, but their classic values are only 0.814, 0.139, and 0.0285 A3, respectively. K+ could strongly decrease PP transportation because K+ had the strongest polarizability. The cationic non-classic polarization strongly decreased the electric field around soil particles, thus strongly decreasing the electrostatic repulsive forces between adjacent soil particles in aggregate, which decreased PP transportation in runoff. K+ cation with larger non-classic polarizability, relative to Na+ and Li+, decreased the PP transportation. The soil electric field and specific ion effects were found to play an important role in DP transportation.
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