Phosphate fertilizer enhancing soil erosion: effects and mechanisms in a variably charged soil

The extensive application of phosphate fertilizers could produce a series of environmental problems by adsorbing on the surface of soil particle and migrating into water during soil erosion. Therefore, this study is to explore the effects of phosphate on soil aggregate stability and soil erosion and to analyze the mechanisms of phosphate enhancing soil erosion from the scope of soil charge density, electric field, and particle interaction. A variable charged soil (0-20 cm) was pre-treated firstly by KCl, K2HPO4, and KH2PO4, respectively. Under the conditions of KCl, K2HPO4, and KH2PO4 solutions with concentrations of 0.0001, 0.001, 0.01, 0.1, and 1 mol L-1, (1) the amounts of soil particle transport and erosion intensities were measured using rainfall simulation with electrolyte solutions instead of rainwater; (2) the aggregate stabilities were measured by weighing the particles and micro-aggregates of < 2, 5, and 10 mu m after soil aggregate breakdown in electrolyte solutions; and (3) the zeta potentials of soil particles were measured in electrolyte solutions. The application of K2HPO4 and KH2PO4 in soil strongly enhanced soil aggregate breakdown and soil erosion, while in KCl application, soil aggregates were stable and erosion did not occur. Moreover, the intensities of soil aggregate breakdown and soil erosion for K2HPO4 application were much stronger than that for KH2PO4 application. Phosphate, especially K2HPO4, strongly increased surface negative charge density of soil particles and thus increased the electrostatic repulsive pressure between adjacent soil particles in aggregates, and as a result, the soil erosion intensity increased. However, the surface charge density was not increased by the increased pH, specific adsorption, and dispersion force adsorption but possibly attributed to a non-classic induction force adsorption arising from the anionic non-classic polarization in the strong electric field around soil particle surface. The application of phosphate decreased aggregate stability and stimulated soil erosion through increasing charge density of particle surface by a new non-classic induction adsorption of phosphate.