Purpose: There is a debate whether microplastic particles released into soils can modify phosphorus bioavailability by altering the soil surface properties. Here, we aim to explore the impact of polyethylene microplastics (PE-MPs) on the adsorption-desorption of inorganic phosphate anions (P) on a volcanic ash soil (VAS). Methods: Batch P adsorption-desorption experiments were conducted in a Chilean VAS with and without 1% (w/w) PE-MPs addition taking P concentrations (KH2PO4 dissolved in 0.01 mol L-1 NaCl background solution) 0.02-6.47 mmol L-1, solid (g):liquid (mL) ratio 1:40, and at a pH range of 4.5 to 10.5 at 20 +/- 1 degrees C temperature. The VAS and VAS/PE-MPs systems were characterized and kinetic and isotherm adsorption data were modelled to predict mechanisms. Results: The Elovich model described the kinetics P adsorption data on VAS with and without 1% PE-MPs (r2 >= 0.985 and chi 2 <= 12). Adsorption isotherms fitted well to the Freundlich model (r2 >= 0.994 and chi 2 <= 6.39), indicating a high heterogeneous surface for both systems. The Freundlich model indicated an increase in P adsorption capacity from 49.55 (mmol kg-1) (L mmol-1)1/n for VAS to 54.66 (mmol kg -1) (L mmol -1)1/n for VAS + 1% PE-MPs. Desorption of P was higher in the VAS + 1% PE-MPs system compared to VAS alone. For both systems, solution pH showed no significant changes in P adsorption on VAS. Scanning electron microscopy-energy dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy results showed that P was bound to PE-MPs through a weak van der Waals force and/or pore-filling mechanism. Conclusion: This study demonstrated that PE-MPs in VAS could modify surfaces available for P adsorption and act as a carrier to enhance P mobility.