Soil water, salt, and microplastics interact during migration: Performance and mechanism

Farmland salinization and microplastics (MPs) pollution are important environmental issues that threaten soil ecosystems. However, the interactions among soil water, salt, and MPs during migration in MPs-contaminated saline soils remain unexplored. To address this, we conducted an infiltration test using loam soils with four different salinity levels and polyethylene MPs concentrations. Experimental results showed that both MPs and salts inhibited water infiltration and that the inhibitory effect of MPs weakened with increasing soil salinity. After infiltration, MPs increased the water content in the shallow soil layer (0–15 cm) and decreased that in the deep layer (15–30 cm). Furthermore, total salt content of MPs-containing soils was higher than that of MP-free soils in the deep layer. Notably, MPs hindered the leaching of Na⁺ and SO₄^2-, and a larger obstruction was observed for Na⁺ compared to SO₄^2- leaching. In addition, MPs migrated more easily in non-saline soils, and higher salinity had a stronger inhibitory effect on MPs migration than lower salinity. Mechanism analysis indicated that MPs influence water movement through their water-repellent properties, changing the soil pore size distribution, and degradation of soil structure. Moreover, salts affect water movement by reducing the mobility of soil water and blocking soil macropores. Additionally, the surface of MPs is negatively charged and can adsorb Na⁺ in soil through electrostatic forces, further influencing salt migration. Furthermore, the presence of salt ions affects the migration ability of MPs by reducing their electrostatic repulsion and increasing the mean hydrodynamic diameter of MPs particles. These findings provide valuable insights for developing healthy soil–crop systems for croplands rich in MPs and salts.