Variability in Characteristics of Water Retention Curve in Polluted Loessial Soils by Polyethylene Microplastics

The occurrence of microplastics (MPs), known as "white pollution", especially in arid and semi-arid areas where plastic mulch is used, can affect the movement of water and solutes, especially the characteristics of the soil water retention curve (SWRC). This study aimed to achieve a better understanding of changing soil moisture characteristic curve in three loessial soils (silty loam, silty clay, and silty clay loam) by applying different rates of Polyethylene (PE) weights (0, 0.5, 1, 2, and 4 g/kg soil) during four months of controlled incubation. SWRC models were fitted to water retention curves; after evaluation, the parameters of the best model were obtained. Finally, the morphological and chemical properties of polyethylene, separately and mixed with soil, were evaluated using instrumental methods such as FTIR, FESEM, and EDX analysis. The effect of MPs on the shape of SWRC were quite evident, indicating their clear impact on the water retention capacity in silty loam < silty clay loam < silty clay soil, as compared to the soil without MPs. Also, PE concentration reduced the water-holding capacity of silty clay soil to a greater extent, relative to silty clay loam and silty loam. In addition, the shape parameter of SWRC was increased significantly, while the saturated water content (theta s) and residual water content (theta r) were decreased significantly, as MPs concentration was increased. Finally, soil texture had a greater effect on SWRC than MPs concentration. This study, thus, emphasizes the potential adverse effects of MPs accumulation on the soil water retention capacity in agricultural fields containing high concentrations of plastic mulch.