Effect of Polypropylene Microplastic on Soil Water Characteristic Curve

Microplastics (MP) are present in the soil mainly due to the use of agricultural mulch films and sewage sludge as fertilisers. They are small particles of less than 5mm in size, whose presence impacts agricultural productivity by modifying the soil pore structure, affecting its water and nutrient retention properties, and altering the soil water characteristic curve (SWCC). The objective of this study was to investigate the effects of the size and concentration of polypropylene (PP) microplastics on the SWCC of silty sand. The soil comprises of 72.2% sand, 27.7% silt and 0.1% clay. Polypropylene particles of size (0–50 µm and 50–100 µm) were added to the soil at three concentrations (0.1%, 0.2%, and 0.4% w/w) in the range reported in the literature. The SWCC was measured using a pressure-plate assembly designed such that the mass balance of water imbibition and exudation can be verified at all stages. A genetic algorithm from python library (pygad) was employed to fit the observed soil moisture data and estimate the parameters of the Van Genuchten (VG) model. As the pressure increased, the MP of size 50-100 µm showed a significant decrease in the soil water holding capacity up to a pressure of 0.7 bar, beyond which there were no significant differences with the SWCC for the control soil without MP. Increasing MP content decreased the soil water retention capacity.  The field capacity decreased by 5.8% at a concentration of 0.4% for larger-sized microplastics. The smaller size MP (0–50 µm) at low concentration of 0.1% did not significantly affect the soil water content, while the accumulation of PP-MP at higher concentrations (0.2% and 0.4%) resulted in a significant decrease in the soil water holding capacity. The differences in microplastic sizes and concentrations led to variations in the SWCC, which was reflected in the variations in the fitted parameters of the VG model. The presence of larger MP may have disrupted the original capillary pore structure and weakened the soil capillarity, contributing to a decline in soil water retention. The small amount of finer MP may have been transported through the soil pores without significantly affecting the water retention properties of the soil. These findings highlight MP potential risk to soil hydraulic properties and its negative impact on plant growth.