Effect of Polyethylene Microplastics on Soil Physical Properties: The Interactive Effect of Particle Size and Content

ObjectiveThe extensive use of plastic film in Xinjiang has led to the accumulation of a large amount of microplastics (MPs) in farmland soil, causing changes in soil physical and chemical properties and affecting crop yields.MethodIn this study, the interaction effect of the particle size and content of polyethylene microplastics (PE-MPs) on soil physical properties was systematically investigated by simulating the residual environment of microplastics in farmland. By designing different microplastic content and particle size treatment test groups and comparing their indicators with the blank control group, the impact of microplastics on soil physical properties was evaluated.ResultThe results showed that PE-MPs significantly changed soil thermal dynamics, structural stability and water transport characteristics through size-content synergism: In terms of temperature control, microplastics reduce thermal conductivity by increasing surface roughness and porosity, and increase the daily average temperature of 5 cm soil layer by 0.97 ℃ (1 700 μm-1% treatment), and the thermal effect decreases with depth (0.67–0.93 ℃ decrease with 25 cm). In terms of structural remodeling, the treatments of large particle size (≥550 μm) and high content (≥0.25%) significantly reduced the bulk density, increased the porosity (55.62%-59.41% in the 1 700 μm treatment group), and promoted the formation of large aggregates (> 0.2 mm) by physical adsorption (93% increase in proportion, mean weight diameter up to 3.70 mm); The water transport characteristics show that the water loss rate of large particle size microplastics (≥550 μm)is 1.2 times higher than that of 48 μm treatment due to the formation of millimeter cracks at low content (0.1%-0.25%), and the particle size effect is covered at high content (1%). Correlation analysis further revealed the nonlinear coupling mechanism of soil parameters. Correlation analysis further reveals the nonlinear coupling mechanism of soil parameters, with a highly significant positive correlation between the proportion of large aggregates and average particle size, and a negative correlation between bulk density and porosity.ConclusionIt is pointed out that when the particle size is > 550 μm and the content is > 0.25%, microplastics may lead to ecological risks such as soil erosion resistance degradation and hydrothermal coupling imbalance by reconstructing pore network and aggregate bridging structure. In the future, it is necessary to combine microbial function and crop physiology studies to quantify the cascade effect of microplastic pollution, and provide cross-scale theoretical support for farmland pollution control and risk assessment.