Influences of different sizes and contents of microplastics on soil macroaggregate formation and properties

Microplastics (MPs) in the soil environment have received particular attention in recent years due to their potential widespread ecological and human health impacts. Given that well-structured soil aggregates can contribute to the abilities of a soil to store carbon, resist erosion and support plant growth, evaluating the impacts of MPs on soil aggregates is important. A major knowledge gap is whether the effects of MPs on aggregates formation and properties depend on MP particle size. This thesis aimed to investigate the effects of different sizes and concentrations of microplastics (MPs) on soil aggregate formation and properties. The study used polyethylene MPs of three different sizes (<35, <125, and <500 μm) mixed with soils and subjected to wet-dry cycles to incorporate the MPs into the aggregates. The findings revealed that smaller MPs (<35 μm) had a more significant impact on aggregate formation, leading to a decrease in the mean weight diameter of newly-formed macroaggregates. Increasing concentrations of the smaller MPs also resulted in a significant reduction in the mean weight diameter of newly-formed macroaggregates. Additionally, the wettability of aggregates decreased with increasing MP concentrations and increased with increasing MP sizes. However, the water stability of aggregates showed a non-monotonic response to the concentrations and sizes of MPs. Furthermore, the study developed practical methods for characterizing the particle density of pristine MPs and detecting their concentrations in the laboratory and proved that ethanol could help laser diffraction method more precisely measure the particle size distribution of MPs. Overall, this study provides evidence that the influence of pristine MPs on soil aggregates increases with decreasing size and higher concentrations. Further research is needed to understand the breakdown process of MPs and the impacts of aged MPs on aggregate properties.