Microplastics decrease soil compressibility but have no major impact on soil physical properties

Microplastic pollution of soils has raised concerns on how microplastics impact soil properties and functions. Impacts of microplastics on soil properties is usually studied by amending soils with microplastics at various concentrations, but little attention has been given on how to compact soils after microplastic incorporation and how microplastics affect soil compressibility. Here, we used the uniaxial compression test to investigate the effects of microplastic type (i.e., granular polyethylene and fibrous polypropylene), size (i.e., 20, 200, 1000 µm for granular polyethylene and 3000 and 5000 µm for fibrous polypropylene) and concentration (i.e., 0.0 %, 0.5 %, 1 % and 2 %) on compression characteristics of a silt loam soil, followed by the evaluation of soil structure, water holding characteristics, and water and gas permeability. Soil compression was significantly affected by microplastic types, size, and concentrations. Granular microplastics increased the void ratio uniformly within the applied stress whereas fibrous microplastics increased the void ratio much more at low stress than at high stress. As a result, fibrous microplastics significantly increased the compression index (C_c) with increasing microplastics concentration. Granular microplastics decreased the swelling index (C_s), making soil less resilient against compaction. However, soil structure, water holding characteristics, and water and gas permeability were not significantly affected, except for the 5000 µm fibrous polypropylene at a concentration of 2 %, where soil porosity increased and soil water holding capacity decreased. These findings highlight the importance in considering soil compressibility, especially for laboratory incubation experiments, when evaluating microplastic effects.