Effects of Microplastics on Soil Hydraulic Properties

Agricultural soils have been recognized as a major reservoir of microplastics, and concerns have arisen about the impacts of microplastics on soil properties and functioning. Here, we investigated the effects of microplastics on hydraulic properties of soils and determine the underlining mechanisms contributing to the effects. We measured the responses of a silt loam soil, a typical agricultural soil, to the incorporation of pristine as well as UV-weathered polypropylene granules and polyester fibers, two common types and shapes of microplastics, in terms of bulk density, saturated hydraulic conductivity, field capacity, permanent wilting point, water holding capacity, wet aggregate size distribution, and contact angle. We mixed polypropylene granules and polyester fibers into soil at different volume-based concentrations to elucidate the effect of microplastic shape. We also studied how weathering (UV and soil burial exposure) will affect the impact of microplastics on soil hydraulic properties. Due to the generally hydrophobic nature of plastic particles, the soil was found to lose some of its water holding capacity when contaminated with plastics. Fibrous microplastics rearrange soil structure and thus trigger more pronounced responses of soil hydraulic properties than granular microplastics. The results of this study provide fundamental knowledge about how microplastics interact with soil matrices and affect soil hydraulic properties, and advance current understanding about the impacts of microplastics on soil health.