Size- and Concentration-Dependent Effects of Microplastics on Soil Aggregate Formation and Properties

Abstract

Plastics fragment and threaten soil ecosystems. Degradation of soil structure is one of the risks. Despite this, data on impacts of different sized microplastics (MPs) on soil aggregates is still lacking. This study systematically investigated the effects of pristine polyethylene powders of different sizes (<35, <125, <500 μm) and concentrations (0, 0.1, 1, 10 wt%) on aggregate formation and their properties for two contrasting soils (woodland soil, WS; agricultural soil, AS). 75-days wet-dry cycles produced newly-formed aggregates in all treatments. MP size and concentration impacted the incorporation of MPs in the aggregates and this varied with aggregate size; the size distribution of aggregates also varied with MP size and concentration. Aggregates produced in soil containing 10 wt% <35 μm MPs had significantly lower MWDs (mean weight diameters) than controls. The wettability of aggregates (>4 mm) reduced with increasing MP exposure concentration and decreasing MP exposure size. MP incorporation decreased the water stability of aggregates (1-2 mm) in WS but increased it in AS. The particle density of the aggregates (>4 mm) significantly decreased with increasing MP concentration, whereas MP size had no effect. Overall, fine MPs (<35 μm) pollution should be more strictly controlled to protect soil aggregated structure.

Keywords: MPs, Soil structure, wettability, water stability, particle density