Size- and concentration-dependent effects of microplastics on soil aggregate formation and properties

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 lacking. This study systematically investigated the effects of pristine polyethylene powders of different sizes (<35, <125, <500 μm) and concentrations (0, 0.1, 1.0, 10 wt%) on aggregate formation and their properties for two contrasting soils (woodland soil, WS; agricultural soil, AS). 75 day wet-dry cycles produced newly-formed aggregates in all treatments. MP size and concentration impacted the incorporation of MPs in 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 aggregates (>4 mm) significantly decreased with increasing MP concentration, whereas MP size had no effect. As MPs breakdown, fragment and become smaller over time, their potential risk to the aggregated structure of soil increases. Current concerns regarding microplastics in soil tend to focus on toxicity and largely ignore effects on soil structure that can inhibit soil functioning. Our study suggests that serious degradation of soil structure due to MP contamination can occur if MPs are small relative to aggregate sizes and are present at high concentrations; coarser MPs are unlikely to change soil structure and aggregate properties even at extreme MP loadings. As MPs degrade and become smaller overtime, their physical impact on soils is likely to increase and thus the potential risk of MPs to the aggregated structure of soil will increase.