The Re-distribution of Pristine and Aged Microplastics (<50 µm) in Soil Aggregate Fractions

Soil aggregates play a pivotal role in soil organic carbon dynamics and microbial activity. However, their influence on the pressing issue of microplastic (MP) contamination in soils remains poorly understood. This lack of attention may be attributed to the inherent complexity and heterogeneity of soil, which renders plastic isolation and identification in soil is particularly challenging. This study aims to investigate MPs redistribution among soil aggregate fractions during the process of soil aggregation. Two soil textures (silt loam and sandy loam) were amended with organic matter (OM) to promote aggregation during a two-month incubation period,  with 0.1 % microplastics powder added to the soils. A self-made pristine and aged LDPE and PET microplastics (<50 µm) were used in this experiment. Subsequently, physical fractionation were implemented to separate the soils into aggregate fraction (macro-aggregate, micro-aggregates and within associated fractions and silt+clay fractions). Organic matter was removed via oxidation to prevent interference with MP analysis. MPs were subsequently extracted through density separation, filtration, and examined using a Keyence VH-Z500 digital microscope. Unexpectedly, even small amounts of MPs significantly influenced soil aggregation, with effects varying by polymer type, weathering state, and soil texture. LDPE was predominantly retained in the micro-aggregate fractions in both soil textures, except for aged LDPE in loam soil, where over 60% accumulated in the silt+clay fraction. Conversely, PET was primarily retained in the macro-aggregates of silt loam soils and the micro-aggregates of sandy loam soils. Furthermore, the redistribution of MPs during soil aggregation exhibited notable differences, with silt loam soils demonstrating the highest degree of MP redistribution. These findings are relevant as soil aggregates provide different levels of physical protection against degradation and mobility, influencing the bioavailability of microplastics and their potential transfer to other environmental compartments.