Microplastic incorporation into soil aggregates: Insights from two-year field experiments in European agricultural topsoils

Agricultural plastic mulch films are widely used in vegetable production to optimise soil temperature, moisture retention and weed control. However, they are also an important pathway for plastics to enter the soil, where they degrade over time into microplastics (MPs). The fate of these MPs in soil is still uncertain, however it is assumed that embedment in soil aggregates will protect MPs from further degradation.The aim of this study was to investigate i) how much of the MPs from biodegradable and conventional films in European topsoils are occluded within soil aggregates, ii) if soil properties control this occlusion, and iii) whether certain sizes and shapes of MPs are favoured for the embedment.To answer these questions, we analysed samples from field plot trials in Finland, Spain and Germany where MPs (< 1 mm) derived from recycled low-density polyethylene and starch - polybutylene adipate terephthalate films were incorporated into topsoil (0-10 cm) at a concentration of 0.05%. Barley was grown there in two consecutive years and soil samples were taken immediately after harvest.Free MPs and MPs embedded in soil aggregates were separated using a combination of plastic extraction (density separation and organic matter digestion) and aggregate separation techniques (ultrasonication and shaking). The size and shape of MPs were analysed using a UNet model applied to digital microscopic images.Our results showed that up to 80% of MPs are embedded in soil aggregates, with the highest proportions found in Spain, followed by Germany and Finland. Significant differences in the distribution of MPs inside and outside aggregates were observed in both Spain and Finland. The clay content had a significant effect on the occlusion of the MP in the aggregates. MPs embedded in aggregates were on average 2.5 times smaller than those outside, with most of them being smaller than 100 µm. We conclude that large portions of MPs are embedded in soil aggregates, how this affect their fate must now be analysed (see Groß et al., (EGU 2025): Microplastic degradation in agricultural soils across Europe: Comparative study of MPs inside and outside soil aggregates over two years).