Microplastic alteration in agricultural soils across Europe: Comparative study of MPs inside and outside soil aggregates over two years

Soils are considered to be a major sink for microplastics (MPs) in the environment, with the application of agricultural mulch films being one of the most important pathways to enter soil. Once in the soil, plastic particles are exposed to various environmental factors leading to MP ageing, characterised by morphological and structural changes. Soil aggregates can play a crucial role for these degradation processes, potentially preserving MP within them.Therefore, the aim of this study was to investigate the degradation differences between MPs originating from mulching films inside and outside of soil aggregates over a two-year exposure period in European agricultural topsoils.To do so, 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 MP and MP embedded in soil aggregates were separated using a combination of plastic extraction and aggregate separation techniques, ensuring that these methods did not alter the surface or structure of the MPs. The degradation state was assessed using a correlative multimodal approach, including scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX), nano-computed tomography (nano-CT) and Fourier transform infrared spectroscopy (FTIR).Exposure to soil resulted in significant ageing effects of MPs, such as surface cracking, increased oxygen content and the formation of new functional group, a higher proportion of pores, and the attachment of microorganisms. Notably, the ageing effects were more pronounced for MPs outside the aggregates compared to those embedded in the aggregates. In addition, differences were observed that were influenced by the specific conditions in each country. The results of this study reflect the complexity of environmental ageing, which depends on the soil conditions in each country. In conclusion, aggregates protect MPs from degradation, favouring plastic accumulation in the soil.