Microplastic distribution and transport in agricultural soils : from field to burrow scale

Micro- and nanoplastics (MNPs) are small plastic particles that are emitted to soils via a range of different pathways with potentially adverse effects on the soil ecosystem. In this thesis, field and laboratory studies were done to investigate the spatial distribution of MNPs in agricultural soils for an improved understanding of input pathways, transport mechanisms and potentially transport limiting factors that modify local exposure levels. Microplastics (MPs) detected in soil profiles taken from long-term field trials revealed that amendments with sewage sludge led to higher exposure to MPs than when conventional mineral fertiliser was applied. Moreover, substantial transport was observed in the field, likely a combined result of macropore transport and bioturbation as influenced by soil management and earthworm activity. Microcosm studies of MNPs with the deep-burrowing earthworm Lumbricus terrestris revealed bioturbation as a relevant transport mechanism both for nanoplastics and MP fibres. Relatively small particles were transported faster, but particle aspect ratio and volume likely play decisive roles for the potential ingestion and subsequent transport rate by earthworms. Collectively, the results highlight the need to consider horizontal and vertical MNP mobility in soils, in particular transport by bioturbation, for future monitoring schemes and mass balances. The comparison of a range of analytical methods to quantify and characterise MNPs revealed further need to optimize detection and recovery, as well as improve conversion methods between particle- and mass-based approaches for achieving reliable exposure estimates. Further investigations are necessary to assess the transport potential of other MNP shapes and sizes, by other earthworm ecotypes and under differing soil conditions and soil management systems.