Bioturbation-driven transport of microplastic fibres in soil

Microplastic pollution of the terrestrial environment and its potential impact on ecosystem services are gaining increasing public and scientific attention. Although the number of investigations on microplastic presence in soil under different management systems is growing, knowledge on the potential mobility of microplastics and their spatial distribution within soil is still lacking. In particular, microplastic fibres are often considered less mobile in soil due to their elongated shape and the resulting potential entanglement in pore structures or aggregated soil particles. While these processes may affect their water-driven transport, biologically-driven transport may still occur unimpeded because particles may be ingested by macrofauna such as earthworms. Micro- and nanoplastic transport by earthworms has been previously observed, however, the effect of particle shape on the transport dynamics is still elusive.For understanding microplastic fibre transport in soil from a mechanistic perspective, we performed a series of process-studies with deep-burrowing earthworms, i.e. Lumbricus terrestris, in microcosms. We utilized metal-doped fibrous polyethylene terephthalate (PET) microplastics (1.27±0.66 mm) for a facilitated detection through acid extraction and subsequent analysis via inductively-coupled plasma mass-spectrometry. Fibres were spiked into the top of the soil columns of the microcosms, which were sampled according to specific depth segments every 7 days for four weeks. As a result, we were able to quantify fibre transport in soil profiles with a temporal resolution. Earthworms were important drivers of vertical transport of microplastic fibres in the soil, with fibres visibly incorporated into burrow walls. Thus, despite their elongated shape and relative larger size, microplastic fibres are not necessarily immobilized by interactions with soil particles, but continuously affected by burrowing soil organisms. Understanding these transport dynamics and the potential spatial distribution of microplastics of different shapes and sizes in the field is crucial in order to support appropriate sampling schemes for monitoring and for obtaining accurate mass estimates of microplastic pollution of terrestrial ecosystems.