Nanoplastic transport in soils by advection and bioturbation

Micro- and nanoplastics are increasingly perceived as an emerging threat to ecosystems. They are emitted to soils through different pathways, including sewage sludge or compost applications in agriculture, or through tire abrasion and degradation of mismanaged waste. Yet, their environmental behaviour and fate in terrestrial ecosystems is still poorly investigated. In order to investigate the potential impact of different transport processes on the redistribution of plastics in natural soils, column leaching tests and bioturbation studies in microcosms were conducted using a natural topsoil and palladiumdoped polystyrene nanoplastics of 256 nm diameter. Under the influence of advection, nanoplastic retention in saturated columns was very limited. Kinetic transport parameters were obtained from saturated column tests by applying inverse modelling in HYDRUS-1D. Derived attachment efficiencies were relatively low, att = 6.25 × 10-4. In unsaturated soils, more representative of prevailing field conditions, nanoplastic mobility through percolating water was very limited. However, the burrowing activity of anecic earthworms, here Lumbricus terrestris, caused a significant redistribution and transport of nanoplastics into deeper soil layers, steadily increasing over the duration of the experiment. Observed spatial and temporal changes in nanoplastic distribution were used to determine bioturbation rates by applying a bioturbation model (kbioturb = 4.5 × 10-11). The bioturbation model systematically underestimated nanoplastics in the lower layers, indicating that further differentiation of the transport modes by soil biota might be necessary. Although mixing by earthworms was slow, the current study suggests that under field conditions bioturbation may be more important than advective transport for nanoplastics in soils. While displacement of nanoplastics likely reduces uptake and risks for terrestrial organisms and crops near the surface, potential effects in deeper soil layers are of yet unknown consequences. A wider array of nanoplastic types and sizes, as well as modes of applications is needed to allow for extrapolation of findings.