Nanoplastic\nTransport in Soil via Bioturbation by Lumbricus terrestris

Plastic pollution\nis increasingly perceived as an emerging threat\nto terrestrial environments, but the spatial and temporal dimension\nof plastic exposure in soils is poorly understood. Bioturbation displaces\nmicroplastics (>1 μm) in soils and likely also nanoplastics\n(<1 μm), but empirical evidence is lacking. We used a combination\nof methods that allowed us to not only quantify but to also understand\nthe mechanisms of biologically driven transport of nanoplastics in\nmicrocosms with the deep-burrowing earthworm Lumbricus terrestris. We hypothesized that ingestion and subsurface excretion drives\ndeep vertical transport of nanoplastics that subsequently accumulate\nin the drilosphere, i.e., burrow walls. Significant vertical transport\nof palladium-doped polystyrene nanoplastics (diameter 256 nm), traceable\nusing elemental analysis, was observed and increased over 4 weeks.\nNanoplastics were detected in depurated earthworms confirming their\nuptake without any detectable negative impact. Nanoplastics were indeed\nenriched in the drilosphere where cast material was visibly incorporated,\nand the reuse of initial burrows could be monitored via X-ray computed\ntomography. Moreover, the speed of nanoplastics transport to the deeper\nsoil profile could not be explained with a local mixing model. Earthworms\nthus repeatedly ingested and excreted nanoplastics in the drilosphere\ncalling for a more explicit inclusion of bioturbation in nanoplastic\nfate modeling under consideration of the dominant mechanism. Further\ninvestigation is required to quantify nanoplastic re-entrainment,\nsuch as during events of preferential flow in burrows.