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Abstract
Plastic pollution is increasingly perceived as an emerging threat to terrestrial environments, but the spatial and temporal dimension of plastic exposure in soils is poorly understood. Bioturbation displaces microplastics (>1 μm) in soils and likely also nanoplastics (<1 μm), but empirical evidence is lacking. We used a combination of methods that allowed us to not only quantify but to also understand the mechanisms of biologically driven transport of nanoplastics in microcosms with the deep-burrowing earthworm Lumbricus terrestris. We hypothesized that ingestion and subsurface excretion drives deep vertical transport of nanoplastics that subsequently accumulate in the drilosphere, i.e., burrow walls. Significant vertical transport of palladium-doped polystyrene nanoplastics (diameter 256 nm), traceable using elemental analysis, was observed and increased over 4 weeks. Nanoplastics were detected in depurated earthworms confirming their uptake without any detectable negative impact. Nanoplastics were indeed enriched in the drilosphere where cast material was visibly incorporated, and the reuse of initial burrows could be monitored via X-ray computed tomography. Moreover, the speed of nanoplastics transport to the deeper soil profile could not be explained with a local mixing model. Earthworms thus repeatedly ingested and excreted nanoplastics in the drilosphere calling for a more explicit inclusion of bioturbation in nanoplastic fate modeling under consideration of the dominant mechanism. Further investigation is required to quantify nanoplastic re-entrainment, such as during events of preferential flow in burrows.
Highlights
Plastics can make their way into soils from diffuse sources, such as mismanaged waste, littering, or as secondary particles from plastic products fragmenting during their use or originating from traffic.[2,4−7] Agricultural soils are exposed to plastics via application of sewage sludge,[8−11] compost,[12] manure,[13] or other biosolids as soil amendments[14] and with irrigation water[4] or when microplastics are released from macroplastics used in agriculture such as mulching films[15] or packaging material.[16]
Water applications to the columns were purposefully small to keep the focus on the contribution of bioturbation to nanoplastics transport opposed to advective transport
Nanoplastic concentrations in the control columns that were only exposed to water applications without earthworms remained below the Pd background concentration in soil layers deeper than 6 cm even after 28 days of treatment (Figure 2), confirming that, as intended, advective transport was not a major factor in our system
Summary
While plastic pollution has been acknowledged as a major challenge for the marine environment,[1] recent material flow estimates suggest that comparably more plastic is emitted to soils.[2,3] Plastics can make their way into soils from diffuse sources, such as mismanaged waste, littering, or as secondary particles from plastic products fragmenting during their use or originating from traffic.[2,4−7] Agricultural soils are exposed to plastics via application of sewage sludge,[8−11] compost,[12] manure,[13] or other biosolids as soil amendments[14] and with irrigation water[4] or when microplastics are released from macroplastics used in agriculture such as mulching films[15] or packaging material.[16]. Effects on terrestrial organisms exposed to micro- or nanoplastics are often expressed as a function of average concentrations in the soil, but the smaller scale spatial distribution of contaminants in soils is often more important than average concentrations for their bioavailability and subsequent effects.[20,33] Our understanding of the terrestrial fate and spatial distribution of microplastics and nanoplastics within the soil profile is still fragmentary,[5,34] making it challenging to reliably assess the exposure of soil organisms or the long-term fate of these particles. The abundance of nonbiodegradable nanoplastics in soils is expected to increase over time as plastic emissions continue and larger particles already present in the soil gradually fragment.[35] Understanding the spatial distribution and mobility of nanosized plastics in soil will become more important in the future
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