Leaching of microplastics enhanced through complex soil meso- and macrofaunal community transport

Microplastics (MPs) are emerging pollutants in soils that have a negative impact on the survival and reproduction of soil organisms. However, very little is known about soil fauna's role as transport agents of MPs into deeper soil layers. Here, we explored the effects of soil meso- (collembolans) and macrofauna (earthworms) on the distribution of linear low-density polyethylene MPs (LLDPE-MPs) down the soil profile in a mesocosm incubation experiment. Sixty PVC columns (10 cm in diameter x 23 cm in height), filled with agricultural soil, received 250 mg of LLDPE-MPs (300-600 μm) and soil organisms: earthworms belonging to three different ecological groups (epigeic, endogeic and anecic), and collembolans (a fixed mixture of epedaphic, hemiedaphic and euedaphic species), either as monocultures or in combination (all earthworm groupings, epigeic + Collembola, endogeic + Collembola, anecic + Collembola and all earthworm groupings + Collembola). Animal treatments without addition of LLDPE-MPs were also established as controls. Results showed that LLDPE-MPs were consistently leached in the control units after 14 days (66.75 LLDPE-MP particles leached in total), implying that infiltrating water alone can transport LLDPE-MPs down through the soil matrix. However, this downward movement of LLDPE-MPs into the leachate was significantly increased by ∼158 % (172.25 LLDPE-MP particles leached) when anecic earthworms were present and increased by ∼172 % (181.5 LLDPE-MP particles leached) when the mixture of the three earthworm ecological groupings were present. Leaching of LLDPE-MPs also increased when Collembola were combined with anecic worms and with the three-earthworm combination by ∼225 % (217 LLDPE-MP particles leached) and ∼276 % (250.75 LLDPE-MP particles leached), respectively (compared to controls). These findings suggest that most prominently anecic burrowing activity and the interactions between macro- and mesofauna enhance the vertical transport of MPs to deeper soil layers and leaching from soil at this limited experimental scale. We suggest that future studies on MP transport in soils include a more complex and representative population of soil organisms to account their collective influence on MP transport, so that predictive modelling for MP transport can faithfully factor in meso- and macrofauna variables.