Interactions between microplastics, cadmium and earthworms

Biodegradable polylactic acid (PLA) mulch has been developed to replace conventional polyethylene (PE) mulch to reduce plastic pollution and the accumulation of plastic mulches derived-microplastics (MPs) in agricultural soil. Cadmium (Cd) is another soil contaminant, and can be adsorbed by MPs. However, the impacts of Cd-MP interactions on earthworm remain highly debated. It is therefore important to assess the ecological risk of the MPs-Cd interaction in farmland.

Laboratory avoidance experiments were carried out to investigate the impacts of plastic mulches derived-MPs and / or Cd on earthworm Lumbricus terrestris avoidance and mortality. The results showed realistic levels of MP or Cd and the combination of them in soils did not impact earthworm avoidance or mortality. However earthworms had a slight preference for PLA compared with PE. Exposure experiments were conducted to further investigate the impacts of these MPs and / or Cd on earthworm behaviour. The results showed in addition to mortality realistic levels of MP in soils did not impact earthworm weight change. PLA reduced earthworm exposure to Cd relative to PE by removing it from solution and reducing its bioavailability. Results of the adsorption experiments showed PLA MPs adsorbed significantly more Cd than PE MPs which consistently support the above result of exposure experiment. Furthermore, the abundance of oxygen-containing functional groups in MPs increased and biofilms developed on MPs during weathering leading to increased adsorption of Cd. Field sampling was conducted at an existing experimental site complementing the Cd-free parts of our avoidance experiments.

These results fill the knowledge gap of the impacts of interaction between Cd and mulch-derived MPs on earthworms in agricultural soil and suggest that the replacement of PE by PLA mulches could lead to a reduction in Cd bioavailability in Cd-bearing arable soils in the short term due to increased adsorption onto MPs.