Impacts of microplastics and heavy metals on the earthworm Eisenia foetida and on soil organic carbon, nitrogen and phosphorus

Abstract Microplastics (MPs) are increasingly being studied because they have become ubiquitous in aquatic and terrestrial environments. For example, little is known about the negative effects of co-contamination by polypropylene microplastic (PP MPs) and heavy metal mixtures on terrestrial environment and biota. This study assesses the adverse effects of co-exposure to PP MPs and heavy metal mixture (Cu 2+ , Cr 6+ and Zn 2+ ) on soil quality and the earthworm Eisenia foetida . Soil samples were collected in the Dong Cao catchment, near Hanoi, Vietnam, and analyzed for changes in extracellular enzyme activity and carbon, nitrogen and phosphorus availability in the soil. We determined the survival rate of earthworms Eisenia foetida that had ingested MPs and two doses of heavy metals (the environmental level – 1X – and its double – 2X). Earthworm ingestion rates were not significantly impacted by the exposure conditions but the mortality rate for the 2X exposure conditions was 100%. Metals-associated PP MPs stimulated the activities of β-glucosidase, β-N-acetyl glucosaminidase and phosphatase enzymes in soil. Principle component analysis showed that these enzymes were positively correlated with Cu 2+ and Cr 6+ concentrations, but negatively correlated with microbial activity. Zn 2+ showed no correlation with soil extracellular enzyme activity or soil microbial activity. Our results showed that co-exposure of earthworms to MPs and heavy metals had no impact on soil nitrogen and phosphorus but caused a decrease in total soil carbon content, with a possible associated risk of increased CO 2 emissions.