Microplastic in soil - Examination of long-term effects and avoidance behaviour in the naturally occurring, endogeic earthworm species Aporrectodea caliginosa

This project was a collaborative project between Roskilde University, the Danish EPA and a consortium of stakeholders coordinated by the Danish Biomass Recycling Association. The project was established based on a common wish, from the project partners and Department of Plant and Environmental Sciences at Copenhagen University, for increasing scientific knowledge on how and to what degree important soil associated organisms may be affected by microplastic particles accumulated in agricultural soil through recycling of organic resources as fertilizers. The overall aim of the project was to examine if the use of organic waste (sewage sludge and composted organic household waste), as fertilizers, affect behavior, survival, growth and reproduction in earthworms; and specifically, if microplastic particles in the fertilizers has a measurable effect on the earthworms. The research question was addressed through two types of experiments: 1) A long-term exposure-study over 12 weeks, where survival, growth, burrowing behavior and reproduction in the grey worm (Aporrectodea caliginosa) was examined, and 2) A number of avoidance experiments with A. caliginosa and/or the compost worm Eisenia veneta, where the worms’ avoidance behavior was examined by registering their choices when provided the possibility to choose between 2-3 different soil-types. Overall, none of the tested treatments (soil/fertilizer types or added microplastic) resulted in negative effects on survival, growth or reproduction of A. caliginosa. Both long-term and avoidance experiments indicated that some fertilizers, e.g., composted household waste and sewage sludge, improve living conditions for the common earthworm species A. caliginosa, probably due to the content and type of organic material in the fertilizers. The avoidance experiments showed that if food quality is the same in the tested soils, the compost worm E. veneta can detect and avoid added microplastic (in a concentration of 0.1% on a weight-basis). However, the experiments also indicated that the two tested earthworm species’ choice of habitat is generally driven by food-availability and -quality rather than the prevalence of physical contaminants, in the form of microplastic, in the soil. Likewise, a similar 12-week exposure study on the compost worm E. veneta, which was carried out at Roskilde University as part of a MSc thesis project (Karling 2018), showed no long-term effects of microplastic in organic resources used as fertilizers, and in addition a study of microplastic effects on Eisenia andrei (Rodriguez-Seijo et al, 2017) support the findings of both of these studies. Together these studies provide the first steps towards understanding potential environmental consequences of fertilizing agricultural soils with organic fertilizers containing microplastics. Based on these projects alone, there is no indication that microplastic present in organic resources pose a risk to earthworm populations when organic resources are used as fertilizers on agricultural soils. On the other hand, a Dutch study from 2016 (Lwanga et al, 2016) did show negative effects on the survival and growth, but not reproduction, of another earthworm species (Lumbricus terrestris) during 60 days of exposure to microplastic through food. The earthworm species used in this study has a feeding strategy that is completely different from the two species used in the present study and the earlier MSc thesis study, which may be part of the reason for the observed differences. However, the exposure concentrations in the Lwanga et al (2016) study were also considerably higher than in the present study, and this is probably the main reason for the different outcomes of the studies. The study on E. andrei by Rodriguez-Seijo et al (2017) did show histopathological effects in the guts of the tested earthworms, starting at a soil concentration of approximately 0.01% (on weight basis), which indicates that the presence of microplastic in soil can be a stress-factor for the worms even if it does not translate into effects at the organism level.Although earthworms are considered to be one of the most important groups of soil macroinvertebrates, in relation to degradation of organic material and the conditioning of soils, it would be relevant to perform similar experiments with other groups of terrestrial invertebrates, for example collembolans or isopods, to examine if these are at risk from microplastic when organic resources are recycled as fertilizers on agricultural land.