Current Research Trends on the Effects of Microplastics in Soil Environment Using Earthworms: Mini-Review

Objectives : The increase in the amount of plastics used has brought convenience to human life, but has also caused various environmental problems such as problems related to microplastics. Microplastics are defined as small plastic particles with a size of under 5 mm. As of now, research on microplastics has been largely focusing on the marine environment, and studies on microplastics in soil environment are on the rise in very recent years. Thus, the environmental impact of microplastics on the soil environment are insufficient than those on the marine environment, and there is a need for more studies. Therefore, this mini-review attempts to examine the studies on microplastics in soil environment focusing on the studies using earthworms as a test species. Methods : Published studies on microplastics in soil environment were identified by searching Web of Science. The focus of this mini-review was to gather the published articles reporting the effect of microplastics on earthworms in the absence and presence of other chemical contaminants. Results and Discussions : The number of studies evaluating the effect of microplastics in soil are significantly increasing, and earthworms are being mostly used as the test species. Experimental conditions were divided into two types: Single exposure to microplastics and combined exposure to microplastics and other chemicals. In most studies, changes in the biomarkers which are related to oxidation stress of earthworms were confirmed, and direct damage to intestinal tissues of earthworms was also observed. However, in the case of the changes of the biomarkers of earthworms, their patterns were different depending on the experimental conditions (i.e., concentrations of microplastics and chemicals, microplastics size and type, exposure period, etc.). Also, the avoidance behavior of earthworms and the internal accumulation of microplastics and co-existing chemicals were not consistent among the studies. Thus, the effects of microplastics on earthworms are not clear yet, although they seem to have some adverse effects. Conclusions : This study showed that the interest in the impact of microplastics in the soil environment has been increased recently, and earthworms are being used mostly in the studies. Yet, there is no standardized method to study the effect of microplastics on earthworms. Studies so far confirmed that microplastics induced changes in and outside of earthworm bodies. However, due to various experimental conditions, it seemed that consistent results could not be confirmed in those researches. For that reason, this study suggests that standardized research methods for microplastics researches using earthworms as a test species need to be established and need to be applied to further studies.