The impacts of microplastics on plants have been extensively researched, yielding a variety of responses: promoting growth, limiting growth, or causing no change in plants. Experimental studies, following basic principles of ecotoxicology, typically use a homogeneous distribution of microplastics in soils, where soil and microplastic are well-mixed. However, in the environment, plastic is not homogeneously distributed. Therefore, we tested whether the distribution of microplastics in soils affects the impact observed on plants. For this purpose, we tested the effect of homogeneously distributed microplastics and heterogeneously distributed microplastics (at different levels) on the growth of spring onions. In addition, the presence of drought was also included in our greenhouse experiment. The results show that the distribution of microplastics (whether it is homogeneous or heterogeneous) affects the growth of spring onions differently, especially the shoot and root mass. First, differences of 21–22 % in shoot mass and 29–38 % in root mass were observed between heterogeneously distributed treatments and the homogeneous treatment. Second, under drought conditions, the effects -particularly on shoot mass and the C:N (carbon:nitrogen) ratio- may differ compared to non-drought. Differences of 30–37 % in shoot mass, and up to 16 % in the carbon/nitrogen ratio were observed between different heterogeneously distributed treatments and the homogeneous treatment in the drought case. In addition, shoot mass and the C:N ratio varied depending on drought conditions. Our results strongly suggest that future experiments on microplastic effects in soil should consider at least vertically heterogeneity of the pollutant to arrive at more realistic effect estimates.
