Extent and effects of microplastic pollution in soil with focus on recycling of sewage sludge and composted household waste and experiences from the long-term field experiment CRUCIAL

Microplastics (MP) occur in household waste products, which can be recycled as fertilizers in agricultural fields. Recycling of waste products has many benefits but concerns of the effects of MP on soil health limit recycling. MP are present in composted household waste and sewage sludge. Sewage sludge contains many small particles (primarily fibers and fragments), whereas compost mainly contains larger fragments (flakes from packaging and bags). Here, we review the extent and possible consequences of MP pollution in soil with focus on waste product recycling. We summarize the results from studies that have measured MP concentration in soil and waste products. We review the possible hazards of MP on soil invertebrates, plant growth and microbial communities based on published studies. We discuss these results in relation to MP quantities measured in agricultural fields and generally find that MP contents in fields are below the MP levels that cause negative effects in most current effects studies. Finally, we present results from the long-term field experiment CRUCIAL, which have received composted household waste and sewage sludge in dosages corresponding to more than 100 years of legal amendment. Experiments with earthworms and quantification of various soil organisms do not indicate that household waste and sewage sludge, including the inherent contaminants, affect soil health negatively. In fact, growth of earthworms and abundances of organisms were often higher in these treatments compared to NPK-fertilized or unfertilized plots, probable due to the content of organic matter in the waste product. Based on these assessments, we conclude that the potential risk of current levels of microplastics in terrestrial environments is low for agricultural soils, but more studies are needed to perform a robust risk assessment.