Effects of long-term organic waste fertilization of agricultural soils on the endogeic earthworm Aporrectodea caliginosa – with special emphasis on soil microplastic pollution

Summary

Recycling of municipal waste products, such as composted household waste and sewage sludge, is beneficial in many respects, especially to establish a sustainable circular economy of nutrients, reduce the loss of valuable nutrients and reduce need for mineral fertilizers in crop production. However, these types of products may also contain various xenobiotics such as heavy metals and organic pollutants, which may compromise soil fertility. Further, the microplastic content in some waste product is an emerging concern. Studies on how these products impact on soil fertility are consequently very important.
Earthworms perform a number of soil services and are therefore very important for soil fertility in many parts of the world. Additionally, earthworms may be susceptible to soil pollution, as they live most of most their lives buried in the soil, ingest soil and pass it through their gut. Earthworms are therefore often used as model organisms in ecotoxicology studies. In this study we use the endogeic earthworm species Aporrectodea caliginosa.
We designed an experiment with the purpose of comparing the effects of long-term fertilization with NPK, cattle manure, composted household waste (at normal and accelerated level) and sewage sludge (at normal and accelerated level) on body volume, burrowing behaviour and reproduction of A. caliginosa. Additionally, we included an experiment where we spiked with high amounts of two different kinds of microplastic (polyethylene (PE)) and polymethylmethacrylat (PMMA)), evaluating the same parameters.
We found no significant effects of the plastic treatments on earthworm fitness, but saw indications that earthworms may be able to detect and avoid plastic particles at high concentrations in the soil. We found significant differences on earthworm fitness in the soil screening experiment, but these effects were not related to pollution level in the soil. Mostly, the earthworms in the treatments considered potentially problematic (i.e., municipal waste products) actually performed better than comparable treatments. We therefore conclude that long-term fertilization with municipal waste products will have no negative impacts on the fitness of A. caliginosa.