What do we know about how the terrestrial multicellular soil fauna reacts to microplastic?

The ubiquitous accumulation of microplastic (MP) particles across all global ecosystems accompanies their uptake into soil food webs. In this review, we analyzed studies on passive translocation, active ingestion, bioaccumulation and adverse effects within the phylogenetic tree of multicellular soil faunal life. The representativity of these studies for natural soil ecosystems was assessed using data on the type of plastic, the shape, the composition, the concentration and the time of exposure. Available studies cover a wide range of soil organisms, with emphasis on earthworms, nematodes, springtails, beetles and lugworms, each focused on well-known model organisms. Thus, about 58 % of the studies used inappropriate concentrations or units, whereas 42 % applied MP concentrations similar to amounts in slightly to very heavily polluted soils. In many cases, however, polystyrene microspheres were used, which represent a combination of plastic type and shape that is easily available but does not reflect the main plastic input into soil ecosystems. In turn, MP fibers are strongly underrepresented compared with their high abundance within contaminated soils. A few studies also examined the comminution of macroplastic by the soil fauna. Further properties of plastic such as aging, coating and additives have been insufficiently documented. Despite these limitations, there is a recurring pattern of active intake followed by a population shift within the gut microbiome and adverse effects on motility, growth, metabolism, reproduction and mortality in various combinations, especially at high concentrations and small particle sizes. For the improvement of future studies, we identified the problems with past experiments, and we recommend that coming studies consider the type, shape, grade of aging, specific concentrations of MP fractions and long-term incubation in both natural and contaminated soils.