Microplastics from biodegradable mulching films affect soil physicochemical properties and earthworm reproduction, but not microarthropod communities

Agricultural mulching films represent a major source of microplastics (MPs; defined as particles 1 μm-5 mm in size) in soils. With a projected exponential increase of the global use of agricultural mulching films, concentrations of MPs in soil are bound to increase. Short-term single species toxicity tests using mulching film-based MPs showed effects on soil invertebrates at high concentrations, up to 5 % (w/w dry soil), as well as on soil physicochemical properties. This study aimed to provide insight into the long-term effects of mulching film-based MPs by simulating an agricultural growing season in a highly controlled mesocosm system called CLIMECS. Eight replicate constructed cores of Lufa 2.2 soil spiked with 0 % (control), 0.025 %, 0.05 %, 0.2 % or 0.8 % starch-polybutadiene adipate terephthalate MPs received a constant springtail community (Heteromurus nitidus, Protaphorura fimata and Sinella curviseta), two species of earthworm (Aporrectodea caliginosa and Lumbricus rubellus), cress (Lepidium sativum) as vegetation cover, and lettuce (Lactuca sativa) as a crop. After 13 weeks incubation, soil pH and smaller soil aggregate fractions were significantly decreased already at the lowest exposure concentration of 0.025 % MPs compared to the control (p < 0.05 and p < 0.001, respectively). Springtail community composition did not show differences between treatments. Earthworm survival was not affected by the MPs, but total earthworm reproduction was lower at 0.2 % and 0.8 % MPs compared to the 0.05 % treatment. This study showed that MPs derived from biodegradable mulching film plastics may affect soil physicochemical properties and earthworm reproduction at environmentally relevant concentrations.