Influence of biodegradable microplastic alteration during composting and drying/rewetting cycles on soil physicochemical properties

Biodegradable microplastics (MPs) may enter soils either as pristine materials during agricultural plastic use or through organic fertilizers after partial degradation during composting. Their impacts on soil properties may be influenced by climatic conditions. The aim of this study was to investigate how (i) MP degradation state (pristine or composted) and (ii) physical stress cycles (alternating freeze–thaw, drying–rewetting) influence soil properties. To this end, one Cambisol was exposed in controlled conditions to three types of MPs at a concentration of 1% (w/w), with four soil incubation replicates: a conventional polyethylene (PE) film, and two biodegradable films, a poly(butylene adipate terephthalate) and starch (PBAT/starch) film, and an ethylcellulose-based (EC-based) film. MPs were introduced either as pristine or after composting, then visible fragments were removed after a 59-day incubation period, to evaluate the effects of incubation on soil physical properties after removal of visible plastic fragments, rather than their physical presence. However, some leachates or small plastic fragments may still be present. Incubation was conducted either under constant conditions or under alternating physical stress cycles. Soil physicochemical and aggregation properties were measured and SOC composition analyzed using Rock-Eval ® pyrolysis. Soil responses varied primarily according to plastic type and degradation state. No significant effects on the measured properties were observed during the 59-day incubation with PE MPs, whether pristine or composted, whereas state-dependent responses were observed for biodegradable MPs. Pristine PBAT/starch and EC-based MPs changed soil pH and electrical conductivity, and increased mean weight diameter (MWD). Composting attenuated several of these effects, particularly for PBAT/starch. For EC-based MPs, composting attenuated responses for mean weight diameter and SOC content, whereas pH and electrical conductivity showed opposite trends between pristine and composted MP treatments. Alternating physical stress cycles selectively modulated specific soil properties in a treatment- and plastic composition-dependent manner, with effects mainly observed for MWD, soil pH, and electrical conductivity.