Plastic mulch debris is increasingly accumulating in agricultural soils worldwide, and their ecological role in reshaping soil detritivores functions remain poorly investigated. Here we demonstrated that terrestrial isopods (Porcellionides pruinosus) actively fragment and transform both fossil-derived low-density polyethylene (LDPE) and biodegradable (MaterBi®) mulch debris into secondary plastics by shaping ecological traits. Using long-term soil microcosms, we performed a multilevel ecological analysis to assess mulch fate, change in soil physicochemical properties, and trait responses at multiple biological scales, including population-level (gregarious behaviour and trophic activity), individual-level (mass gain, body water content, assimilation efficiency), and biochemical traits (glycogen, protein, and lipid content). Terrestrial isopods generated both micro- and nanoplastic particles from both mulch types and redistributed them through faecal pellets into soil. Plastic exposure disrupted social aggregation networks, reduced coordinated feeding activity, and reshaped energy allocation pathways. Biodegradable mulch promoted higher fragmentation and assimilation efficiency, whereas LDPE lowered glycogen reserves and
increased lipid accumulation, indicating metabolic stress. Multivariate trait analyses revealed significant reorganisation of ecological functioning between polymer types and plastic-free microcosms, with behavioural and metabolic traits strongly coupled to environmental changes, and individual performances conformed with the environmental change. This study represents the first integrative assessment of how soil detritivores respond to and potentially mediate the transformation of plastic debris across multiple ecological levels. Our findings identify soil detritivores as active mediators of plastic fragmentation and biogeochemical cycling, highlighting large plastic debris as an overlooked driver of functional reorganisation in terrestrial ecosystems. Furthermore, the multilevel approach suggests that higher-hierarchical ecological responses may be more sensitive and representative indicators of mulch contamination.

Project funding from the Ministry of Universities and Research as part of the call to review the final rankings of the PRIN 2022 call for proposals for the project entitled “NANOplastics Toxicity Evaluation and Risk (management) in teRrestrial Agro-Ecosystems (NanoTERRAE)” Protocol number 20229KSJAT/