Co-presence of black soldier fly frass, soil-biodegradable mulch films, and earthworms: effects on film biodegradation, ecotoxicity, and microbial community

Background: Soil-biodegradable mulch films (SBF) are used increasingly to enhance crop yields while addressing soil pollution and disposal issues. Another sustainable practice gaining attention is the use of soil amendments derived from waste, such as insect frass from Hermetia illucens. However, the combined effects of these elements on soil ecotoxicity and microbiota remain unclear. To investigate their impact, an incubation experiment was conducted, testing the presence/co-presence of a commercial SBF (10 g kg−1), H. illucens frass (50 g kg−1), and Eisenia fetida earthworms in 250 g of dry soil, maintained at 50% water-holding capacity. Results: After 22 days, SBF biodegradation reached 9.5% but increased to 62% with H. illucens frass and 52% with E. fetida, indicating that frass altered microbial communities and that earthworms enhanced soil perturbation, accelerating SBF breakdown. Ecotoxicological analysis confirmed that the tested SBF was not harmful to E. fetida, as no mortality or genotoxic damage was observed. Hermetia illucens frass also enhanced earthworm biomass, highlighting its potential as a beneficial soil amendment. The addition of soil-biodegradable mulch film, H. illucens frass, and earthworms altered soil microbial composition and functionality, reducing α diversity but enriching decomposition- and nutrient-cycling taxa like Bacillaceae and Lichtheimiaceae, enhancing soil fertility. Conclusions: These findings highlight potential synergy between insect frass and SBF in promoting soil health and accelerating SBF biodegradation in soil. Future research should focus on optimizing application rates and assessing long-term effects, with particular attention to the potential phytotoxic effects of frass on crops. © 2025 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.