Microplastic exposure reduces plant biomass and chlorophyll content with no detectable safety advantage for biodegradable polymers

Biodegradable plastics are increasingly promoted as environmentally preferable substitutes for conventional polyethylene, yet whether the microplastics they release are less harmful to plants remains unresolved. We systematically reviewed and meta-analysed 31 studies (323 effect sizes) comparing biodegradable and conventional microplastics for effects on terrestrial plant growth, chlorophyll content and genotoxicity, using random-effects models with robust variance estimation. Microplastic exposure significantly reduced plant biomass and growth (− 19.2%, 95% CI [− 29.5, − 7.3]) and chlorophyll content (− 19.5%, 95% CI [− 30.4, − 6.8]). Reductions were numerically larger for biodegradable than for conventional microplastics (biomass − 22.1% vs. − 13.3%; chlorophyll − 26.8% vs. − 10.1%), but the between-class difference was not statistically significant. Co-exposure with heavy metals or other stressors more than doubled the biomass reduction (− 40.1% vs. − 16.2%). Aged biodegradable microplastics showed a non-significant trend towards genotoxic and cytotoxic responses, based on only two to three studies. Heterogeneity was extreme (I 2 ≈ 96–100%), but effects were robust in sensitivity analyses. These results provide no evidence that biodegradable microplastics are safer for plants than conventional ones, although a non-significant difference is not evidence of equivalence; co-occurring stressors intensify phytotoxicity, and ageing may introduce genotoxic potential, pending confirmation. These findings bear directly on environmental evaluation of biodegradable mulch films.