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

This project contains the protocol, extracted dataset, risk-of-bias appraisal and analysis code for a systematic review and meta-analysis of the effects of microplastics on terrestrial plants, with a specific focus on whether biodegradable polymers are less harmful than conventional ones. Biodegradable mulch films based on polylactic acid (PLA), PBAT, PBS and starch blends are widely promoted as environmentally preferable substitutes for low-density polyethylene. Because complete biodegradation requires conditions rarely met in agricultural soils, these films fragment into biodegradable microplastics that may accumulate to an extent comparable to conventional debris. Whether that substitution genuinely lowers risk to crops was unresolved: primary studies report contradictory findings, and no quantitative synthesis of this evidence existed. The review searched PubMed and Web of Science Core Collection from inception to March 2026, supplemented by backward citation searching, and included 31 primary experimental studies yielding 323 effect sizes. Growth/biomass and chlorophyll content were analysed as log response ratios and genotoxicity endpoints as Hedges' g, using random-effects models with robust variance estimation clustered by study. Three moderators were specified in advance: polymer degradability class, ageing state, and co-stressor background. Microplastic exposure reduced plant biomass and growth by 19.2% (95% CI −29.5 to −7.3) and chlorophyll content by 19.5% (−30.4 to −6.8). No significant difference in harm was detected between biodegradable and conventional microplastics; reductions in the biodegradable class were numerically larger but the contrast was not statistically significant, and given the dispersion of true effects this is an undetected difference rather than demonstrated equivalence. Co-exposure with heavy metals or other stressors more than doubled the biomass reduction relative to clean exposure (−40.1% vs −16.2%). A non-significant trend toward genotoxicity with aged biodegradable microplastics rested on only two to three studies and is reported as preliminary. Heterogeneity was extreme (I² ≈ 96–100%), so conclusions rest on the direction of effects rather than their magnitude. The findings provide no evidence that biodegradable microplastics are safer for plant health than conventional ones, with implications for the environmental evaluation of biodegradable mulch films. This is a retrospective registration, created after completion of all analyses and after acceptance for publication. It is a permanent record of methods and materials, not a preregistration.