Meta-analysis reveals contrasting effects of biodegradable and conventional microplastics on earthworm fitness, physiology, and gut microbiota

Microplastics (MPs) have become widespread contaminants in terrestrial ecosystems and may pose risks to soil fauna, particularly earthworms, a key group of soil ecosystem engineers. Because laboratory ecotoxicological studies have demonstrated that MPs disrupt earthworm physiological and cellular functions, then we must establish whether earthworms respond consistently to the conventional MPs and newer types of biodegradable MPs. Here, we conducted a quantitative meta-analysis of peer-reviewed studies published from database inception to 8 December 2025 to assess the effects of conventional and biodegradable MPs on earthworm fitness, physiology, and gut microbiota. A total of 2124 observations extracted from 102 articles were evaluated for the effect of MPs on 15 indicators of earthworm. Overall, MPs exposure reduced survival and growth rates, increased oxidative stress and oxidative DNA damage, and inhibited gut bacterial diversity and richness. Meta-regression analysis indicated that conventional MPs exhibited stronger dose-dependent toxicity patterns, characterized by increased catalase (CAT), and peroxidase (POD) activities, elevated malondialdehyde (MDA), and decreased survival and growth at higher doses. In contrast, biodegradable MPs primarily affected reproduction, CAT activity and glutathione (GSH) level in a size-dependent manner. Among the MPs and experiment features, the MP shape, dose, and exposure duration were most likely to cause earthworm responses, with MDA, 8-hydroxy-2'-deoxyguanosine (8-OHdG), and reactive oxygen species (ROS) consistently acting as sensitive physiological indicators of MPs exposure in earthworms. Overall, MP pollution poses substantial risks to earthworms and gut microbial communities, and biodegradable MPs do not eliminate the earthworm biological responses. This suggests that substituting biodegradable MPs for conventional MPs might not improve earthworm survival, growth and reproduction, which has implications for earthworm populations and community ecology.