Microplastics and Soil Greenhouse Gas Emissions: A Critical Reflection on Meta-Analyses

Microplastics (MPs) are widespread in agricultural soils and may influence emissions of greenhouse gases (GHGs), though their specific impacts remain uncertain. This study utilized conventional and network meta-analysis to evaluate the effects of long-term exposure to environmentally relevant MP concentrations. Results showed that, compared with uncontaminated soil, polypropylene increased CH4 emissions by 2.01 times, while polyethylene terephthalate reduced them to 0.47 times. Polybutylene adipate terephthalate (PBAT) and polylactic acid/PBAT increased CO2 emissions by 3.13 and 2.70 times, respectively; polyethylene raised N2O emissions by 1.52 times. Other MPs had no significant impact. Most types of MPs increased GHG emissions, with conventional MPs contributing more to N2O emissions and biodegradable MPs contributing more to CO2 emissions. Biodegradable plastics were not necessarily more environmentally friendly in terms of global warming potential. Moreover, the study summarized issues in current meta-analyses targeting MPs, emphasizing the need to prioritize MP types, account for correlations and interactions among moderators, ensure effect size independence, and address potential “P-hacking” during the discretization of continuous variables. This deepens our comprehension of the environmental impacts of MPs and provides valuable guidance for future meta-analyses to accurately assess effects of MPs.