Could soil microplastic pollution exacerbate climate change? A meta-analysis of greenhouse gas emissions and global warming potential

Microplastics pollution and climate change are primarily investigated in isolation, despite their joint threat to the environment. Greenhouse gases (GHGs) are emitted during: the production of plastic and rubber, the use and degradation of plastic, and after contamination of environment. This is the first meta-analysis to assess underlying causal relationships and the influence of likely mediators. We included 60 peer-reviewed empirical studies; estimating GHGs emissions effect size and global warming potential (GWP), according to key microplastics properties and soil conditions. We investigated interrelationships with microbe functional gene expression. Overall, microplastics contamination was associated with increased GHGs emissions, with the strongest effect (60%) on CH emissions. Polylactic-acid caused 32% higher CO emissions, but only 1% of total GWP. Phenol-formaldehyde had the greatest (175%) GWP via 182% increased NO emissions. Only polystyrene resulted in reduced GWP by 50%, due to NO mitigation. Polyethylene caused the maximum (60%) CH emissions. Shapes of microplastics differed in GWP: fiber had the greatest GWP (66%) whereas beads reduced GWP by 53%. Films substantially increased emissions of all GHGs: 14% CO, 10% NO and 60% CH. Larger-sized microplastics had higher GWP (125%) due to their 9% CO and 63% NO emissions. GWP rose sharply if soil microplastics content exceeded 0.5%. Higher CO emissions, ranging from 4% to 20%, arose from soil which was either fine, saturated or had high-carbon content. Higher NO emissions, ranging from 10% to 95%, arose from soils that had either medium texture, saturated water content or low-carbon content. Both CO and NO emissions were 43%-56% higher from soils with neutral pH. We conclude that microplastics contamination can cause raised GHGs emissions, posing a risk of exacerbating climate-change. We show clear links between GHGs emissions, microplastics properties, soil characteristics and soil microbe functional gene expression. Further research is needed regarding underlying mechanisms and processes.