Effect of microplastics on soil greenhouse gas emissions: A global meta-analysis study

Microplastics (MPs) emerged as a critical global pollutant, yet their effects on soil greenhouse gas (GHG) emissions remain uncertain. This meta-analysis evaluates the effects of MPs exposure on GHG emissions and identifies key influencing factors. Regardless of any influencing factors, MPs exposure decreased N₂O emissions by 28.5 %, while increased CO₂ and CH₄ emissions by 0.07 % and 28.6 %, respectively. However, these changes were statistically insignificant. The factors such as MPs concentration, shape, and type, initial soil type and pH, and the presence of additional additive were identified to significantly influence N₂O emission response. The most substantial increase in N₂O emissions occurred when MPs exposed to silt soils (+55.8 %), whereas the greatest inhibition was observed in soils with fertilizer addition (-48.8 %). CO₂ emission response was significantly influenced by MPs size and shape, initial soil pH and type, experimental duration, and co-additives, with the MPs exposure in sand soils exhibiting the highest increase (+20.7 %) and the exposure of fiber MPs causing the largest reduction (-40.4 %). Additionally, MPs shape and initial soil pH and type were found to significantly affect CH₄ emission response. The model selection analysis revealed that the response ratio [ln (RR)] of nirS gene to MPs exposure and MPs concentration were the most critical factors influencing N₂O emissions, whereas the size, concentration, and type of MPs, ln (RR) of Chitinase, initial soil pH, additional additive, and experimental duration were the most important influencing factors for CO₂ emissions. Overall, this study highlights the high uncertainty associated with the response of GHG emission to MPs exposure due to the complex interplay of abiotic and biotic processes mediated by MPs under varying conditions. In the future, extensive studies across diverse conditions of MPs (e.g., type, shape, and concentration) and soil (e.g., texture and pH) are still urgently needed.