Global hierarchical meta-analysis of microplastic-induced changes in the soil nitrogen cycle

Microplastics (MPs) significantly disrupt soil nitrogen (N) cycling by altering physicochemical properties and microbial communities, affecting fertility and crop productivity. However, most studies are short-term and lack comprehensive evaluations of MPs type, size, and concentration. This global meta-analysis assesses MPs' impact on soil N cycling, explores the influence mechanisms of MP characteristics, and to inform precise management strategies. Results show that high concentrations (>1%) and small particle sizes (1-100 μm) of MPs simultaneously increased total carbon (TC, +22.3%), dissolved organic carbon (DOC, +23.0%), and microbial biomass carbon (MBC, +23.5%) through a dual mechanism of physical adsorption and carbon supply from degradation, thereby providing readily available carbon sources for soil microorganisms. Although MPs had no significant effect on total nitrogen (TN), they markedly increased NH₄⁺-N (+36.5%) and promoted NO₃^--N consumption (-26.7%). This pattern was more pronounced in warm (10-30 °C) and humid (>400 mm) climatic regions, likely due to enhanced denitrification under high temperature and moisture conditions. Functional gene responses were tightly coupled with these chemical changes: MPs of 1-100 μm significantly upregulated denitrification genes nirK (+49.2%) and nosZ (+35.0%), directly driving NO₃^--N reduction, whereas large (1000-5000 μm) and ultrafine (<1 μm) MPs preferentially stimulated nitrification-related genes (AOA/AOB-amoA, +19.7-35.0%), corresponding to NH₄⁺-N accumulation. Biodegradable MPs further released labile carbon, resulting in a greater increase in nirK abundance (+24.2%) than non-biodegradable MPs, thereby explaining their stronger denitrification-promoting effects. Overall, MP concentration, particle size, and biodegradability jointly regulate soil nitrogen cycling by first altering carbon availability and microenvironmental conditions, and subsequently directing nitrification or denitrification processes, with these effects being amplified under warm and humid climates. These findings highlight MPs as ecological regulators with far-reaching impacts beyond traditional physical pollutants and offer vital insights for sustainable N management and soil pollution mitigation.