Aged polyethylene microplastics reduce CO2 emissions by altering carbon degradation genes rather than soil chemical properties at different aggregate scales

The accumulation of microplastics (MPs) in agricultural soil can affect soil CO 2 emissions by changing microbial functions. However, little is known about how aged MPs affect CO 2 emissions and its mechanism at soil aggregate scale. Therefore, large macroaggregates (>2 mm), small macroaggregates (0.25–2 mm), microaggregates (0.053–0.25 mm), and silt and clay (<0.053 mm) were separated in a pot experiment involving polyethylene microplastics (PE) treated at different application times and a reference (CK), while CO 2 emissions, carbon (C) degradation function genes, and soil chemical properties were evaluated at the end of the mineralization incubation. Results showed PE aging for 2 months (SPE) stimulated CO 2 emissions by 12.8–31.7 %, while PE aging for 2 years (LPE) decreased CO 2 emissions by 0.5–24.9 % among all aggregation sizes. Meanwhile, CO 2 emissions and impact hotspots occurred in smaller soil aggregates, with the degree of impact depending on MP aging. Specifically, LPE significantly decreased soil aggregates pH by 0.1–0.2 units ( p < 0.05), while SPE increased soil aggregates pH by 0.02–0.13 units. Although the presence of MPs did not significantly affect soil nutrient and mineral content, SPE significantly increased the labile C degradation genes related to starch, hemicellulose and cellulose, whereas LPE effectively reduced these genes. Furthermore, partial least squares path model revealed labile C degradation genes have more contribution on soil aggregate CO 2 emissions than soil chemical properties, which were the main drivers of CO 2 emissions. This study provides new insights for evaluating MP aging on soil mineralization in cultivated soil under different aggregation sizes. • Effects of pristine and microplastic (MP) on CO 2 emissions in aggregation sizes were evaluated. • Both MP aging and aggregation sizes significantly affected soil CO 2 emissions. • Soil CO 2 emissions were positive correlated with C degradation genes and soil pH. • MP aging affected CO 2 emissions by altering soil microbes rather than chemical properties.