Impact of microplastics on CO2 emissions in paddy soils: The role of cultivation duration and microbial community

Microplastics (MPs) are increasingly recognized for their potential to alter soil carbon dynamics, yet their effects on carbon dioxide (CO₂) emissions and associated microbial mechanisms remain unclear in paddy soils with varying cultivation histories. Using a microcosm experiment, we investigated how polyethylene MPs (0 %, 0.01 %, 0.1 %, 0.5 %, and 1 %) influence CO₂ emissions, carbon-degrading genes, and microbial communities in soils with different rice cultivation durations (3, 15, and 40 years). MPs did not exhibit a dose-response relationship with CO₂ emissions in the soils cultivated for ≤15 years, but 1 % MPs increased emissions by 5–8 % across all soils (p < 0.05). In the soil cultivated for 40 years, even 0.01 % MPs stimulated the release of CO₂, as the exposure of MPs (0.01–1.0 %) significantly increased CO₂ emissions per unit of soil organic carbon by 6–9 %. The abundance of abfA and sga genes linked to labile carbon degradation correlated positively with CO₂ emissions in 3- and 15-year soils, respectively, whereas these genes were not able to explain the change of CO₂ emissions in 40-year soils. Microbial communities were more significantly shaped by cultivation duration than by MPs, with 15-year soil showing a markedly lower oligotrophic-to-copiotrophic microbial ratio than 3- and 40-year soils. Network analysis identified potential hosts of carbon-degrading genes, including Herminiimonas (3- and 40-year soils) and Pelagibacterium, Undibacterium, Fulvivirga, and Muriicola (15- and 40-year soils). Our findings demonstrate that 1 % MPs significantly enhance CO₂ emissions from paddy soils, whereas the threshold for MP-induced soil organic carbon decomposition decreases to 0.1 % with prolonged cultivated, suggesting an increased sensitivity of cultivated soils to MPs contamination over time.