[Effects of Microplastic Input on N₂O Production and Pathways in Rice-wheat Rotational Cropland Soils in Subtropical...

The paddy-wheat rotation system is a key grain production model in China, playing a vital role in ensuring national food security. However, due to high nitrogen (N) inputs, it has become a major source of agricultural N₂O productions. Agricultural practices such as plastic film mulching, organic fertilizer application, and sewage irrigation have exacerbated microplastic pollution in croplands. Microplastics in soil can alter physicochemical properties and microbial processes, thereby affecting N₂O productions. However, the mechanisms by which microplastic pollution influences N₂O productions in paddy-wheat rotation systems remain unclear. In this study, we investigated the effects of inputs of conventional petroleum-based polyethylene (PE) and fully biodegradable polylactic acid (PLA) microplastics on soil physicochemical properties, nitrification-denitrification rates, N₂O productions, and their production pathways in typical paddy-wheat rotation soils of a subtropical hilly region. The results showed that microplastic treatments significantly altered soil properties and nitrogen transformation processes. During incubation, soil pH decreased, while concentrations of ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) increased. Soil organic matter (SOM) significantly increased under 5% PE/PLA treatments, with PLA exerting a stronger influence on soil parameters than PE. Soil potential nitrification rate (PNR) increased, while potential denitrification rate (PDR) showed a decreasing trend, with PLA having a more pronounced effect. N₂O productions exhibited a rise-and-fall pattern, with emissions under PE significantly higher than the control (CK) during days 15-30 (P<0.05). At the same concentration, PLA induced higher N₂O productions than PE (P<0.05). Heterotrophic denitrification was the dominant N₂O source (64.5%-76.5%), while 1% PE enhanced autotrophic nitrification. Regarding functional genes, 1% PE significantly increased the abundance of amoA-AOA, whereas PLA suppressed its expression during days 15-31. Both 1% and 2.5% PE/PLA treatments reduced the abundance of amoA-AOB, while nirK and nirS abundances were generally higher under PLA than PE treatments. Partial least squares path modeling (PLS-PM) revealed that denitrification genes, soil properties, and PDR had significant effects on N₂O productions, collectively explaining 77% of the variation in N₂O productions. Overall, PLA microplastics enhanced denitrification processes more strongly than PE, resulting in greater N₂O productions from the soil.