Effects of aged microplastics on paddy soil properties and greenhouse gas emissions under laboratory aerobic conditions

Microplastics (MPs) formed after changes in chemical or physical properties may alter soil properties, which in turn may affect microbial activities and greenhouse gas (GHG) emissions. However, few studies have focused on the effects of aged MPs changes on soil properties and greenhouse gas emissions. Therefore, we aimed to investigate the impact of MPs with different aging times on soil GHG emissions and dissolved organic carbon (DOC). Low-density polyethylene (PE) and polylactic acid (PLA) were treated with ultraviolet (UV) irradiation for 0–2 weeks. Soil was incubated with PE or PLA 1% (w/w) concentration at 60% water holding capacity (WHC) for 35 days. Emissions of nitrous oxide (N2O) and carbon dioxide (CO2) were measured on days 0, 1, 3, 5, 7, 14, 21, 28, and 35. Results showed that CO2 and N2O emissions were higher (p < 0.05) in MPs-amended treatments than those without MPs and increased with MPs age. The addition of virgin PE did not affect soil DOC content, whereas aged PE and all PLA additions significantly increased soil DOC content on day 0, probably because UV irradiation caused the degradation of MPs to smaller molecules. In addition, aged MPs addition altered DOC spectral characteristics on day 7, possibly because aged PE and PLA promote microbial decomposition of organic matter by altering soil properties. Changes in soil DOC content and specific ultraviolet absorbance (SUVA) by aged PE and PLA probably promoted the emissions of CO2 and N2O compared to virgin MPs or soil only. Our study revealed that aged PE and PLA promote GHG emissions from soil by changing DOC contents and qualities.