Dose effect of polyethylene microplastics on nitrous oxide emissions from paddy soils cultivated for different periods

The presence of microplastics (MPs) under flooded conditions is beneficial for nitrifiers and denitrifiers to produce nitrous oxide (NO), but their dose effect remains unclear. This study evaluated the impact of different doses of polyethylene (PE) MPs on the release of NO from paddy soils cultivated for different years. Compared with unpolluted soils, low doses of MPs (≤ 0.1%) had a negligible influence on NO emissions, and high amounts of MPs (≥ 0.5%) significantly (p < 0.05) increased NO emissions from the paddy soils cultivated for 3, 15 and 40 years by 2.5-4.3, 3.9-8.5 and 8.9-27.7 times, respectively. Moreover, an exponential model indicated that a 0.2% concentration of PE MPs appeared to be the dose threshold that accelerated the release of NO from the all soils. Increased MP concentrations accelerated NO emissions by affecting microbial functional genes involved in NO production and reduction, but microbial taxonomic attributes involved in nitrogen cycling played an insignificant role in controlling NO emissions. Overall, our results indicated that high doses (≥ 0.5%) of PE MPs essentially accelerated the emission of NO from rice soils, and a longer cultivation period (40 years) enhanced the positive effect of MPs on NO emissions.