Retention and migration of microplastics in stepped paddy fields: A study on microplastic dynamics in the special...

The stepped paddy fields (SPFs) are important for food security and sustainable development. The unique spatial structure and complex hydrological processes in this system make it difficult to understand the migration of pollutants. In this study, microplastic pollution was investigated in the water and soil from Ziquejie SPFs, China. Samples were taken according to different stages of rice cultivation, different altitudes, and soil depths. Before rice planting, the average abundance in the water and soil samples was 1.3 ± 1.1 items/L and 292.2 ± 260.8 items/kg, respectively. After rice harvesting, the average abundance in soil increased to 495.37 ± 175.46 items/kg. More microplastics were found in depths of 0-15 cm than that of 15-30 cm, but the difference was not significant. Major microplastics were small (<1 mm) polyethylene (PE) and polypropylene (PP) fibers, with the main color of blue and transparent. The study found that SPFs hindered the migration of microplastics in irrigation water and hydraulic conditions affected the accumulation of microplastics. The edge areas in paddy fields where the water flowed slowly and were prone to vortices made it easier for microplastics to stay. Most of the microplastics accumulated in the upper SPF. After rice harvesting, the overall abundance of microplastics in the SPFs increased. Fibers and fragments exhibited different characteristics in migration. This study attempts to draw attention to the ecological risks caused by microplastic pollution in SPFs, especially in the upper paddy field and the effluent. The results are helpful for the protection of the SPF ecosystem and provide valuable references for future research.