Agricultural plastic Legacy in river Sediments: Abundance, oxidation, and occurrence characteristics of microplastics across rural China

Microplastics (MPs), as emerging contaminants, remain relatively understudied in agricultural river systems, particularly regarding their occurrence, oxidation degree, and associated risks. This study conducted a comprehensive investigation into the occurrence status of microplastics across 25 agricultural rivers in Henan Province, China. Results showed that the average abundance of MPs in agricultural river sediments reached 1016.2 ± 1264.98 items/kg, which is between that in urban-dominant rivers (2836.6 ± 7313.8 items/kg) and lakes/reservoirs (379.5 ± 346.6 items/kg), while being slightly higher than in marine environments (818.2 ± 2166.0 items/kg). The polymer composition was dominated by polypropylene (PP, 39.6 %) and polyethylene (PE, 36.9 %). Most MPs were < 0.5 mm (80 %), with particles (45.0 %) and fragments (34.9 %) as the main shapes, and transparent MPs (56.3 %) the most common. Spatial analysis revealed higher MP abundance in areas with intensive agriculture. Carbonyl Index (CI) analysis showed a high degree of oxidation (CI > 0.3) in 73.9 % of samples, reflecting oxidative aging characteristics, likely due to riverine environmental conditions and the prolonged retention of MPs. Risk assessment indicated that the study area overall exhibited a moderate pollution level. This study provides a scientific basis for MP pollution control in agricultural watersheds, recommending enhanced source control of agricultural non-point source pollution (e.g., agricultural plastic mulch residues) to protect rural aquatic ecosystem health.