Runoff and accumulation of microplastics derived from polymer-coated fertilizer in Japanese paddy fields

Polymer-coated fertilizers, widely used in rice cultivation in Japan, contribute to reactive nitrogen management and agricultural productivity but are a source of microplastics in the environment. Here, we investigated microplastics derived from polymer-coated fertilizer (microcapsule) runoff in Japanese paddy fields at 38 sites to quantitatively assess the behavior of microcapsules in paddy fields and to estimate the total amount of runoff and accumulation in Japan. We also examined the factors causing variations in the amount of runoff among paddy fields. Between 61% and 100% of microcapsule runoff during the irrigation period occurred between puddling and rice transplanting, with concentrations ranging from 2-482 mg/m2 in paddy fields. Water management practices and wind direction and speed explained the difference in runoff between plots. The total amount of microcapsules discharged from Japanese paddy fields during the irrigation season was estimated to be between 17 and 6,291 t (median 1,157 t) from the loads obtained in this study. According to fertilizer statistics and our results, total microcapsule accumulation on agricultural land in Japan was estimated to be 75,623 t. These results suggest that paddy fields in Japan will remain a long-term source of marine microplastics.