The input–output balance of microplastics derived from coated fertilizer in paddy fields and the timing of their discharge during the irrigation season

Polymer-based microcapsules derived from coated fertilizers are not recovered after use. Therefore, they are a source of microplastics to the agricultural lands and coastal areas of Japan. In this study, we investigated the input–output balance of microcapsules in three paddy fields and the timing of microcapsule discharge from the fields with the aim of developing effective techniques to reduce microcapsule discharge. Microcapsules were discharged from the paddy fields primarily during puddling, when the weir plate was overflowed, and when surface drainage was implemented. About 50% of the total discharge during the irrigation period occurred during puddling, which is a process for leveling paddy fields. Therefore, contamination of the surrounding environment by microcapsules from paddy fields can be effectively reduced by preventing the release of microplastics from paddy fields during puddling. We also showed that the total microcapsule discharge cannot be controlled solely by irrigation water management, such as by adjusting the height of the weir plate. We found that about 0.067–0.076% of the total number of microcapsules accumulated in the soil of the paddy fields was discharged during the irrigation season in 2020. Furthermore, 70% of the microcapsules discharged from one field in 2020 had resided in the soil for at least two years. The use as fertilizer coatings of biodegradable polymers that would degrade completely in the soil within a few years could therefore substantially reduce the amount of microplastics released into the ocean from agricultural fields, and their development is thus urgently needed.