Retention, Degradation, and Runoff of Plastic-Coated Fertilizer Capsules in Paddy Fields in Fukushima and Miyagi Prefectures, Japan: Consistency of Capsule Degradation Behavior and Variations in Carbon Weight and Stable Carbon Isotope Abundance

Abstract

Paddy field runoff containing plastic capsules that are used to coat fertilizers has been receiving increased attention. However, the behavior of these capsules, especially their degradation behavior, has not been extensively investigated. We divided the capsules in runoff into two categories: “floating capsules after ploughing” and “floating capsules discharged via pipes at the exits of paddy fields”. The behaviors of the capsules in both types of runoff were monitored in 2022 and 2023 at four paddy fields in Fukushima and Miyagi prefectures in northern Japan. Sampling of capsules in paddy biomass and soil, and comparisons of capsule weight to biomass weight showed that a decrease in plastic capsule weight reflected a decrease in capsule runoff. However, the emergence of clear effects showed a delay of 2 to 3 years, as explained by carbon isotopic analyses. The decrease in the weight of the plastic capsules could be attributed to a combination of capsule degradation and the release of urea inside the capsules, which was also explained by carbon isotopic analyses. Three types of degraded capsules were found: shrunken, broken, and spherical. Statistically significant differences among the weights of each type found.