Flotation of polymer‑coated fertilizer capsules in paddy fields: Drivers and implications for microplastic discharge control

Polymer-coated fertilizer capsules are a relevant source of agricultural microplastics in paddy fields, yet controls on their mobilization during puddling remain insufficiently quantified. We investigated capsule flotation across 40 rice paddy fields in Ishikawa Prefecture, Japan. Accumulated capsule stocks in soil and water depth jointly controlled capsule mobilization to the water surface during puddling. Water depth explained a proportion of variation comparable to or greater than that explained by soil capsule concentration, indicating strong hydrological control on mobilization. The proportion of floating capsules increased with increasing water depth, and segmented regression identified a change point at approximately 8 cm under the conditions of this study, suggesting a shift in flotation behavior. Comparative analysis among fields with different management histories, together with polymer-specific indicators of soil residence time, suggested that longer soil residence is associated with lower flotation potential. Accordingly, recently accumulated capsules exhibited greater mobility than long-residence particles. These findings demonstrate that microplastic mobilization during puddling is governed by the interaction between accumulated particle stocks, hydrological conditions, and time-dependent particle properties. Consequently, agricultural microplastic discharges cannot be predicted from accumulated particle stocks alone but require explicit consideration of hydrological controls on mobilization. Water management during puddling therefore represents a practical leverage point for mitigating discharges.