Plastic-coated controlled-release fertilizers and biodegradable plastics, such as polylactic acid (PLA) and poly(butylene adipate-co-terephthalate) (PBAT), are increasingly used in agriculture to reduce labor. However, the transport of their fragments from agricultural watersheds to semi-enclosed coastal bays, and their subsequent deposition in bottom sediments, remain poorly understood. This study evaluates the seasonal transport, spatial distribution, settling flux, and sedimentary accumulation of polymer-coated fertilizer (PCF) and biodegradable plastic fragments from agricultural drainage rivers to a semi-enclosed inner bay in Japan, where water exchange is relatively limited.

Microplastic (MP) particles (>20μｍ) were collected from river surface waters, sediment traps, and surface sediments. To enhance detection reliability, we refined the pretreatment procedures, validated by a spike-and-recovery test, and performed particle identification using micro-Fourier transform infrared spectroscopy (micro-FTIR). Utilizing an expanded spectral library covering 20 reference polymers, including in-house spectra of PCFs, PLA, and PBAT, this approach successfully discriminated target synthetic polymers from other MPs and natural materials.

The results showed that PCF and PLA fragments were widely detected in agricultural surface rivers year-round, appearing during both the fertilizer application season and the non-cropping period. This suggests that these particles persist in soils or riverbeds and are remobilized by hydrological disturbances rather than being cleared via short-term runoff. These fragments were predominantly found in the sub100μｍ range, indicating the critical role of fine fragments in transport dynamics. Furthermore, the detection of these fragments in a sediment core suggests their long-term preservation in the inner bay. By comparing riverine fluxes, sediment-trap deposition, and sediment inventories, this presentation demonstrates whether the innermost bay functions as a definitive sink for agricultural microplastics.