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Researchers at Tokyo Metropolitan University set out to understand how polymer-coated fertilizer (PCF) used on farmland eventually reaches beaches and ocean waters. By examining PCF debris collected from shorelines across Japan, they found that very little fertilizer plastic returns to land through rivers. Only about 0.2% of the PCFs applied in nearby areas were detected on beaches near river mouths. However, the picture changes dramatically when farmland is connected to the ocean by canals. In those cases, as much as 28% of the fertilizer plastic was found washing back onto shore. These results point to beaches as a potentially important but overlooked “sink” in the global movement of plastic pollution.
Plastic pollution in the ocean threatens marine life, ecosystems, and human health. Scientists estimate that roughly 90% of the plastic that has entered the ocean is no longer visible at the surface. Much of it is believed to have settled on the seafloor or become trapped in various environmental “sinks.” To reduce the growing problem of plastic waste, researchers are trying to untangle how plastic travels from where it is used on land to where it ultimately ends up in the ocean.
Fertilizer Coatings as a Major Source of Microplastics
Polymer-coated fertilizer (PCF) has emerged as a significant contributor to microplastic pollution. These fertilizers are wrapped in a thin plastic layer that slows the release of nutrients, allowing them to last longer in the soil. PCFs are commonly used for rice farming in Japan and China and are also applied to crops such as wheat and corn in the U.S., U.K., and Western Europe. Previous studies have shown that 50-90% of the plastic debris found on Japanese beaches originates from these fertilizer coatings. Despite this, scientists have had limited understanding of how PCFs move from farmland into waterways and how that journey influences where the plastic ultimately accumulates.
How Water Pathways Shape Plastic Pollution
The research team, led by Professor Masayuki Kawahigashi and Dr. Dolgormaa Munkhbat, conducted extensive surveys of fertilizer plastic deposits in different coastal settings. They examined 147 survey plots across 17 beaches, focusing on locations near river mouths and areas where agricultural land drains directly into the sea.
Their findings revealed sharp contrasts. Near river mouths, the amount of PCF found on beaches accounted for less than 0.2% of the fertilizer used in surrounding fields. About 77% of the material remained on farmland, while the remaining 22.8% was carried out to sea. In areas with direct drainage from fields to the ocean, however, 28% of the fertilizer plastic ended up back on nearby beaches. The researchers concluded that waves and tidal forces play a key role in pushing these plastics ashore, turning beaches into temporary storage sites for microplastics. Since most PCFs that leave fields enter rivers, the majority of these plastic capsules ultimately go “missing.”
Why Some Plastics Stop Washing Back
The researchers also observed physical changes in many of the fertilizer microplastics collected from beaches. Many particles showed noticeable reddening and browning. Using Energy-Dispersive X-ray Spectroscopy (EDX), the team detected added layers of iron and aluminum oxide on the plastic surfaces. These materials may increase the weight of the capsules, making them less likely to be carried back to shore by waves.
Although many questions remain about how plastic pollution moves through the environment, this study provides an important step forward. By mapping how fertilizer plastics travel from land to sea, the researchers have helped shed light on how PCFs contribute to the global problem of missing plastics.
