Scientists at Tokyo Metropolitan University have identified polymer-coated fertilizers as a significant source of ocean microplastics, with pathways from farmland directly influencing how much reaches shorelines. Their study reveals that direct drainage from fields to the sea results in far higher beach accumulation than river transport. This work sheds light on the elusive fate of plastics in marine environments.
Polymer-coated fertilizers, widely used in agriculture to control nutrient release, are contributing substantially to microplastic pollution in oceans. Researchers from Tokyo Metropolitan University, led by Professor Masayuki Kawahigashi and Dr. Dolgormaa Munkhbat, analyzed debris from 147 survey plots across 17 beaches in Japan. They focused on how these plastics move from paddy fields to coastal areas.
The study highlights stark differences in transport routes. Near river mouths, less than 0.2% of applied fertilizer plastics were found on beaches, with 77% remaining on farmland and 22.8% entering the sea. In contrast, areas where agricultural land drains directly into the ocean via canals showed up to 28% of the plastics washing back ashore. Waves and tides appear to trap these particles temporarily on beaches, acting as overlooked sinks in the plastic pollution cycle.
These findings are particularly relevant for regions like Japan and China, where such fertilizers are common for rice farming, and the US, UK, and Western Europe for crops like wheat and corn. Previous research indicates that 50-90% of plastic debris on Japanese beaches originates from these coatings. The team also noted physical alterations in collected particles, including reddening and browning, with Energy-Dispersive X-ray Spectroscopy revealing iron and aluminum oxide layers that may increase density and reduce reshore movement.
Overall, the research explains part of the mystery surrounding the 90% of ocean plastics that vanish from the surface, likely settling in seafloors or other sinks. Published in Marine Pollution Bulletin in 2026, the study underscores the need to track land-to-sea plastic pathways to mitigate environmental threats to marine life and human health.
