Tokyo, Japan – Researchers from Tokyo Metropolitan University have studied how polymer-coated fertilizer (PCF) applied to fields ends up on beaches and in the sea. They studied PCF deposits on beaches around Japan, finding that only 0.2% of used PCFs are washed into rivers and returned to the coastline. When there are canals connecting fields to the sea, this rises to 28%. Their findings highlight a potentially significant "sink" in the global circulation of plastics.
Plastic marine pollution poses a serious threat to wildlife, ecosystems, and human health. It is estimated that around 90% of the plastic that has flowed out to sea has disappeared from the sea surface, accumulated on the sea floor or any number of other "sinks." To effectively reduce the amount of "missing plastics," scientists have been studying the complex ways by which plastic material is transported from its point of use to the sea.
Polymer-coated fertilizer (PCF) is a major source of microplastic pollution. Certain fertilizers are coated in a thin layer of plastic to delay the release of chemicals, making it last longer. They are widely used in Japan and China for rice cultivation, as well as for wheat, corn, and other crops in the U.S., U.K., and Western Europe. In fact, it has been shown that 50-90% of plastic debris found on beaches in Japan is derived from PCFs. Yet, the way in which PCFs are carried from land to sea, and how that affects its eventual disappearance, is not well understood.
A team of researchers from Tokyo Metropolitan University, led by Professor Masayuki Kawahigashi and Dr. Dolgormaa Munkhbat, surveyed the amount of PCFs ending up on beaches across different environments. They focused on beaches near river mouths and direct drainage points from agricultural fields to the sea, surveying 147 plots across 17 beaches. Near river mouths, they estimated that the PCFs found on beaches there amount to less than 0.2% of what was used in surrounding areas. With 77% staying on fields, the remaining 22.8% wash out to sea. On the other hand, surveys around direct drainage points from agricultural land to the sea showed that 28% end up back on the beach. The team concluded that waves and tidal action help them wash back onto land, making beaches a temporary sink for microplastics. Given that most PCFs lost from fields end up in rivers, the majority of these plastic capsules end up going "missing."
The team also noticed that many of the PCF microplastics they found showed significant reddening and browning. Analysis with Energy-Dispersive X-ray Spectroscopy (EDX) revealed newly added particles of iron and aluminum oxide, which may be weighing the capsules down, making them less likely to wash back to shore. While many challenges remain in understanding the complex transport of a major pollutant, the team's survey is a key first step in tracing how PCFs contribute to the global challenge of missing plastics.
