Microplastic coatings from agricultural fertilizers are turning up on beaches in surprising quantities, according to new research from Tokyo Metropolitan University, Japan. The study is the first to quantify how much of this pollution originates from farmland and returns to shore, offering new insight into the mystery of "missing plastics" in the ocean.
Polymer-coated fertilizers (PCFs), commonly used to control nutrient release in crops such as rice, wheat, and corn, are increasingly recognized as a major source of environmental microplastics. While these 3-6 mm capsules improve agricultural efficiency, their persistent plastic shells can leak into nearby waterways during irrigation, eventually reaching the ocean.
To trace their journey, the researchers conducted a large-scale survey of 147 plots across 17 Japanese beaches, examining both physical proximity to river mouths and the presence of direct drainage from paddy fields. Capsules were collected from strandlines and analyzed using Fourier-transform infrared (FTIR) spectroscopy and energy-dispersive X-ray fluorescence (EDX). FTIR was used to confirm polymer types – mostly polyethylene and polyurethane – while EDX helped quantify surface degradation through the presence of iron and aluminum oxides.
The results showed dramatic differences in accumulation depending on the transport pathway. Beaches near river mouths received less than 0.2 percent of applied PCFs – suggesting most fertilizer plastics are lost to the ocean. In contrast, beaches connected to paddy fields via direct drainage canals accumulated as much as 28 percent of the applied microplastics, likely due to easier transit and wave-driven deposition.
Capsules collected from strandlines also showed signs of aging. FTIR spectra revealed chemical changes indicative of oxidation and photodegradation, while EDX analysis linked color changes to surface accumulation of iron oxides, which may reduce buoyancy and influence further transport. The findings suggest beaches act as temporary sinks, where capsules accumulate briefly before being carried back to sea.
With an estimated 84,000-112,000 tons of PCFs applied to Japanese fields since the 1970s, the study highlights an overlooked contributor to marine microplastic pollution.
The researchers call for better monitoring of agricultural plastics and suggest their methodology – including multistrandline sampling and spectroscopic analysis – could be applied more broadly to track land-based plastic leakage.
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