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Tracing Microplastics from Polymer-Coated Urea: Urban to Agricultural Pathways and Environmental Fate
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Researchers evaluated off-site transport of microplastics from polymer-coated slow-release urea fertilizer across urban and agricultural settings, finding that surface application in agricultural fields released 39.3 g/ha of visible MPs per runoff event, while subsurface banding and incorporation reduced losses by 90% or more — making application method the key factor in limiting fertilizer-derived plastic pollution.
Polymer-coated urea (PCU) is widely used as a slow-release nitrogen (N) fertilizer because it improves N use efficiency and reduces losses through leaching, denitrification, and volatilization. However, the polymer coating raises concerns about potential microplastic (MP) transport to surface waters following fertilizer application. This thesis evaluates the off-site transport of MPs and associated nutrient and sediment losses from PCU in both urban landscape and agricultural systems. In the urban landscape study, treatments included sod, mulched beds, and water-efficient ground cover receiving uncoated urea, PCU, or liquid slow-release N at 225 kg N ha⁻¹ (with reduced rates for beds and ground cover) compared to an unfertilized control. In the sweet corn agricultural field study, treatments included three fertilizer application methods (incorporated, subsurface band, or surface) with the same fertilizer sources applied at 245 kg N ha⁻¹ and an unfertilized control. Across both studies, runoff water was collected from 1–2% slopes following four simulated flooding events over two years and analyzed for nitrate-N (NO₃– N), sediment, and visible and microscopic MPs. Sediment losses varied by system, with ground cover > mulched beds > sod in urban landscapes, while no significant differences were observed among fertilizer placement methods in the agricultural field. NO₃–N losses were lowest in vegetated ground cover in the urban study and were reduced by slow-release fertilizers compared to conventional urea in the agricultural study. Visible MP transport was minimal in sod and ground cover but significantly higher in mulched beds in the urban system (2.9 g ha⁻¹). In contrast, the agricultural study showed the greatest MP transport from surface-applied PCU (39.3 g ha⁻¹), while subsurface banding and incorporation substantially reduced losses (3.9 and 1.2 g ha⁻¹, respectively). Across both studies, there was minimal evidence of microscopic particles consistent with PCU-derived polymers. Overall, results demonstrate that vegetation and subsurface fertilizer placement are key factors in reducing sediment and polymer-associated MP transport. While PCU provides agronomic benefits in N efficiency, surface exposure, whether through mulch-based landscapes or surface fertilizer application, significantly increases the risk of off-site transport of fertilizer coatings into runoff systems.
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