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Tire wear particles induce a functional trade-off in bioretention systems: Coupled effects on nitrogen removal and greenhouse gas emissions
Summary
Tiny bits of tire rubber that wash into roadside rain gardens (a common green infrastructure used to filter stormwater) can throw off the delicate microbial balance that normally removes nitrogen pollution from the water, making these systems less effective at cleaning runoff before it reaches rivers and streams. Interestingly, the same rubber particles also reduced greenhouse gas emissions from these systems, showing there's a trade-off — you can't fully win on water quality and climate benefits at the same time. As tire particles keep building up in urban environments, this research suggests we may need to redesign these green infrastructure systems to ke
Bioretention systems are widely adopted nature-based solutions (NbS) for mitigating urban stormwater pollution, yet their long-term functional stability is increasingly undermined by the accumulation of tire wear particles (TWPs). This study moves beyond descriptive performance assessment to investigate the underlying biogeochemical mechanisms governing the response of bioretention media to dynamic TWP stress (0, 1, 10, and 100 mgL). Our results reveal a sophisticated functional trade-off induced by TWPs: although nitrogen removal efficiency was significantly compromised-with NH-N and total nitrogen (TN) removal rates decreasing by up to 14.64% and 11.02%, respectively-the system's net global warming potential was concurrently mitigated, achieving a 7.41-61.26% reduction in CO-equivalent emissions. Mechanistically, partial least squares path modeling (PLS-PM) identified that TWPs trigger a metabolic bottleneck in the nitrogen cycle. The accumulation of TWPs significantly inhibited hydroxylamine oxidoreductase (HAO) and hydroxylamine reductase (HyR) activities (path coefficient: -0.742, p < 0.01) and suppressed the abundance of nitrifying bacteria (e.g., Nitrospira). Crucially, metagenomic insights demonstrated that TWPs redirected the nitrogen flux by upregulating the nrfA gene, thereby facilitating dissimilatory nitrate reduction to ammonium (DNRA) as an alternative pathway. These findings demonstrate that high TWP levels impair nitrogen removal through multi-interface synergy, highlighting potential ecological risks. This study provides a novel predictive framework for managing emerging microplastic pollutants in green infrastructure, offering actionable insights for optimizing the multifunctional ecosystem services of urban NbS under anthropogenic stress.