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Microplastics can Alter the Nitrogen Cycle in Surface Flow Constructed Wetlands
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Both conventional PVC and biodegradable PLA microplastics disrupted nitrogen cycling in constructed wetlands, with PVC causing ammonium accumulation and PLA acting as a carbon source, while both enhanced biofilm formation and enzyme activities at higher concentrations. These polymer-specific effects on nitrogen dynamics reveal that microplastic contamination poses distinct and complex threats to the water treatment functions of natural and engineered wetlands.
The accumulation of microplastics (MPs) in wetlands poses threats to aquatic ecosystems. However, the impacts of MPs on the nitrogen cycle in surface flow constructed wetlands (SFCWs) remain unclear. This study investigated the impacts of polyvinyl chloride (PVC, conventional) and polylactic acid (PLA, biodegradable) MPs at concentrations of 0%, 0.1%, 1.0%, and 5.0% on nitrogen dynamics and key nitrogen cycle enzyme activities in SFCWs. Results showed that MPs altered nitrogen concentrations by decreasing total nitrogen (TN) in the overlying water while increasing organic nitrogen (Norg). Notably, PVC MPs caused ammonium nitrogen (NH4+-N) accumulation due to hindered oxygen diffusion, while PLA MPs decreased it by serving as carbon sources. Additionally, MPs promoted microbial colonization and increased biofilm biomass in sediments. Oxygen penetration depth decreased with increasing MP concentrations. The enzymatic analysis further demonstrated that MPs significantly enhanced the activities of key nitrogen cycle enzymes, including ammonia monooxygenase (AMO), hydroxylamine oxidoreductase (HAO), nitrate reductase (NR), nitrite reductase (NIR), and hydrazine synthase (HZS), particularly at higher concentrations. Overall, our findings fill the gap in understanding polymer-specific MP impacts on wetland nitrogen cycling and provide a scientific basis for mitigating ecological risks and safeguarding wetland functional stability.
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A study of conventional PVC and biodegradable PLA microplastics in constructed wetland sediments found both types altered microbial diversity and community composition, with PVC enriching denitrifying bacteria and PLA promoting methanogenic pathways at concentrations as low as 1%. These microbial shifts in wetland sediments could disrupt nitrogen and carbon cycling in ecosystems that serve as critical water quality filters.
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Experiments in free water surface constructed wetlands showed that higher operating water levels significantly improved microplastic removal efficiency — up to 70% in summer — by promoting particle settling and reducing resuspension of larger fragments. Optimizing water management in constructed wetlands offers a cost-effective, ecologically integrated approach to reducing microplastic discharge from treated wastewater into downstream aquatic environments.
Effects of microplastics on carbon and nitrogen cycling in coastal wetlands: A critical meta-analysis
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This meta-analysis of over 1,000 data points from 34 studies found that microplastics significantly altered certain carbon and nitrogen cycling indicators in coastal wetlands under laboratory conditions, but tidal simulation experiments more representative of real environments showed no significant effects. The findings caution against over-interpreting lab results and highlight that tidal dynamics and vegetation factors are often missing from experimental designs.
Microplastics in Wetland Ecosystem: A Complex Nexus and Way Forward
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This review examines the distribution, fate, and ecological impacts of microplastics in wetland ecosystems, which serve as both sinks and potential secondary sources of microplastic contamination. Wetlands filter water supplies and support biodiversity, so microplastic accumulation there directly threatens aquatic food webs, drinking water quality, and the communities that depend on these ecosystems.
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