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Effects of microplastics on inorganic nitrogen dynamics in surface water sediments under different disturbance intensities
Summary
Laboratory experiments showed that microplastics in sediments alter nitrogen cycling in freshwater systems in ways that depend strongly on concentration: low levels boosted ammonium release, while high levels suppressed it and amplified nitrate consumption. These disruptions to the nitrogen cycle could affect water quality and aquatic productivity, especially in systems that are frequently disturbed by dredging or flooding.
Microplastics (MPs) increasingly accumulate in sediments, and frequent anthropogenic disturbances (e.g., dredging) in freshwater systems make it essential to investigate their influence on inorganic nitrogen dynamics under disturbed conditions. Disturbance simulation experiments were conducted to investigate how MPs affect nitrogen migration and transformation in aquatic environments. At low MP concentrations (1 %), MPs enhanced ammonium (NH) desorption at the surface by 92 %. In contrast, higher concentrations (5 %) impeded sediment upward migration and reduced NH desorption by 64 %. MPs also showed a dose-dependent effect on NO consumption, with 5 % MPs increasing it by 104 % at the surface and by 21 % at the sediment-water interface. The consumed NO is largely converted to NH via dissimilatory NO reduction to NH (DNRA), with MPs influencing this process by altering sediment resuspension and redox conditions. NH variation across MP treatments diminished as disturbance intensity (DI) increased. Three-factor analysis of variance (ANOVA) confirmed that DI was the dominant driver of NH variation (p = 0.010) rather than MPs.
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