Impacts of microplastics on the nitrogen transformation in hyporheic zones with different grain sizes.

Hyporheic zones (HZs) play a pivotal role in nitrogen transformation and serve as reservoirs for microplastics (MPs) in rivers. While the effects of MPs on nitrogen transformation have gained wide attention, their general patterns and mechanisms remain poorly understood in natural environments. Here, we compared the effects of non-biodegradable polyvinyl chloride (PVC) and biodegradable polylactic acid (PLA) MPs on inorganic nitrogen transformation in simulated HZs with sediments of different grain sizes under water exchange. The results showed that exposure to PVC MPs had little effect on inorganic nitrogen in the HZs with mixed sediments, whereas PLA markedly decreased nitrate. Within the PLA treatment, fine-grained sediment decreased nitrate by 84.6% and increased nitrite by 90.7% compared with the mixed-grain condition, accompanied by oxygen-deficient conditions. Microbial community composition and nitrogen-related functional responses also differed across sediment grain size conditions. In the PVC treatment, fine-grained sediment enhanced AMO, NAR, and NIR activities and showed higher abundances of nitrification- and denitrification-related functional genes. A similar grain size response was observed in the PLA treatment. Hence, PVC and PLA produced distinct nitrogen transformation responses across sediment grain size conditions, accompanied by differences in oxygen dynamics, microbial community composition, enzymatic activities, and functional gene abundances.