Mitigating greenhouse gas emissions in constructed wetlands under intermittent aeration: insights from solid-phase biological carbon source utilization.

Artificial aeration has been considered to be the most effective method to enhance the nitrogen removal efficiency of constructed wetlands (CWs) under low carbon-nitrogen (C/N) ratio conditions. However, it is worth noting that enhancing nitrogen removal in intermittently aerated CWs often exacerbates greenhouse gas (GHG) emissions making the wetlands become a carbon source. The synergistic effect of using a biodegradable polymer, poly (butylene succinate) (PBS), as a solid carbon source to address this challenge remains obscure. This study established three vertical flow CWs: a non-aerated control (CWB), an intermittently aerated system (CWA), and an intermittently aerated system amended with PBS (CWP). Results demonstrated that CWP achieved superior total nitrogen removal efficiency (94.28 ± 4.14%) without secondary COD pollution. Concurrently, CWP drastically mitigated GHG emissions, reducing the total global warming potential by 64.9% and 70.8% compared to CWA and CWB, respectively, primarily by substantially reducing nitrous oxide emissions. Mechanistically, PBS addition significantly enriched key denitrification genes, elevating the nosZ/nirS + nirK ratio by over 38%. This fostered a beneficial microbial community distinct from the controls, enriching for efficient heterotrophic nitrification-aerobic denitrification genera such as Acidovorax and Propionivibrio. Furthermore, metabolic pathway analysis revealed that CWP not only promoted complete denitrification but also coordinately upregulated the entire aerobic methane oxidation pathway, enhancing the processing of formaldehyde. This study demonstrates that integrating PBS into intermittently aerated CWs is an effective strategy to simultaneously achieve high-efficiency nutrient removal and profound GHG mitigation for low C/N wastewater.