Mechanistic study on C/N ratio-driven nitrogen transformation pathways and microbial community dynamics in constructed wetland-microbial fuel cell systems

Abstract

The influent carbon-to-nitrogen (C/N) ratio serves as a key parameter that affects pollutant removal and electricity generation in constructed wetland-microbial fuel cell (CW-MFC) systems. This study built a CW-MFC system and set six different influent C/N ratio gradients (2, 3, 5, 7, 11, 15). It systematically examined how the C/N ratio influences pollutant removal efficiency, electrochemical performance, and microbial community structure. Results showed that chemical oxygen demand (COD) removal rate increased with higher C/N ratios and reached 90.06% at C/N = 15. Total nitrogen (TN) removal rate showed nonlinear changes and peaked at 75.27% when C/N = 3. Ammonium nitrogen (NH₄⁺-N) and total phosphorus (TP) removal rates achieved optimal values at C/N = 5 (86.19%) and C/N = 7 (89.44%), respectively. Nitrogen speciation analysis indicated that short-cut nitrification–denitrification occurred more readily at C/N = 5 and 7, with higher nitrite nitrogen (NO₂⁻-N) accumulation. For electrochemical performance, output voltage and power density rose with increasing C/N ratios, but coulombic efficiency dropped sharply to 0.25% at C/N = 15. This drop suggests that excessive carbon sources reduced electron transfer efficiency. Microbial community analysis revealed Proteobacteria as the dominant phylum. Abundances of functional genera like Geobacter and Denitratisoma shifted significantly with C/N ratio changes. These shifts drove alterations in nitrogen transformation pathways and electrochemical behaviors. The C/N ratio regulated system performance through a three-tier cascade involving redox environment shifts, electron flux repartitioning, and microbial community succession. The study demonstrates that moderate C/N ratios (3–7) can synergistically optimize nitrogen removal, electricity generation, and microbial functions. It provides a theoretical basis for operational regulation in CW-MFC systems.