Quantitative response relationships between nitrogen transformation rates and nitrogen functional genes in a tidal flow constructed wetland under C/N ratio constraints

• Single-stage tidal flow CW achieved promising results comparable to hybrid systems.
• C/N ratio showed significant effects on treatment performance and N removal pathways.
• Different N removal processes were coupled at molecular level (functional genes).
• NH4+−NNH4+−N transformation rate was collectively determined by the seven functional genes.
• TN transformation rate was primarily determined by amoA and anammox.

The present study explored treatment performance and nitrogen removal mechanisms of a novel tidal flow constructed wetland (TF CW) under C/N ratios ranging from two to 12. High and stable COD (83–95%), NH4+−NNH4+−N (63–80%), and TN (50–82%) removal efficiency were simultaneously achieved in our single-stage TF CW without costly aeration. Results showed that a C/N ratio exceeding six was required to achieve complete denitrification without NO2−−NNO2−−N and NO3−−NNO3−−N accumulation in the system. Molecular biological analyses revealed aerobic ammonia oxidation was the dominant NH4+−NNH4+−N removal pathway when the C/N ratio was less than or equal to six. However, when the C/N ratio was greater than six, anammox was notably enhanced, resulting in another primary NH4+−NNH4+−N removal pathway, in addition to the aerobic ammonia oxidation. Quantitative response relationships between nitrogen transformation rates and nitrogen functional genes were established, and these relationships confirmed that different nitrogen transformation processes were coupled at the molecular level (functional genes), and collaboratively contributed to nitrogen removal in the TF CW. Specifically, NH4+−NNH4+−N transformation rates were collectively determined by amoA, nxrA, anammox, narG, nirS, nirK, and nosZ; and TN removal was influenced primarily by amoA and anammox.

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