Distribution patterns of nitrogen micro-cycle functional genes and their quantitative coupling relationships with nitrogen transformation rates in a biotrickling filter

•Distribution patterns of N functional genes were quantitatively assessed.•N functional genes enriched in different depth gradients of the biofilter.•DNRA coupled with nitrification and denitrification was a pathway for N removal.•Nas showed a negative relationship with NH4+ removal and NO3− accumulation.

The present study explored the distribution patterns of nitrogen micro-cycle genes and the underlying mechanisms responsible for nitrogen transformation at the molecular level (genes) in a biotrickling filter (biofilter). The biofilter achieved high removal efficiencies for ammonium (NH4+-N) (80–94%), whereas nitrate accumulated at different levels under a progressive NH4+-N load. Combined analyses revealed the anammox, nas, napA, narG, nirS, and nxrA genes were the dominant enriched genes in different treatment layers. The presence of simultaneous nitrification, ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) were the primary factors accounted for the robust NH4+-N treatment performance. The presence of DNRA, nitrification, and denitrification was determined to be a pivotal pathway that contributed to the nitrate accumulation in the biofilter. The enrichment of functional genes at different depth gradients and the multi-path coupled cooperation at the functional gene level are conducive to achieving complete nitrogen removal.

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