Quantitative characterization and genetic diversity associated with N-cycle pathways in urban rivers with different...

The nitrate reduction contributions of denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) remain largely unknown especially in the context of river remediation. In this research, the quantitative differentiation of these three nitrate-reduction processes with different remediation conditions was done by the joint use of microbial analysis and nitrogen isotope-tracing. The experiments were done in simulated river systems with 100-day operations. The results of isotope-tracing showed that the respective N-removal contribution of denitrification was 85.88%-92.46% and 83.49%-84.73% in urban river with aeration and addition of Ca(NO₃)₂, whereas anammox became the same important (contribution of 49.35%-57.85%) with denitrification for nitrogen removal at a high C/N (Chemical oxygen demand/total nitrogen) ratio of 20. Besides, DNRA only occurred at a C/N ratio of 10 with high-level ammonium accumulation (11.20 ± 0.61 mg/L). Microbial analyses indicated that Ca(NO₃)₂ injection could promote not only the relative abundance of Proteobacteria (from 47.66% to 59.52%) but also the abundance of hzsB (from (4.66 ± 0.40) × 10⁴ copies·g^-1 to (2.66 ± 0.12) × 10⁵ copies·g^-1). Moreover, Ca(NO₃)₂ injection showed significantly positive correlation with Candidatus Jettenia of hzsB and Thiobacillus of all the denitrification functional genes including narG, norB, nosZ and nirS. The C/N ratio showed significantly positive correlation with Azoarcus of nirS (r = 0.941, p < 0.01) and Alloactinosynnema of hzsB (r = 0.941, p < 0.01). It was worth noting that Thiobacillus dominated in N-transformation processes, which underlined the need for the coupling of N transformation with other elements such as sulfur for better understanding and manipulating N cycling in urban rivers.