Spatial distribution of potential nitrogen reduction rates and associated microbial communities revealed by metagenomic analysis in Yangtze River sediments

Understanding the intricacies of nitrogen reduction processes and the composition of associated microbial communities is crucial for illuminating the reactions of ecosystems and their functions to persistent nitrogen inputs. To enhance research on the nitrogen reduction process, we determined the potential rates, quantified the relevant genes, and analyzed the macro factors in the sediments of the Yangtze River. The results showed that dissimilatory reduction of nitrate to ammonium (DNRA) dominated the N-reduction processes in the Yangtze River sediment, with average rates of 0.89 ± 0.71 nmol N g^-1 h^-1. Meanwhile, denitrification and anammox rates were 0.73 ± 0.74 and 0.07 ± 0.07 nmol N g^-1 h^-1, respectively. The Three Gorges Dam (TGD) caused higher potential rates (nmol N g^-1 h^-1) of denitrification (1.38), anammox (0.12), DNRA (1.48), and N₂O depletion (1.49 nmol g^-1 h^-1) in the Three Gorges Reservoir (TGR) compared to other river reaches. The average copy numbers (copies·g^-1) of nrfA (2.96 × 10⁶), narG (8.17 × 10⁵), nirS (6.10 × 10⁶), nosZ (2.77 × 10⁶), and hzsB (3.68 × 10⁵) in TGR sediments were higher than those in the other reaches. The TGD's interception of fine sediments and nutrients enhanced microbial gene abundance, thereby favoring N-reduction processes and resulting in N₂O depletion in reservoir sediments. Moreover, the TGD caused a decreased contribution gap between DNRA and denitrification in the TGR (2%) compared with the upper (35%) and lower (18%) reaches, while causing predominant anammox (50%) in the middle reach. Metagenomic results suggested that sediment particle size, along with organic carbon and inorganic nitrogen concentrations, influenced N reduction rates by affecting narG, norB and C, nrfA and H, and hzsB and C. This study reveals the spatial pattern of the N-reduction rate in the Yangtze River sediments and quantitatively defines the intensity of dam effects on sediment N-reduction rate.