Multi-element coupling effect of nitrogen cycling in an intensively dam-controlled river system

Inland aquatic systems play a crucial role in the global nitrogen (N) cycle. This study focused on the Shaying River Basin, which is characterized by intensive damming, base-flow deficiencies and high N loading. Metagenomics sequencing and molecular ecological network analysis were used to conduct a comparative analysis of N cycling and its coupling effects with carbon (C) and sulfur (S) cycling in different stagnant habitats. Our major findings are listed below. (1) Compared with free-flowing habitats, overlying water in stagnant habitats (reservoirs and sluices) had higher abundances of mineralization and organic nitrogen synthesis genes, and lower abundances of denitrification and nitrification genes. These results indicate that damming enhances the conversion between organic and inorganic N but weakens inorganic N removal. The superposition of high N concentrations also led to inhibition of N removal. (2) The topology of molecular ecological networks showed differentiated coupling effects between cycling of N with C or S. Specifically, methane metabolism in reservoirs with low N concentrations promoted N removal, and the co-occurrence between N and S cycling enhanced the simultaneous removal of N and S. Conversely, co-exclusion between N with C or S cycling functional groups hindered nitrification and denitrification in sluices with high N concentrations. (3) For sediment, a high N concentration enhanced the potential of mineralization, nitrification, denitrification, and anaerobic ammonium oxidation to facilitate inorganic N removal. The co-exclusion between C and N cycling consistently inhibited denitrification, anaerobic ammonium oxidation, and N fixation in different habitats. Co-occurrence of organic S transformation, S mineralization, S reduction genes and N mineralization, organic N synthesis, and denitrification genes promoted N removal. In conclusion, high N concentrations of overlying water and dam-induced stagnation impeded N removal, while the coupling of N with C or S cycling showed different effects on N removal in different stagnant habitats.