Dissimilatory Nitrate/Nitrite Reduction Processes in River Sediments Across Climatic Gradient: Influences of Biogeochemical Controls and Climatic Temperature Regime

Dissimilatory nitrate/nitrite reduction processes play an important role in controlling nitrogen loading in river environments. However, the relative importance of climatic temperature regime and biogeochemical controls to dissimilatory nitrate/nitrite reduction processes remains unclear. We used nitrogen isotope tracer approach to investigate geographical variabilities of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) in river sediments from temperate to tropical climates of China. Denitrification, anammox, and DNRA varied greatly across the climatic gradient, with potential rates of 1.47–25.7, 0.54–3.4, and 0.15–7.17 nmol N g⁻¹ h⁻¹, respectively. Mean measured rates throughout the sampling sites were 9.73 nmol N g⁻¹ h⁻¹ for denitrification, 1.29 nmol N g⁻¹ h⁻¹ for anammox, and 1.61 nmol N g⁻¹ h⁻¹ for DNRA. Denitrification and DNRA rates increased significantly from temperate to tropical climates, while no significantly spatial difference was observed for anammox rates along the climatic gradient. Mean annual temperature, total organic carbon, dissolved organic carbon, pH, NH₄ ⁺, NO₃ ^–, C/N, Fe²⁺, and functional genes were the crucial factors affecting denitrification, anammox, and DNRA. High dissolved organic carbon and NO₃ ^– availability determined nitrogen removal capacity in river sediments. Mean annual temperature was the most important factor explaining the geographical variances of denitrification and DNRA, while the critical predictor of anammox variance was sediment pH along the climatic gradient. Our results highlight that biogeochemical controls and climatic temperature regime are important coregulators affecting the geographical variabilities of dissimilatory nitrate/nitrite reduction processes in river sediments at the continental-scale variation.