Unravelling Coupled\nHydrological and Geochemical Controls\non Long-Term Nitrogen Enrichment in a Large River Basin

Many groundwater and surface water bodies around the\nworld show\na puzzling and often steady increase in nitrogen (N) concentrations,\ndespite a significant decline of agricultural N inputs. This study\nuses a combination of long-term hydrogeochemical and hydraulic monitoring,\nmolecular characterization of dissolved organic matter (DOM), column\nexperiment, and reactive transport modeling to unravel the processes\ncontrolling N-reactive transport and mass budgets under the impacts\nof dynamic hydrologic conditions at a field site in the central Yangtze\nRiver Basin. Our analysis shows that the desorption of ammonium (NH<sub>4</sub><sup>+</sup>) from sediments via cation exchange reactions\ndominates N mobilization and aqueous N concentrations, while the mineralization\nof organic N compounds plays only a minor role. The reactive transport\nmodeling results illustrate the important role of cation exchange\nreactions that are induced by temporary NH<sub>4</sub><sup>+</sup> input and cation concentration changes under the impact of both\nseasonal and long-term hydrologic variations. Historically, cation\nexchangers have acted as efficient storage devices and mitigated the\nimpacts of high levels of NH<sub>4</sub><sup>+</sup> input. The NH<sub>4</sub><sup>+</sup> residing on cation exchanger sites later acts\nas a long-term N source to waters with the delayed desorption of sediment-bound\nNH<sub>4</sub><sup>+</sup> induced by the change of hydrologic conditions.\nOur results highlight the complex linkages between highly variable\nhydrologic conditions and NH<sub>4</sub><sup>+</sup> partitioning\nin near-surface, river-derived sediments.