Unravelling Coupled Hydrological and Geochemical Controls on Long-Term Nitrogen Enrichment in a Large River Basin.

Many groundwater and surface water bodies around the world show a puzzling and often steady increase in nitrogen (N) concentrations, despite a significant decline of agricultural N inputs. This study uses a combination of long-term hydrogeochemical and hydraulic monitoring, molecular characterization of dissolved organic matter (DOM), column experiment, and reactive transport modeling to unravel the processes controlling N-reactive transport and mass budgets under the impacts of dynamic hydrologic conditions at a field site in the central Yangtze River Basin. Our analysis shows that the desorption of ammonium (NH) from sediments via cation exchange reactions dominates N mobilization and aqueous N concentrations, while the mineralization of organic N compounds plays only a minor role. The reactive transport modeling results illustrate the important role of cation exchange reactions that are induced by temporary NH input and cation concentration changes under the impact of both seasonal and long-term hydrologic variations. Historically, cation exchangers have acted as efficient storage devices and mitigated the impacts of high levels of NH input. The NH residing on cation exchanger sites later acts as a long-term N source to waters with the delayed desorption of sediment-bound NH induced by the change of hydrologic conditions. Our results highlight the complex linkages between highly variable hydrologic conditions and NH partitioning in near-surface, river-derived sediments.