Biogeochemical Processes of Dissolved Nitrogen in the Backwater Zone of a Tributary in Three Gorges Reservoir, China: Implications from the Hydrologic Processes and Isotopic Tracing

Since eutrophication occurred frequently in the backwater zone of tributaries in the Three Gorges Reservoir (TGR) during the hydrological management of the dam, biogeochemical processes of dissolved nitrogen and driving factors in the backwater zone are pressed for clarity. In this study, the concentration of dissolved nitrogen species in stratified water sampling from a typical backwater zone of tributary in TGR was determined, and stable isotopes of water were used to quantify the influence of hydrological processes on the dynamics of dissolved nitrogen. Influenced by heavy rainfall before sampling, the average value of total dissolved nitrogen (TDN) in river water was 2.23 +/- 0.48 mg/L, 1.6 times of the seasonal average. Probably due to assimilation and denitrification, the NO3--N concentration was extremely low in stratified water. The dissolved organic nitrogen (DON, 0.84-2.87 mg/L) dominated the composition of dissolved nitrogen species. Combined with H-O isotopes and optimized mathematical models, surface and middle water was recharged by the TGR mainstream within 15 km from the estuary, providing up to 93.5% of the DON concentration, while about 77.3% of DON in bottom water may be contributed from the sediment release. The abnormally increased proportion of DON in TDN may increase the turnover pressure of nitrogen internal circulation and the risk of algal bloom. For another perspective, NH4+-N had high potential health risks to the local residents according to the estimate of hazard quotient. Overall, this study reveals that hydrological processes controlled by the dam have an important impact on the dissolved nitrogen cycle, and long-term monitoring of nitrogen dynamics in this typical river is required.