Fate of dissolved inorganic nitrogen in turbulent rivers: The critical role of dissolved oxygen levels

Dissolved inorganic nitrogen (DIN) is considered the main factor that induces eutrophication in water, and is readily influenced by hydrodynamic activities. In this study, a 4-year field investigation of nitrogen dynamics in a turbulent river was conducted, and a laboratory study was performed in the approximately homogeneous turbulence simulation system to investigate potential mechanisms involved in DIN transformation under turbulence. The field investigation revealed that, contrary to NO(3)(-)dynamics, the NH+4 concentrations in water were lower in flood seasons than in drought seasons. Further laboratory results demonstrated that limitation of dissolved oxygen (DO) caused inactive nitrification and active denitrification in static river sediment. In contrast, the increased DO levels in turbulent river intensified the mineralization of organic nitrogen in sediment; moreover, ammonification and nitrification were activated, while denitrification was first activated and then depressed. Turbulence therefore decreased NH4+ and NO2- concentrations, but increased NO(3 )(-)and total DIN concentrations in the overlying water, causing the total DIN to increase from 0.4 mg/L to maximum of 1.0 and 1.7 mg/L at low and high turbulence, respectively. The DIN was maintained at 0.7 and 1.0 mg/L after the 30-day incubation under low and high turbulence intensities (epsilon) of 3.4 x 10(-4) and 7.4 x 10(-2) m(2)/s(3), respectively. These results highlight the critical role of DO in DIN budgets under hydrodynamic turbulence, and provide new insights into the DIN transport and transformation mechanisms in turbulent rivers.

Automated summary

This study investigates the transformation mechanisms of dissolved inorganic nitrogen (DIN) under hydrodynamic turbulence. Field investigation reveals that NH+4 concentrations are lower in flood seasons compared to drought seasons, while laboratory results demonstrate that limitation of dissolved oxygen (DO) causes inactive nitrification and active denitrification in sediment. Turbulence decreases NH4+ and NO2- concentrations, but increases NO(3 )(-)and total DIN concentrations in the overlying water.