The trade-off effects of water flow velocity on denitrification rates in open channel waterways

Abstract

Ditches and river networks play crucial roles in nitrogen (N) removal within aquatic ecosystems, and their effectiveness depends on various biogeochemical and hydraulic factors. Among these, water flow velocity, as a hydraulic factor, has been relatively underexplored due to the limited N removal quantifying method in flowing open channel waterway. In this study, we used argon as a tracer to calibrate the diffusion coefficient of N2. We then applied the gas diffusion coefficient method to assess how water flow velocity affects denitrification rates. Our results provide new insights into the trade-off role of water flow velocity in N removal through denitrification across a range of flow velocities (0, 1, 4, 6, 10 cm s−1) in open channel waterway. This trade-off effect arises from the interplay between increased NO3− transfer and rising dissolved oxygen levels as water flow velocity increases, resulting in an initially increased and subsequently declined denitrification rate with increased water flow velocities. The denitrification rates reached maximum and minimum values at the water flow velocity of 4 cm s−1 and 0 cm s−1, respectively, regardless of the presence of vegetation. Average denitrification rates ranged from 36.8 to 358.8 µmol N2 m−2h−1 in unvegetated sediments and from 133.0 to 383.5 µmol N2 m−2h−1 in vegetated sediments within the spectrum of water flow velocities. In addition, we observed significantly higher denitrification rates at night than during the day, likely due to the photosynthesis and respiration processes associated with plant metabolism. These findings of this study contribute to the understanding of how water flow velocity affects denitrification rates and provide valuable information for the simulation and restoration efforts aimed at enhancing the N removal capacity of river networks.