Impacts of river stage fluctuations on river corridor nitrous oxide source and sink dynamics

Riparian hyporheic zones regulate nitrogen cycling and nitrous oxide (N₂O) emissions. However, the specific influence of periodic river stage fluctuations, particularly dam-induced variations, on these processes driving N₂O source-sink dynamics remains inadequately understood. We developed a coupled flow and reactive transport model to quantify how periodic river stage fluctuations and sediment properties govern riparian N₂O dynamics. Results reveal critical fluctuation amplitudes of 0.5-0.9 m that triggers a shift from net N₂O sink to source, driven by redox oscillations. Higher stage amplitudes enhance nitrate influx but reduce nitrate removal efficiency. Hydraulic conductivity regulates the rate of water and solute transport and thereby enhancing nitrogen transformations and optimizing net N₂O emissions. Elevated longitudinal dispersivity deepens incomplete denitrification and N₂O reduction hotspots by optimizing biogeochemical coupling between nitrate supply and anaerobic consumption. The net accumulation of N₂O is primarily governed by a kinetic decoupling within denitrification, where biogeochemical variables such as high solute concentrations and enzymatic parameters favor early N₂O production over its final reduction. Concurrently, hydrological dynamics exert a critical control, as longer water-level fluctuation periods enhance process rates by extending reaction windows, while losing river conditions promote N₂O production through increased substrate delivery but lead to transport-limited, inefficient removal. These findings suggest that dynamic hydrologic forcing may convert riparian corridors into significant N₂O emission hotspots, warranting control in river management.