N2O dynamics in the hyporheic zone due to ripple migration

Abstract

The migration rate of ripples along the streambed has a profound effect on the hyporheic exchange and nitrogen dynamics. However, how the denitrification intermediate N2O is released in mobile bedforms still remain unclear. In this study, the ripple domains are established based on the sediment stability criteria by using the mean channel flow velocity U and the median sediment grain size D50. A two-dimensional numerical model coupling hyporheic flow, ripple migration and N2O transformation is proposed to investigate how ripple celerity affects N2O dynamics in streambeds. The results show that there are different patterns for the release of N2O from migrating ripples with different medium grain sizes D50 (0.06–0.3 mm). N2O can be released via upwelling of anoxic pore water from the deeper area in immobile or slow-mobile ripples. However, rapid ripple migration and the resulting fast vertical flow reversal can suppress the net pore water displacement and lead to the formation of a redox seal, which separates the deeper anoxic area from the above oxic area and inhibits N2O emissions. The magnitude of the nutrients concentration gradient and the N2O flux between these two areas depend only on the dispersive and diffusive transport. Therefore, the controls of N2O emission flux hence shift from the pumping- dominated regime in stable ripples to a grain size dominated regime of the migrating ripples. The possible implications about the effects of hydrodynamic changes due to the migration of ripples are presented. Our results emphasize that knowledge of N2O production-consumption hotspots within streambeds is essential to understand the mechanisms of N2O emissions from the sediment–water interface.