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The magnitude and mechanisms of nitrous oxide (NO) release from rivers and streams are actively debated. The complex interactions of hydrodynamic and biogeochemical controls on emissions of this important greenhouse gas preclude prediction of when and where NO emissions will be significant. We present observations from column and large-scale flume experiments supporting an integrative model of NO emissions from stream sediments. Our results show a distinct, replicable, pattern of nitrous oxide generation and consumption dictated by subsurface (hyporheic) residence times and biological nitrogen reduction rates. Within this model, NO emission from stream sediments requires subsurface residence times (and microbially mediated reduction rates) be sufficiently long (and fast reacting) to produce NO by nitrate reduction but also sufficiently short (or slow reacting) to limit NO conversion to dinitrogen gas. Most subsurface exchange will not result in NO emissions; only specific, intermediate, residence times (reaction rates) will both produce and release NO to the stream. We also confirm previous observations that elevated nitrate and declining organic carbon reactivity increase NO production, highlighting the importance of associated reaction rates in controlling NO accumulation. Combined, these observations help constrain when NO release will occur, providing a predictive link between stream geomorphology, hydrodynamics, and NO emissions.

Dissolved oxygen (DO) concentrations and consumption rates are primary indicators of heterotrophic respiration and redox conditions in the hyporheic zone (HZ). Due to the complexity of hyporheic flow and interactions between hyporheic hydraulics and the biogeochemical processes, a detailed, mechanistic, and predictive understanding of the biogeochemical activity in the HZ has not yet been developed. Previous studies of microbial activity in the HZ have treated the metabolic DO consumption rate constant (KDO) as a temporally fixed and spatially homogeneous property that is determined primarily by the concentration of bioavailable carbon. These studies have generally treated bioactivity as temporally steady state, failing to capture the temporal dynamics of a changeable system. We demonstrate that hyporheic hydraulics controls rate constants in a hyporheic system that is relatively abundant in bioavailable carbon, such that KDO is a linear function of the local downwelling flux. We further demonstrate that, for triangular dunes, the downwelling velocities are lognormally distributed, as are the KDO values. By comparing measured and modeled DO profiles, we demonstrate that treating KDO as a function of the downwelling flux yields a significant improvement in the accuracy of predicted DO profiles. Additionally, our results demonstrate the temporal effect of carbon consumption on microbial respiration rates.

Nitrous oxide (N2O) is a potent greenhouse gas with an estimated 10% of anthropogenic N2O coming from the hyporheic zone of streams and rivers. However, difficulty in making accurate fine‐scale field measurements has prevented detailed understanding of the processes of N2O production and emission at the bedform and flowline scales. Using large‐scale, replicated flume experiments that employed high‐density chemical concentration measurements, we have been able to refine the current conceptualization of N2O production, consumption, and emission from the hyporheic zone. We present a predictive model based on a Damköhler‐type transformation (τ̃) in which the hyporheic residence times (τ) along the flowlines are multiplied by the dissolved oxygen consumption rate constants for those flowlines. This model can identify which bedforms have the potential to produce and emit N2O, as well as the portion and location from which those emissions may occur. Our results indicate that flowlines with τ̃up (τ̃ as the flowline returns to the surface flow) values between 0.54 and 4.4 are likely to produce and emit N2O. Flowlines with τ̃up values of less than 0.54 will have the same N2O as the surface water and those with values greater than 4.4 will likely sink N2O (reference conditions: 17C, surface dissolved oxygen 8.5 mg/L). N2O production peaks approximately at τ̃ = 1.8. A cumulative density function of τ̃up values for all flowlines in a bedform (or multiple bedforms) can be used to estimate the portion of flowlines, and in turn the portion of the streambed, with the potential to emit N2O.

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