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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.