Hyporheic Source and Sink of Nitrous Oxide

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. Plain Language Summary Nitrous oxide (N2O) is a potent greenhouse gas that has been increasing its atmospheric warming impact. Globally, an estimated 10% of the N2O that is released to the atmosphere comes from rivers and streams. These emissions are strongly correlated to nitrogen compounds from industrial and agricultural runoff. However, not all streams that are impacted emit N2O. Clearly, streams have some control over the chemical activity that occurs within their banks. However, the large‐scale studies that have provided the global estimates of N2O emissions have not been able to pinpoint the mechanisms that control emissions or where within a steam emissions originate. Most of the chemical activity in rivers and streams occurs within the sediments directly adjacent to the stream flow. This volume of saturated sediments is called the hyporheic zone. Surface water flows into the hyp