Controls on Nitrous Oxide Emissions from the Hyporheic Zones of Streams

The magnitude and mechanisms of nitrous oxide (N₂O) 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 N₂O emissions will be significant. We present observations from column and large-scale flume experiments supporting an integrative model of N₂O 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, N₂O emission from stream sediments requires subsurface residence times (and microbially mediated reduction rates) be sufficiently long (and fast reacting) to produce N₂O by nitrate reduction but also sufficiently short (or slow reacting) to limit N₂O conversion to dinitrogen gas. Most subsurface exchange will not result in N₂O emissions; only specific, intermediate, residence times (reaction rates) will both produce and release N₂O to the stream. We also confirm previous observations that elevated nitrate and declining organic carbon reactivity increase N₂O production, highlighting the importance of associated reaction rates in controlling N₂O accumulation. Combined, these observations help constrain when N₂O release will occur, providing a predictive link between stream geomorphology, hydrodynamics, and N₂O emissions.