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.