Controls on Nitrous Oxide Emissions from the Hyporheic Zones of Streams

The magnitude and mechanisms of nitrous oxide (N 2O) 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 N 2O emissions will be significant. We present observations from column and large-scale flume experiments supporting an integrative model of N 2O 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, N 2O emission from stream sediments requires subsurface residence times (and microbially mediated reduction rates) be sufficiently long (and fast reacting) to produce N 2O by nitrate reduction but also sufficiently short (or slow reacting) to limit N 2O conversion to dinitrogen gas. Most subsurface exchange will not result in N 2O emissions; only specific, intermediate, residence times (reaction rates) will both produce and release N 2O to the stream. We also confirm previous observations that elevated nitrate and declining organic carbon reactivity increase N 2O production, highlighting the importance of associated reaction rates in controlling N 2O accumulation. Combined, these observations help constrain when N 2O release will occur, providing a predictive link between stream geomorphology, hydrodynamics, and N 2O emissions.