Controls on Nitrous Oxide Emissions from the Hyporheic Zones of Streams

Abstract

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.

Hyporheic Source and Sink of Nitrous Oxide

“…The primary factors that determine whether a particular flowline will emit N 2 O are the residence time and biological activity level (reflected by K DO ) along that flowline, with the latter being influenced by the distribution and reactivity of organic carbon and denitrifying bacterial communities. Our results support and quantify the conceptual model proposed by Quick et al () that short or long flowlines are unlikely to emit N 2 O. Short flowlines may have not started denitrification, thus will not produce N 2 O but pass through the dissolved concentration from the flow.…”

Residence Time Controls on the Fate of Nitrogen in Flow‐Through Lakebed Sediments

“…This transport limitation on denitrification is likely an important control on the fate of the excess N pollution leaving both ponds through the SWI to the regional aquifer. The potential denitrification rates in these groundwater flow‐through ponds will depend upon NO 3 − and DOC availability as those are prerequisites for the onset of anoxic conditions as seen in Experiment 4 and other freshwater SWI studies of riparian and hyporheic zones in river corridors (Baker et al, 2000; Quick et al, ; Zarnetske et al, ). In the lake SWI, the O 2 removal rates systematically increased with subsequent acetate additions, suggesting increased aerobic processing of DOC that depleted dissolved O 2 (Hedin et al, ).…”

“…It should be noted that for all the experiments except Experiment 5, the rate of O 2 removal was greater than the 1:1 O 2 :C molar ratio predicted by the stoichiometry of aerobic respiration (Findlay & Sobczak, ). This suggests that aerobic respiration was utilizing an alternative organic carbon source, such as POC in the sediments (Quick et al, ; Sawyer, ). This is supported by the decrease in POC mass with depth in the ambient sediment cores (Figure ).…”

“…Of note, total DOC does not accurately reflect carbon limitations, as it is the labile forms of DOC that have been shown in other SWIs to be removed preferentially at early times. This process concentrates more recalcitrant DOC compounds along the flow path (Lansdown et al, ; Quick et al, ; Zarnetske et al, ), increasing the likelihood of DOC limitation, with implications for N reactions at these long residence times. For example, under DOC‐limited conditions and with sufficient NO 3 − , N 2 O accumulates faster than it is reduced to produce N 2 because additional carbon electron donors are needed to complete the second half of the denitrification reaction (Hedin et al, ; Quick et al, ).…”

Probabilistic Forecasting of Nitrogen Dynamics in Hyporheic Zone

“…To be specific, we consider a kinetic system in which DO is consumed because of aerobic respiration, ammonium acts as a source of nitrate during nitrification, while denitrification occurs sequentially as nitrate reduction (i.e., is converted to N 2 O) and N 2 O reduction to N 2 . The overall production of nitrogen gases (N gas ) in the form of both N 2 O and N 2 is regulated by the denitrification kinetics (Marzadri et al, ), while the proportion of produced N 2 O (as opposed to N 2 ) is given by the N 2 O yield, N y (Beaulieu et al, ; Quick et al, ). These biogeochemical processes are microbially mediated by an idealized microbial community uniformly distributed within the hyporheic sediment.…”