Nonlinear Response of Riverine N₂O Fluxes to Oxygen and Temperature

Abstract

One-quarter of anthropogenically produced nitrous oxide (N2O) comes from rivers and estuaries. Countries reporting N2O fluxes from aquatic surfaces under the United Nations Framework Convention on Climate Change typically estimate anthropogenic inorganic nitrogen loading and assume a fraction becomes N2O. However, several studies have not confirmed a linear relationship between dissolved nitrate (NO3-) and river N2O fluxes. We apply recursive partitioning analysis to examine the relationships between N2O flux and NO3-, dissolved oxygen (DO), temperature, land use and surficial geology in the Grand River, Canada, a seventh-order river in an agricultural catchment with substantial urban population. Results suggest that N2O flux is high when hypoxia exists. Temperature, not NO3-, was the primary correlate of N2O flux when hypoxia does not occur suggesting NO3- is not limiting N2O production and further increases in NO3- may not lead to comparable increases in N2O flux. This work indicates that a linear relationship between NO3- and N2O is unlikely to exist in most agricultural and urban impacted river systems. Most N2O is produced during hypoxia so quantifying the extent of hypoxia is a necessary first step to quantifying N2O fluxes in lotic systems. Predicted increases in riverine hypoxia via eutrophication and increased temperature due to climate change may drive nonlinear increases in N2O production.

Role of surface and subsurface processes in scaling N2O emissions along riverine networks

“…Although it is understood that microbially mediated denitrification is responsible for a large proportion of N 2 O production in riverine networks (6, 10, 11), quantifying these emissions is challenging because of a lack of high-resolution field data and inadequate parameterization of the dominant biogeochemical processes responsible for N 2 O production at scales ranging from the individual reach to the river network. In addition to the uncertainty associated with predictions of N 2 O emissions from streams and rivers, recent studies suggest that global emissions from riverine networks presented in the most recent Intergovernmental Panel on Climate Change report are likely underestimated (6,(12)(13)(14). This uncertainty results in part from the dependence of biogeochemical reactions on complex interactions occurring at the reach scale among the water column, benthic, and hyporheic zones.…”

Indirect nitrous oxide emissions from streams within the US Corn Belt scale with stream order

“…Uncertainty in the EF 5r can be attributed to a scarcity of studies (21,22), poorly constrained water-air gaseous exchange relationships (23,24), and high variability in river morphology (25,26). Further, the EF 5r assumes a linear relation between nitrate in water and N 2 O emissions (14), the validity of which is the subject of considerable debate (27)(28)(29)(30). Finally, N 2 O fluxes derived from simple gas exchange models have been shown to underestimate the flux if stream channel hydraulics (i.e., stream flow velocity) are ignored (31), highlighting that stream chemistry alone is not an accurate predictor of N 2 O fluxes.…”

Proper Interpretation of Dissolved Nitrous Oxide Isotopes, Production Pathways, and Emissions Requires a Modelling Approach

“…The Grand River is a seventh-order, 300 km long river that drains 6800 km 2 in southern Ontario, Canada, into Lake Erie, see [36], [37], [51]. There are 30 WWTPs in the catchment and their cumulative impact can be observed via the increase in artificial sweeteners in the river [52].…”