The role of river ecosystems in contributing to N2O emissions under climate change: a modeling approach

Recent research has revealed the key role played by riverine ecosystems in contributing to climate change through the emissions of three important greenhouse gases (GHG): carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Among them, the quantification of riverine N2O emissions is critical because its warming capability is 300 times higher than that of CO2, making it the leading ozone depleting gas at present. Riverine N2O emissions depend on the complex interplay between the dissolved inorganic nitrogen (DIN) inputs to the river system and the nitrification-denitrification processes occurring along hydrologic pathways connecting surficial (i.e., water column and benthic zone) and sub-surficial (i.e., hyporheic zone) compartments of the fluvial system. These processes, and the strength of these hydro-biogeochemical interactions among these different compartments, are also influenced by extreme and prolonged low flow events (i.e., drought conditions), whose frequency and intensity are exacerbated by climate change. To investigate which riverine compartments contributed the most to N2O emissions, and under which hydrological conditions, we applied a process-based model to two watersheds with contrasting land use and land cover. The model allows scaling of local processes to the watershed scale. Results to date demonstrate how N2O emissions can change moving along the different reaches that compose a river network (i.e., from headwaters to mainstem) according to the relevant forcing (i.e., climate, hydrology, river morphology, biogeochemistry, land use, drought severity, etc.). In combination, these dynamic factors shape N2O emission signatures from river ecosystems over space and time.