Global riverine nitrous oxide emissions: The role of small streams and large rivers.

Nitrous oxide, N2O, is the leading cause of stratospheric ozone depletion and one of the most potent greenhouse gases (GHG). Its concentration in the atmosphere has been rapidly increasing since the green revolution in the 1950s and 1960s. Riverine systems have been suggested to be an important source of N2O, although their quantitative contribution has been estimated with poor precision, ranging between 32.2 and 2100 GgN2O - N/yr. Here, we quantify reach scale N2O emissions by integrating a data-driven machine learning model with a physically-based upscaling model. The application of this hybrid modeling approach reveals that small streams (those with widths less than 10 m) are the primary sources of riverine N2O emissions to the atmosphere. They contribute nearly 36 GgN2O - N/yr; almost 50% of the entire N2O emissions from riverine systems (72.8 Gg2O - N/yr), although they account for only 13% of the total riverine surface area worldwide. Large rivers (widths wider than 175 m), such as the main stems of the Amazon River (~ 6 GgN2O - N/yr), the Mississippi River (~ 2 GgN2O - N/yr), the Congo River (~ 1 GgN2O - N/yr) and the Yang Tze River (~ 0.7 GgN2O - N/yr), only contribute 26% of global N2O emissions, which primarily originate from their water column. This study identifies, for the first time, near-global N2O emission and NO3 removal hot spots within watersheds and thus can aid the development of local- to global-scale management and mitigation strategies for riverine systems with respect to N2O emissions. The presented framework can be extended to quantified biogeochemical, besides N2O emissions, processes at the global scale.

Power law scaling model predicts N2O emissions along the Upper Mississippi River basin.

Nitrous oxide (N2O) is widely recognized as one of the most important greenhouse gases, and responsible for stratospheric ozone destruction. A significant fraction of N2O emissions to the atmosphere is from rivers. Reliable catchment-scale estimates of these emissions require both high-resolution field data and suitable models able to capture the main processes controlling nitrogen transformation within surface and subsurface riverine environments. Thus, this investigation tests and validates a recently proposed parsimonious and effective model to predict riverine N2O fluxes with measurements taken along the main stem of the Upper Mississippi River (UMR). The model parameterizes N2O emissions by means of two denitrification Damköhler numbers; one accounting for processes occurring within the hyporheic and benthic zones, and the other one within the water column, as a function of river size. Its performance was assessed with several statistical quantitative indexes such as: Absolute Error (AE), Nash-Sutcliffe efficiency (NSE), percent bias (PBIAS), and ratio of the root mean square error to the standard deviation of measured data (RSR). Comparison of predicted N2O gradients between water and air (ΔN2O) with those quantified from field measurements validates the predictive performance of the model and allow extending previous findings to large river networks including highly regulated rivers with cascade reservoirs and locks. Results show the major role played by the water column processes in contributing to N2O emissions in large rivers. Consequently, N2O productions along the UMR, characterized by regulated flows and large channel size, occur chiefly within this surficial riverine compartment, where the suspended particles may create anoxic microsites, which favor denitrification.

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

Riverine environments, such as streams and rivers, have been reported as sources of the potent greenhouse gas nitrous oxide ([Formula: see text]) to the atmosphere mainly via microbially mediated denitrification. Our limited understanding of the relative roles of the near-surface streambed sediment (hyporheic zone), benthic, and water column zones in controlling [Formula: see text] production precludes predictions of [Formula: see text] emissions along riverine networks. Here, we analyze [Formula: see text] emissions from streams and rivers worldwide of different sizes, morphology, land cover, biomes, and climatic conditions. We show that the primary source of [Formula: see text] emissions varies with stream and river size and shifts from the hyporheic-benthic zone in headwater streams to the benthic-water column zone in rivers. This analysis reveals that [Formula: see text] production is bounded between two [Formula: see text] emission potentials: the upper [Formula: see text] emission potential results from production within the benthic-hyporheic zone, and the lower [Formula: see text] emission potential reflects the production within the benthic-water column zone. By understanding the scaling nature of [Formula: see text] production along riverine networks, our framework facilitates predictions of riverine [Formula: see text] emissions globally using widely accessible chemical and hydromorphological datasets and thus, quantifies the effect of human activity and natural processes on [Formula: see text] production.