Alessandra Marzadri, Daniele Tonina, Alberto Bellin

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. Here, we test and validate a recently proposed parsimonious, yet effective, model to predict riverine N2O fluxes along the main stem of the Upper Mississippi River (UMR). The model parameterize 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. 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, we infer that along the UMR, characterized by regulated flows and large channel size, N2O production occurs chiefly within this surficial riverine compartment, where the suspended particles may create anoxic microsites, which favor denitrification.