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

Nitrous oxide, NO, 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 NO, although their quantitative contribution has been estimated with poor precision, ranging between 32.2 and 2100 GgNO - N/yr. Here, we quantify reach scale NO 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 NO emissions to the atmosphere. They contribute nearly 36 GgNO - N/yr; almost 50% of the entire NO emissions from riverine systems (72.8 GgO - 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 GgNO - N/yr), the Mississippi River (~ 2 GgNO - N/yr), the Congo River (~ 1 GgNO - N/yr) and the Yang Tze River (~ 0.7 GgNO - N/yr), only contribute 26% of global NO emissions, which primarily originate from their water column. This study identifies, for the first time, near-global NO emission and NO 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 NO emissions. The presented framework can be extended to quantified biogeochemical, besides NO emissions, processes at the global scale.