Nitrous oxide emission from North America based on bottom-up and top-down approaches: trends, drivers, and comparison
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Nitrous oxide (N2O) is the third-most-important long-lived greenhouse gases and its atmospheric concentration has increased by ~22% compared to the level in the preindustrial era. Anthropogenic activities, especially nitrogen addition to agricultural soils, are the major sources excess N2O to the atmosphere. Here, we synthesize the North American N2O emissions based on bottom-up (BU: Terrestrial Biosphere Models, emission inventories, and statistical models) and top-down (TD: three inversion frameworks) approaches over the period of 1980-2016. The regional land N2O emissions estimated by BU is ~1.7 (0.9-3.0) Tg N2O-N yr-1, which is ~0.2 Tg N2O-N yr-1 higher than that by TD with a much smaller uncertainty range (1.1-1.5 Tg N2O-N yr-1) over 2007-2016. Natural soil emission (~0.6 Tg N2O-N yr-1) averagely accounts for 37% of regional total amount and shows no changing trend. Compared to the 1980s, the anthropogenic emission has increased by 17%, in which agricultural soils play the most important role associated with large additions of N fertilizer, typically in the United States and Canada. As a result, indirect effects (i.e., N leaching and runoff) of upstream N addition has increased with an average of ~0.1 Tg N2O-N yr-1 over the recent decade. By contrast, fossil fuel and industry N2O emissions show a continuous decrease since the 1990s despite of ~59% increases in Mexican industry emissions based on the Emission Database for Global Atmospheric Research (EDGAR). Nitrous oxide emissions due to atmospheric N deposition has not changed, nor emissions from the effects of environmental changes (i.e., climate, elevated carbon dioxide, land use and land cover change) during 1980-2016. This assessment with the most recent estimates indicates that a mitigation strategy is essential to reduce North American N2O emissions from agriculture and its indirect effects.