Global distribution of nitrous oxide production and N inputs in freshwater and coastal marine ecosystems

Abstract

This study examines N 2 O emissions from aquatic environments globally, particularly as they are affected by anthropogenic activity. The global distribution of N 2 O production in rivers and estuaries was modeled as a function of nitrification and denitrification rates, which were related to external nitrogen (N) inputs. N loading rates were estimated as a function of environmental parameters in the watersheds using two existing models that we adapted for global databases. Model estimated export of dissolved inorganic nitrogen (DIN) by world rivers to estuaries in 1990 is 20.8 Tg N yr−1; approximately 75% is estimated to be anthropogenic. DIN export to the Atlantic and Indian Oceans is similar (5.4 Tg N yr−1 and 4.6 Tg N yr−1, respectively); inputs to the Pacific are approximately 50% greater. China and southeast Asia account for over 50% of DIN export by world rivers. Globally, anthropogenic DIN export is predominately attributed to fertilizer N, followed by sewage and atmospheric deposition. About 8% of the total N inputs to the terrestrial environment can be accounted for as DIN export by rivers. Worldwide N 2 O emissions from rivers (55%), estuaries (11%), and continental shelves (33%) are calculated to be 1.9 Tg N yr−1. For rivers and estuaries, approximately 90% of N 2 O emissions are in the northern hemisphere in line with the regional distribution of DIN export by rivers. China and India account for about 50% of N 2 O emissions from rivers and estuaries. About 1% of the N input from fertilizers, atmospheric deposition, and sewage to watersheds is lost as N 2 O in rivers and estuaries. Globally, rivers and estuaries could account for approximately 20% of the current global anthropogenic N 2 O emissions and are similar in magnitude to a number of previously identified sources including direct emissions of N 2 O from soils induced by anthropogenic N inputs.