Land use drives nitrous oxide dynamics in estuaries on regional and global scales

Urban and agricultural development of coastal catchments is known to increase dissolved nitrogen inputs into estuaries; however, much less is known about how land use influences the production of the powerful greenhouse gas nitrous oxide (N2O). Here, we assess dissolved N2O dynamics in four nearby estuaries across a regional land use gradient and summarize the literature to put the observations into global perspective. During summer dry conditions, N2O saturation ranged from 131.4% +/- 45.0% in the most pristine system (28% modified) to 198.6% +/- 52.3% within the most modified urban system (91% modified). The N2O saturation in the wetter winter campaign was higher and more variable than the summer dry campaign (range 84.7-677.7%) likely due to direct transport of N2O into the estuaries from catchment runoff and/or produced through denitrification fueled by high nitrate inputs. During both seasons, N2O was lowest in areas adjacent to fringing mangroves and highest in upstream fresh/saltwater mixing areas. Coupling our results with previously published N2O data from 50 estuarine systems worldwide revealed that estuarine N2O increases concomitantly with catchment modification, dissolved inorganic nitrogen availability, and decreasing oxygen concentrations. Based on these results, a 1% increase in anthropogenic modification to global catchments (i.e., agricultural development and/or urbanization) may increase estuarine N2O saturation by 2.6% +/- 1.2%. These findings indicate that future estuarine N2O emissions are likely to increase as anthropogenic modification of coastal catchments intensifies.