Estuaries as Sources and Sinks of N₂O Across a Land Use Gradient in Subtropical Australia

Intensifying agricultural production and coastal urbanization are increasing nitrogen (N) loads to estuaries, potentially increasing emissions of the greenhouse gas nitrous oxide (N₂O). Here we present a first assessment of how changes in land use intensity affect estuarine N₂O fluxes. We measured N₂O concentrations over marine-freshwater transects in the wet and dry seasons in eight subtropical estuaries selected for differences in land use intensity. Daily estuary N loads ranged from 0.5 ± 0.4 kg N km⁻² d⁻¹ (minimally impacted) to 51 ± 30 kg N km⁻² d⁻¹ (highly impacted), corresponding to higher concentrations of all inorganic N species (nitrate, ammonium, and N₂O) in the highly impacted estuaries. Net N₂O fluxes from the eight estuaries ranged from −20 μg N₂O-N m⁻² d⁻¹ (sink) to +300 μg N₂O-N m⁻² d⁻¹ (source). However, neither N concentrations nor N loads explained the variations in N₂O fluxes. Instead, seasonal differences in freshwater flushing times increased either N₂O uptake (minimally impacted systems) or N₂O efflux (moderately impacted systems) relative to N load. The lack of relationship between freshwater flushing times (kinetics) and N₂O fluxes from the highly impacted estuaries, combined with evidence for both low carbon quality and phosphorous limitation in those systems, suggests that N₂O emissions from highly impacted estuaries were controlled by stoichiometry rather than kinetics. This study shows that estuaries can shift from net sinks to sources of N₂O as land use intensity increases but that the magnitude of this switch cannot be predicted based on N loads alone. (Figure presented.).