Estuaries as Sources and Sinks of N2O Across a Land Use Gradient in Subtropical Australia. Issue 5 (29th May 2018)

• Record Type: Journal Article
• Authors: Wells, Naomi S.; Maher, Damien T.; Erler, Dirk V.; Hipsey, Matthew; Rosentreter, Judith A.; Eyre, Bradley D.
• Is Part Of: Global biogeochemical cycles. Volume 32:Issue 5(2018:May)
• Page Start: 877
• Page End: 894
• Publication Date: 2018-05-29
• Subjects: Greenhouse gasses -- stoichiometry -- land‐use -- reactive nitrogen -- sub‐tropics -- residence time; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2017GB005826
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Intensifying agricultural production and coastal urbanization are increasing nitrogen (N) loads to estuaries, potentially increasing emissions of the greenhouse gas nitrous oxide (N2 O). Here we present a first assessment of how changes in land use intensity affect estuarine N2 O fluxes. We measured N2 O 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 N2 O) in the highly impacted estuaries. Net N2 O fluxes from the eight estuaries ranged from −20 μg N2 O‐N m −2 d −1 (sink) to +300 μg N2 O‐N m −2 d −1 (source). However, neither N concentrations nor N loads explained the variations in N2 O fluxes. Instead, seasonal differences in freshwater flushing times increased either N2 O uptake (minimally impacted systems) or N2 O efflux (moderately impacted systems) relative to N load. The lack of relationship between freshwater flushing times (kinetics) and N2 O fluxes from the highly impacted estuaries, combined with evidence for both low carbon quality and phosphorous limitation in those systems, suggests that N2 O 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 N2 O as land use intensity increases but that the magnitude of this switch cannot be predicted based on N loads alone. Key Points: Estuaries switch from sinks to sources of the greenhouse gas nitrous oxide as land use intensity (degradation) increases Water residence time determines the magnitude of either N2 O uptake (undisturbed systems) or N2 O efflux (degraded systems) Hydrologic control of nitrous oxide fluxes is superseded by stoichiometric limitations in highly degraded estuaries