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

Abstract

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.

Seasonal and spatial controls on N2O concentrations and emissions in low-nitrogen estuaries: Evidence from three tropical systems

“…While the overall area-weighted flux was positive, negative water-air fluxes (as low as -2.2 µmol m -2 d -1 ) were observed in the middle section of the transect (Table 485 2). 2018; Wells et al, 2018), a salt marsh (Daniel et al, 2013), and some monsoonal estuaries in India (Rao and Sarma, 2013). However, N 2 O undersaturation is not observed in all pristine estuaries.…”

“…Figure 11: (a) Average annual NO -3 concentration (µmol L -1 ) vs N 2 O water-air flux (µmol m -2 day -1 ; modified from (Murray et al, 2015). (b) Data from the Johnstone River Estuary, Constant Creek, and the Fitzroy River Estuary, plotted against data presented in (Murray et al, 2015) along with results from several mangrove creeks (Barnes et al, 2006;Maher et al, 2016;Murray et al, 2018) and southeast Queensland estuaries (Wells et al, 2018). Australian estuary channel (Wells et al, 2018)…”

“…Denitrification occurs in anoxic environments, such as anoxic layers of soil or sediment (Davidson and Swank, 1986;Firestone et al, 1979;Thomas et al, 1994), while nitrification occurs in both sediment (Jenkins and Kemp, 1984;Usui et al, 2001) and the water column (Damashek et al, 2016;Iriarte et al, 1998;Somville, 1984), where O 2 is available. 25 High concentrations of nitrate (NO -3 ) in estuarine waters are linked to increased N 2 O water-air emissions (Murray et al, 2015), although this may be attenuated by the estuarine flushing time (Wells et al, 2018). High NH + 4 concentrations are also known to enhance N 2 O production (De Wilde and De Bie, 2000).…”

“…Additionally, most N 2 O water concentration sampling has been undertaken in temperate or higher-latitude estuaries, with only 11 water-air N 2 O emission studies in estuaries located between 25°S and 25°N (Table 1). Waters low in dissolved inorganic nitrogen (DIN) can be a net sink 35 for N 2 O, particularly in small mangrove-lined creeks (Erler et al, 2015;Maher et al, 2016;Wells et al, 2018;Murray et al, 2018). N 2 O undersaturation may be prevalent in mangrove creeks, but the extent and seasonal dynamics of N 2 O undersaturation are not well understood due to the limited number of studies and lack of seasonal sampling.…”

“…However, most estuarine N 2 O surveys employ discrete sampling at coarse reso-45 lution, which may not capture the full range of variation. Continuous sampling, employing real-time N 2 O concentration monitoring, is emerging as a way of investigating spatial variability in estuaries at a very fine scale (Bange et al, 1998;Brase et al, 2016;Mueller et al, 2016;Wells et al, 2018), however there has only been one such sampling campaign in a tropical estuary (Mueller et al, 50 2016).…”