Tidal controls on N2O production and emission

There is growing recognition that aquatic environments emit significant quantities of the greenhouse gas nitrous oxide (N₂O). Yet while work in the inland 'river-continuum' shows that interactions between hydrologic transport and biogeochemical reactions define N₂O emission, N₂O dynamics in downstream coastal waters defined by tides rather than unidirectional flow remain poorly understood. Here we aimed to redress this gap by measuring changes in hydrology (flow velocity, water depth, salinity, and ²²²Rn as an indicator of surface water - groundwater interactions), nitrogen transformations (NO₂^- concentrations and isotopic composition, N₂O isotopic composition and site preference, SP, and N₂:Ar ratios), and N₂O concentrations over a range of tidal intensities, from a tidal freshwater creek and river to a brackish river reach and the marine estuary mouth. We found that relatively small tidal fluctuations (0.7 m - 0.8 m height) in the freshwater creek, not day-night cycles, drove diurnal shifts in both the amount of N₂O (160 nM at low tide to 130 nM at high tide) and its biologic source (SP-N₂O from 16 to 32) in the freshwater tidal creek. The larger tidal river mirrored this pattern, with N₂O emission fluctuations from 20 µmol m^-2 min^-1 (high tide) to 80 µmol m^-2 min^-1 (low tide) coinciding with shifts in δ¹⁵N-N₂O (±3 ‰), NO₂^- concentrations (0.3 nM to <0.01 nM), and N₂ (± 50 µM). Although locations with stronger tidal currents generally also had stronger tidal variations in reactive nitrogen species abundance and isotopic composition, these fluctuations could not be explained by freshwater-marine source mixing. By combining high resolution sensor data with measurements of highly reactive nitrogen isotopes, we provide the first confirmation that tidal pumping alters N₂O production and reduction even tens of kilometres inland from marine waters. Understanding these hydrologic controls on biological N₂O cycling in tidal reaches will be critical for predicting coastal emissions.