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

Estuarine N 2 O emissions contribute to the atmospheric N 2 O budget, but little is known about estuary N 2 O fluxes under low dissolved inorganic nitrogen (DIN) conditions. We present high-resolution spatial surveys of N 2 O concentrations and water-air fluxes in three low-DIN (NO 3 − < 30 µ mol L −1 ) tropical estuaries in Queensland, Australia (Johnstone River, Fitzroy River, Constant Creek) during consecutive wet and dry seasons. Constant Creek had the lowest concentrations of dissolved inorganic nitrogen (DIN; 0.01 to 5.4 µ mol L −1 of NO 3 − and 0.09 to 13.6 µ mol L −1 of NH 4 + ) and N 2 O (93–132% saturation), and associated lowest N 2 O emissions (– 1.4 to 8.4 µ mol m −2 d −1 ) in both seasons. The other two estuaries exhibited higher DIN inputs and higher N 2 O emissions. The Johnstone River Estuary had the highest N 2 O concentrations (97–245% saturation) and emissions (– 0.03 to 25.7 µ mol m −2 d −1 ), driven by groundwater inputs from upstream sources, with increased N 2 O input in the wet season. In the Fitzroy River Estuary, N 2 O concentrations (100–204% saturation) and emissions (0.03–19.5 µ mol m −2 d −1 ) were associated with wastewater inputs, which had a larger effect during the dry season and were diluted during the wet season. Overall N 2 O emissions from the three tropical estuaries were low compared to previous studies, and at times water-air N 2 O fluxes were actually negative, indicating that N 2 O consumption occurred. Low water column NO 3 − concentration (i.e. < 5 µ mol L −1 ) appears to promote negative water-air N 2 O fluxes in estuary environments; considering the number of estuaries and mangrove creeks where DIN falls below this threshold, negative water-air N 2 O fluxes are likely common.