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

Abstract

Estuarine N2O emissions contribute to the atmospheric N2O budget, but little is known about estuary N2O fluxes under low dissolved inorganic nitrogen (DIN) conditions. We present high-resolution spatial surveys of N2O concentrations and water-air fluxes in three low-DIN (NO3 - < 30 {micro}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 {micro}mol L-1 of NO3 - and 0.09 to 13.6 {micro}mol L-1 of NH4+) and N2O (93-132% saturation), and associated lowest N2O emissions (- 1.4 to 8.4 {micro}mol m-2 d-1) in both seasons. The other two estuaries exhibited higher DIN inputs and higher N2O emissions. The Johnstone River Estuary had the highest N2O concentrations (97-245% saturation) and emissions (- 0.03 to 25.7 {micro}mol m-2 d-1), driven by groundwater inputs from upstream sources, with increased N2O input in the wet season. In the Fitzroy River Estuary, N2O concentrations (100-204% saturation) and emissions (0.03-19.5 {micro}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 N2O emissions from the three tropical estuaries were low compared to previous studies, and at times water-air N2O fluxes were actually negative, indicating that N2O consumption occurred. Low water column NO3 - concentration (i.e. < 5 {micro}mol L-1) appears to promote negative water-air N2O fluxes in estuary environments; considering the number of estuaries and mangrove creeks where DIN falls below this threshold, negative water-air N2O fluxes are likely common.