Spatial and Seasonal Variability of Hydroxylamine Concentrations in a Human-Impacted Estuary Off Southeast China

Hydroxylamine (NH₂OH) is an important intermediate of nitrification and can contribute to nitrous oxide (N₂O) production through abiotic and biotic pathways. NH₂OH concentrations and associated biogeochemical controls in estuarine water and sediments remain poorly understood. Here, we investigated spatial and seasonal variability of NH₂OH concentrations in water and sediments and determined the correlation between NH₂OH and N₂O in the Min River Estuary off southeast China. NH₂OH concentrations in water and sediments ranged from 0.22 to 140 nmol N L⁻¹ and from 0.17 to 9.22 μmol N kg⁻¹, respectively. High differences in NH₂OH concentrations between sediments and water could drive diffusion of NH₂OH from sediments to water. NH₂OH concentrations were significantly higher in warm than in cold seasons, suggesting that NH₂OH production is highly dependent on temperature. Upper estuary (urban section) showed higher NH₂OH concentrations in water and sediments, followed by lower and middle estuary. Temperature, dissolved oxygen, and pH were the major factors driving spatial and seasonal variability in NH₂OH concentrations. Water dissolved N₂O concentrations and air-water N₂O fluxes varied between 1.52 and 55.9 nmol N L⁻¹ and between −57.7 and 78.7 nmol N m⁻² h⁻¹, respectively, with switches from net sinks in spring to net sources of N₂O emissions in other seasons. Although N₂O concentrations and fluxes were linearly correlated with water NH₂OH concentrations, the contribution of NH₂OH to N₂O production could not be addressed. Therefore, the dynamics of NH₂OH concentrations are a useful tool to assess potential production and consumption pathways of N₂O in estuarine and coastal ecosystems.