Major role of ammonia-oxidizing bacteria in N₂O production in the Pearl River estuary

Nitrous oxide (N2O) has significant global warming potential as a greenhouse gas. Estuarine and coastal regimes are the major zones of N2O production in the marine system. However, knowledge on biological sources of N2O in estuarine ecosystems remains controversial but is of great importance for understanding global N2O emission patterns. Here, we measured concentrations and isotopic compositions of N2O as well as distributions of ammonia-oxidizing bacterial and archaeal amoA and denitrifier nirS genes by quantitative polymerase chain reaction along a salinity gradient in the Pearl River estuary, and we performed in situ incubation experiments to estimate N2O yields. Our results indicated that nitrification predominantly occurred, with significant N2O production during ammonia oxidation. In the hypoxic waters of the upper estuary, strong nitrification resulted in the observed maximum N2O and ΔN2Oexcess concentrations, although minor denitrification might be concurrent at the site with the lowest dissolved oxygen. Ammonia-oxidizing β-proteobacteria (AOB) were significantly positively correlated with all N2O-related parameters, although their amoA gene abundances were distinctly lower than ammonia-oxidizing archaea (AOA) throughout the estuary. Furthermore, the N2O production rate and the N2O yield normalized to amoA gene copies or transcripts estimated a higher relative contribution of AOB to the N2O production in the upper estuary. Taken together, the in situ incubation experiments, N2O isotopic composition and concentrations, and gene datasets suggested that the high concentration of N2O (oversaturated) is mainly produced from strong nitrification by the relatively high abundance of AOB in the upper reaches and is the major source of N2O emitted to the atmosphere in the Pearl River estuary.