Sedimentary processes dominate nitrous oxide production and emission in the hypoxic zone off the Changjiang River estuary.

Coastal oceans, known as the major nitrous oxide (NO) source to the atmosphere, are increasingly subject to eutrophication and concurrent near-bottom hypoxia. The natural nitrogen cycle is likely to be altered markedly in hypoxic coastal oceans. However, the processes responsible for NO production and emission remain elusive because of lacking field rate measurements simultaneously conducted in the water column and sediment. Here, we quantified NO production rates using a N-labeled technique in the water-column and surface sediments off the Changjiang (Yangtze) River estuary, the largest hypoxic zone in the Pacific margins. Our results showed that the estuarine surface sediments were the major source for NO production, accounting for approximately 90% of the total water-column accumulation and consequent efflux of NO in the hypoxic zone, whereas the water-column nitrification and denitrification combined only contributed <10%. More importantly, the coupling of nitrification and denitrification at the presence of abundant supply and remineralization of labile organic matter was the main driver of the NO release from the sediment-water interface in this region. This study highlights the dominant role of benthic processes occurring at the sediment-water interface controlling the coastal NO budget, as the anthropogenic eutrophication and hypoxia are expanding in coastal oceans.