Major role of ammonia-oxidizing bacteria in N 2O production in the Pearl River estuary

Ma, Li; Lin, Hua; Xie, Xiabing; Dai, Minhan; Zhang, Yao

Nitrous oxide (inline-formulaN2O) has significant global warming potential as a greenhouse gas. Estuarine and coastal regimes are the major zones of inline-formulaN2O production in the marine system. However, knowledge on biological sources of inline-formulaN2O in estuarine ecosystems remains controversial but is of great importance for understanding global inline-formulaN2O emission patterns. Here, we measured concentrations and isotopic compositions of inline-formulaN2O 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 inline-formulaN2O yields. Our results indicated that nitrification predominantly occurred, with significant inline-formulaN2O production during ammonia oxidation. In the hypoxic waters of the upper estuary, strong nitrification resulted in the observed maximum inline-formulaN2O and inline-formulaΔinline-formulaN2Oinline-formulaexcess concentrations, although minor denitrification might be concurrent at the site with the lowest dissolved oxygen. Ammonia-oxidizing inline-formulaβ-proteobacteria (AOB) were significantly positively correlated with all inline-formulaN2O-related parameters, although their amoA gene abundances were distinctly lower than ammonia-oxidizing archaea (AOA) throughout the estuary. Furthermore, the inline-formulaN2O production rate and the inline-formulaN2O yield normalized to amoA gene copies or transcripts estimated a higher relative contribution of AOB to the inline-formulaN2O production in the upper estuary. Taken together, the in situ incubation experiments, inline-formulaN2O isotopic composition and concentrations, and gene datasets suggested that the high concentration of inline-formulaN2O (oversaturated) is mainly produced from strong nitrification by the relatively high abundance of AOB in the upper reaches and is the major source of inline-formulaN2O emitted to the atmosphere in the Pearl River estuary.

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Ma, Li / Lin, Hua / Xie, Xiabing / et al: Major role of ammonia-oxidizing bacteria in N2O production in the Pearl River estuary. 2019. Copernicus Publications.

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