Abstract
Abstract. 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.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas with global warming potential 298 times that of carbon dioxide (CO2) on a 100 years timescale and contributes to stratospheric ozone depletion as a major precursor of free radicals (Ravishankara et al, 2009)
The Spearman correlations and redundancy analysis in this study indicate that high nutrient and Total suspended material (TSM) concentrations and low-Dissolved oxygen (DO) and low-pH conditions were favorable for relatively high abundance of ammonia-oxidizing bacteria (AOB) in the upper estuary, which is consistent with our previous Pearl River estuary study that found high TSM concentrations and low DO and pH influenced substrate availability and AOB distribution (Hou et al, 2018)
This study explored the relative contributions of AOB and ammonia-oxidizing archaea (AOA) in producing N2O in the Pearl River estuary by combining isotopic compositions and concentrations of N2O, distributions and transcript levels of AOB and AOA AMO subunit A (amoA) and denitrifier nirS genes, and incubation estimates of nitrification and N2O production rates
Summary
Nitrous oxide (N2O) is a potent greenhouse gas with global warming potential 298 times that of carbon dioxide (CO2) on a 100 years timescale and contributes to stratospheric ozone depletion as a major precursor of free radicals (Ravishankara et al, 2009). The main processes responsible for N2O emissions are microbial transformation of ammonia, nitrite, and nitrate through nitrification and denitrification (Butterbach-Bahl et al, 2013). N2O is released as a byproduct during nitrification via incomplete oxidation of hydroxylamine (NH2OH) to nitrite (NO−2 ) by ammonia-oxidizing bacteria (AOB) (Stein, 2011). This process may be enhanced under suboxic conditions (Naqvi et al, 2010). While no equivalent of the hydroxylamine oxidoreductase that catalyzes N2O formation through NH2OH oxidation has been found in ammonia-oxidizing archaea (AOA) (Hatzenpichler, 2012), recent studies indicated
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