AUTHOR=Li Yuhong , Luo Yang , Liu Jian , Ye Wangwang , Zhang Jiexia , Zhan Liyang 

TITLE=Sources and sinks of N2O in the subtropical Jiulong River Estuary, Southeast China

JOURNAL=Frontiers in Marine Science

VOLUME=10

YEAR=2023

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2023.1138258

DOI=10.3389/fmars.2023.1138258

ISSN=2296-7745

ABSTRACT=<p>Nitrous oxide (N<sub>2</sub>O) is one of the most important greenhouse gases and contributes to the depletion of ozone in the stratosphere. Estuaries are areas of intensive biological production and associated N<sub>2</sub>O emissions through both denitrification and nitrification processes. The spatial and temporal variations of N<sub>2</sub>O in the Jiulong River Estuary, a subtropical estuary, were explored to evaluate sources and sinks of N<sub>2</sub>O in this area. The estuary was found to be a strong source of N<sub>2</sub>O, its saturation in the surface water ranged from 113 to 2926% relative to the ambient atmospheric concentrations, showing great temporal and spatial variations and was influenced by multiple factors such as the concentration of dissolved inorganic nitrogen (DIN, i.e., <inline-formula><mml:math display="inline" id="im1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>,<inline-formula><mml:math display="inline" id="im2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>, and <inline-formula><mml:math display="inline" id="im3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>), salinity and dissolved oxygen. N<sub>2</sub>O concentrations were at a high level in upper estuary but reduced to the lower parts of the estuary. Groundwater input could be another contributor to N<sub>2</sub>O in the estuary. Almost all N<sub>2</sub>O within the estuary was released into the atmosphere rather than being transported to the bay. The N<sub>2</sub>O flux in the estuary (mean 597 μmol/m<sup>2</sup>/d) was at the higher end of the range observed in estuaries worldwide due to the very high DIN loads in the Jiulong River Estuary. Our data indicate that the N<sub>2</sub>O saturation in the estuary continues to increase, although the DIN inputs began to decline in 2011, which might be relate to the improved environmental conditions with increased oxygen concentrations. N<sub>2</sub>O production pathways have changed from predominantly denitrification in the past toward significant production from nitrification in the present. Further investigation is needed to better understand the behavior of N<sub>2</sub>O in the Jiulong River Estuary.</p>