Quantitatively deciphering the roles of sediment nitrogen removal in environmental and climatic feedbacks in two subtropical estuaries.

• Denitrification dominate nitrogen removal in sediments from subtropical estuary. • Particulate organic matter links nitrogen transform between water and sediments. • Estuarine sediments are weak in eliminating riverine nitrogen due to limited area. • Sedimentary denitrification directs a more than half of daily sea-air N 2 O flux. Sedimentary denitrification and anaerobic ammonium oxidation (anammox) are two microbially-mediated nitrogen removal pathways with distinct climatic feedbacks. Estuaries receive large fluxes of anthropogenic nitrogen and serve as hotspots for nitrogen loss. Applying 15N isotope pairing technique and sediment intact core incubation in two subtropical estuaries, the Yangtze River Estuary (YRE) and Jiulong River Estuary (JRE), we show that denitrification predominates the sedimentary nitrogen loss with a minor contribution (8.6 ± 7.5%) from anammox. Particulate organic matter degradation sustains the sedimentary nitrogen removal linking the nitrogen transformations between water column and sediment. Our results indicate that estuarine sediments exhibit high areal nitrogen removal rate, but play a relatively weak role in eliminating the nitrogen inputted from river basin due to the limited area. The riverine excess nitrogen will eventually enter into the adjacent continental shelf and be removed via phytoplankton assimilation-sedimentation-degradation-coupled nitrification-denitrification. In addition, sedimentary denitrification causes 1.8 ± 2.2% of nitrogen flow towards nitrous oxide (N 2 O) production and the derived N 2 O release flux accounts for 59% and 65% of the daily sea-air N 2 O emission in the YRE and JRE, respectively. These findings contribute to a better understanding of estuarine sedimentary nitrogen removal and associated climate feedbacks, and to the parameterization of Earth system models. [Display omitted] [ABSTRACT FROM AUTHOR]