Program

Estuary is a receptor and a reactor as well for anthropogenic nitrogen (N) drained from upstream catchment. Estuary sediment has great potential to remove anthropogenic nitrogen, including NH4+, NO2- and NO3-, thus mitigate coastal eutrophication. Sedimentary nitrogen removal processes include traditional denitrification and anaerobic ammonium oxidation (anammox). By using 15N-labeling technique, we identified microbial pathways of N removal in Jiulong River Estuary (JRE), a hypereutrophic estuarine ecosystem in Fujian, China in December 2015. Verification of anammox and estimation of potential denitrification and anammox rates slurry incubation with different 15N-labeled substrates. While the genuine N removal rates were determined via intact sediment core incubation with 15NO3- under near in situ conditions. Results from both potential and genuine rates showed that denitrification was ~100x anammox in term of  N removal rate. The genuine denitrification and anammox rates ranged of 3.3-13.1 μmol N m-2 h-1 and 0.03-0.25 μmol N m-2 h-1, respectively. However, potential rates for denitrification and anammox were in range of 103.4 to 540.8 and 1.3 to 4.3 μmol N m-2 h-1, respectively, ~30-40 times higher than in situ rates. Furthermore, results show that JRE sediment is a substantial source of N2O (0.05-4.25 μmol N m-2 h-1) with maximum yield of 32%. Genuine denitrification rates were positively correlated with nitrate concentrations in porewater and sediment organic carbon contents, suggesting that denitrification is controlled by nitrate supply and organic matter availability. Our findings also indicate that nitrification in may fuel denitrification by providing additional nitrate.