The Pearl River Estuary (PRE) has long received tremendous amounts of anthropogenic nitrogen, and is facing severe environmental problems. Denitrification and anaerobic ammonium oxidation (anammox) are known to be two major nitrogen removal pathways in estuarine sediments. Through the use of slurry and intact sediment core incubations, we examined the nitrogen removal pathways and quantified the in situ denitrification and anammox with associated gaseous nitrogen production rates. Sedimentary nitrogen removal was predominated by denitrification (93-100%) relative to a minimal contribution (<7%) from anammox. Among the detected environmental factors, salinity, bottom water NOx- (nitrate and nitrite) concentration, sedimentary organic matter and dissolved oxygen consumption rates showed good correlations with denitrification and anammox rates. Sedimentary nitrogen loss was mainly supported by endogenic coupled nitrification-denitrification (6.0 ± 1.5 × 106 mol N d-1), with water-column-delivered NOx- (2.1 ± 0.6 × 106 mol N d-1) as the secondary source. Such results suggested that sedimentary nitrogen removal involved mainly particulate organic form (allochthonous or autochthonous) deposited onto sediments, rather than inorganic forms in overlying water. Meanwhile, total N2O production from sediments was estimated to be 7.3 ± 2.1 × 104 mol N d-1, equivalent to ~35% of the daily N2O emissions in the PRE.
Keywords: Anammox; Coupled nitrification-denitrification; Denitrification; Isotope pairing technique; Nitrous oxide.
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