Organic matter decomposition sustains sedimentary nitrogen loss in the Pearl River Estuary, China

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 NO_x^- (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 NO_x^- (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.