Nitrogen dynamics at the sediment-water interface in a tropical reservoir

Nitrogen dynamics at the sediment-water interface were examined in a tropical reservoir, Upper Peirce Reservoir, Singapore to provide information on the relative importance of each process and evaluated the spatial differences of nitrogen dynamics. The intact sediment cores were collected from three locations with different depths and distances to the artificial aeration system. The flow-through system combined with the isotope pairing technique was applied to estimate the nitrogen fluxes, potential denitrification rates and anammox rates. A mass balance model was developed and calibrated with the experimental results, and calculated mineralization, nitrification and dissimilatory nitrate reduction process rates. The sediment had a strong mineralization rate (13.39 mmol m(-2) d(-1)) at shallow section, thus, had obvious NH4-N fluxes (17.61 mmol m(-2) d(-1)). The total N-2 removal rates through denitrification and anammox processes ranged from 0.08 to 0.28 mmol m(-2) d(-1), relatively lower than that in estuaries and coastal systems. Denitrification was performed as the main loss pathway, dominating up to 98% of total N-2 removal rates. The coupled nitrification-denitrification (D-n) was dominant as compared to denitrification that happened from water column nitrate (D-w). Nitrification and N2 removal rates increased with water column depth. The artificial aeration around the deeper section might have contributed to the bacteria activities, and thus might stimulate high microbial processes converting NH4-N to NO3-N and bioactive nitrogen to N-2 gas. The discovery of nitrogen transformation pathways in tropical freshwater systems are proposed to be incorporated into future conceptual models of global nitrogen cycles. (C) 2014 Elsevier B.V. All rights reserved.