Coupling of denitrification and anaerobic ammonium oxidation with nitrification in sediments of the Yangtze Estuary: Importance and controlling factors

Coupling of denitrification and anaerobic ammonium oxidation (anammox) with nitrification is an important mechanism for controlling nitrogen removal from aquatic ecosystems. In this work, we determined ambient rates of denitrification and anammox and their coupling links to nitrification in sediments of the Yangtze Estuary along a salinity gradient, using continuous-flow experiments combined with N-15 isotope pairing technique. The rates of total denitrification and coupled nitrification-denitrification ranged from 0.44 to 7.03 mu mol N m(-2) h(-1) and from below the detection limit to 4.26 mu mol N m(-2) h(-1), respectively. The rates of total anammox and coupled nitrification-anammox varied from 0.09 to 1.21 mu mol N m(-2) h(-1) and from below the detection limit to 0.73 mu mol N m(-2) h(-1), respectively. It is shown that high availability of NO3- in bottom water as well as ferrous iron and organic carbon in sediments was beneficial to the uncoupled nitrogen removal processes, while low NO3- concentration in bottom water promoted the coupled nitrification-nitrogen removal processes. Along the salinity gradient, the coupled nitrification-nitrogen removal processes were the main nitrogen removal pathway in the Yangtze Estuary, and their contribution to total nitrogen removal was affected by NO3- concentrations, dissolved oxygen and salinity of sampling sites. Overall, this study improves the understanding of nitrate transformation processes in the estuarine and coastal environments.