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 ¹⁵ N isotope pairing technique. The rates of total denitrification and coupled nitrification-denitrification ranged from 0.44 to 7.03 μmol N m ⁻² h ⁻¹ and from below the detection limit to 4.26 μmol N m ⁻² h ⁻¹ , respectively. The rates of total anammox and coupled nitrification-anammox varied from 0.09 to 1.21 μmol N m ⁻² h ⁻¹ and from below the detection limit to 0.73 μmol N m ⁻² h ⁻¹ , respectively. It is shown that high availability of NO ₃ ⁻ in bottom water as well as ferrous iron and organic carbon in sediments was beneficial to the uncoupled nitrogen removal processes, while low NO ₃ ⁻ 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 NO ₃ ⁻ concentrations, dissolved oxygen and salinity of sampling sites. Overall, this study improves the understanding of nitrate transformation processes in the estuarine and coastal environments.