Sediment nitrate reduction processes in response to environmental gradients along an urban river-estuary-sea continuum

Sediment denitrification (DEN), anaerobic ammonium oxidation (Anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are three important nitrate (NO₃⁻) reduction pathways in aquatic ecosystems. These processes modify nitrogen (N) loadings from land to the ocean, with important implications on the management of coastal eutrophication. While NO₃⁻ reduction has been studied intensively for various types of habitats, studies on its distributions along river-estuary-sea continua remain scarce. In this study, we examined these three pathways along a N-laden urban river-estuary-sea continuum comprised of three types of habitats (urban river, estuary, and adjacent sea) in the densely populated Shanghai-East China Sea area. The potential DEN, Anammox, and DNRA rates decreased seaward both in summer and winter in response to decreasing sediment organic matter (OM, 20 to 7 to 7 mg C g⁻¹), ferrous oxide (9 to 2.7 to 2.8 mg Fe g⁻¹), and bottom water dissolved inorganic nitrogen (543 to 112 to 21 μM). Among these pathways, DEN remained a major component (~69.6%) across habitats, while Anammox (47.9%) rivaled DEN (48.3%) in the urban river in winter. N retention index (NIRI), the ratio between retained and removed NO₃⁻, ranged from 0 to 0.5 and increased downstream. Together, these results suggest that the decreasing gradients of OM and inorganic matter shape the distribution of NO₃⁻ reduction along the continuum, reflecting the diminishing impact of the river and human inputs from the urban river to the ocean. Our results highlight the importance of taking a continuum perspective in N cycling studies and emphasize the role of urban rivers as N removal hotspots, which should be a focus of research and management.