Nitrogen mineralization/immobilization dynamics across the river-estuary-sea continuum: Effects of organic matter and microorganisms

N mineralization and immobilization are important N cycling pathways. While they are widely studied in individual coastal habitats, they are rarely compared across different habitats along the river-estuary-sea continuum. We addressed this gap by investigating gross nitrogen mineralization (GNM) and gross ammonium immobilization (GAI) in urban rivers, estuary, and adjacent sea of the Yangtze River-Estuary-East China Sea system. We sampled 30 stations during winter and summer, quantifying GNM and GAI rates using enriched ¹⁵N stable isotopes. We observed a 65 % decrease in total organic C concentrations and a three-fold increase in fungi/bacteria ratios from river to sea. Along the gradient, GNM decreased from 5.41 to 2.41 μg N g⁻¹ d⁻¹ and GAI decreased from 6.08 to 3.27, with their ratios generally >1, indicating nitrogen limitation. Redundant analyses identified temperature and Fe as significant environmental variables. This study highlights the importance of cross-habitat comparisons to N cycling studies in coastal systems.