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.

中文翻译：

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河流-河口-海洋连续体的氮矿化/固定化动力学：有机物和微生物的影响


氮矿化和固定化是重要的氮循环途径。虽然它们在单个沿海栖息地中得到了广泛研究，但很少在沿河流-河口-海洋连续体的不同栖息地中对它们进行比较。我们通过调查长江-河口-东海系统城市河流、河口和邻近海域的总氮矿化 （GNM） 和总铵固定化 （GAI） 来解决这一差距。我们在冬季和夏季对 30 个站点进行了采样，使用富集的 15N 稳定同位素量化了 GNM 和 GAI 速率。我们观察到总有机碳浓度降低了 65%，从河流到海洋的真菌/细菌比率增加了三倍。沿梯度，GNM 从 5.41 μg N g-1 d-1 下降到 2.41 μg，GAI 从 6.08 下降到 3.27，它们的比率通常为 >1，表明氮有限。冗余分析将温度和 Fe 确定为重要的环境变量。本研究强调了沿海系统中跨栖息地比较与氮循环研究的重要性。