Nitrogen mineralization and immobilization in sediments of the East China Sea: Spatiotemporal variations and environmental implications

Nitrogen (N) mineralization and immobilization are important processes of N biogeochemical cycle in marine sediments. This study investigated gross N mineralization (GNM) and NH₄ ⁺ immobilization (GAI) in the sediments from the East China Sea (ESC), using ¹⁵N stable isotope dilution technique. Results show that measured rates of GNM and GAI ranged from 0.04 to 6.1 µg N g⁻¹ d⁻¹ and from undetectable to 9.82 µg N g⁻¹ d⁻¹, respectively. In general, both GNM and GAI rates were significantly greater in summer as compared to winter, and the high rates occurred mainly in the muddy area and increased gradually from the Yangtze Estuary to Zhe-Min Coastal muddy areas. The GNM and GAI processes were related closely to sediment temperature, pH, ammonium (NH₄ ⁺), nitrate (NO₃ ⁻), total organic carbon (TOC), and total nitrogen (TN) contents in the muddy area, while they were associated tightly with sediment temperature, pH, NH₄ ⁺, TOC, TN, sulfide, and Fe(III) concentrations in the sandy area. In addition, the total mineralized and immobilized N in the East China Sea (ECS) were estimated to be approximately 2.1 × 10⁶ t N yr⁻¹ and 2.7 × 10⁶ t N yr⁻¹, respectively. Overall, these results highlight the importance of N mineralization and immobilization in controlling the N budget in the ECS and improve the understanding of both processes and associated controlling mechanisms in the coastal marine ecosystem.