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Journal of Tropical Oceanography

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Analysis of nitrogen source and influencing factors in mangrove sediments of the Qingjiang Estuary (Zhejiang) using stable isotopes

TAN Qingbi1, GUO Hongying1, SUN Siqi1, ZHONG Yuhang1, NIE Hui2, NIU Lixia1,3,4   

  1. 1. School of Ocean Engineering and Technology, Sun Yat-sen University(Guangzhou)/Southern Laboratory of Ocean Science and Engineering, Zhuhai 519000, China;

    2. Zhejiang Water Conservancy and Hydropower College, Hangzhou 310018, China;

    3. Institute of Estuarine and Coastal Research, Guangdong Provincial Engineering Research Center of Coasts, Islands and Reefs, Guangzhou 519000, China;

    4. Guangdong Provincial Key Laboratory of Information Technology for Deep Water Acoustics, Zhuhai 519000, China


  • Received:2025-08-06 Revised:2025-10-06 Accepted:2025-10-17

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Abstract: The Qingjiang Estuary in Yueqing Bay is located at the northernmost limit of mangrove distribution in China, situated in an ecological transition zone between temperate and subtropical regions, and holds significant ecological reference value. Field sampling and analyses were conducted in typical vegetation zones including mangroves (planted in 2019 and 2020), Spartina alterniflora stands, and mixed vegetation areas. Key parameters including total nitrogen (TN), total organic carbon (TOC), and total organic matter (TOM) in sediments were analyzed. Combined with δ¹³C and δ¹⁵N stable isotope characteristics and a multi-source mixing model (MixSIAR in R), the major sources of nitrogen and organic matter were identified. The results showed that nitrogen and carbon contents in estuarine sediments exhibited an overall decreasing trend from west to east, reflecting spatial gradient differences in terrestrial input intensity. Nitrogen forms and concentrations varied significantly among different vegetation types. In S. alterniflora areas, weaker plant root activity, lower productivity, and faster organic matter decomposition rates resulted in limited retention capacity for exogenous organic matter and nitrogen. The mangrove ecosystem demonstrated stronger regulatory functions in nitrogen cycling compared to S. alterniflora. Nitrogen source analysis revealed that other terrestrial inputs were predominant, accounting for 33.65%, followed by mangroves (C₃ plants) contributing 25.28%, marine sources contributing 23.78%, and S. alterniflora (C₄ plants) having relatively minor influence at 17.33%. This study elucidates the relationship between vegetation types and spatial nitrogen distribution in the Qingjiang Estuary, and explores the key ecological functions of mangroves in regional nitrogen cycling processes. The findings provide theoretical basis and practical guidance for the protection and restoration of mangrove ecosystems across different climate zones and for nitrogen pollution control in estuarine environments.

Key words: Qingjiang estuary, Sediment, Stable isotopes, Mangrove, Total organic matter