Dissolved organic matter characteristics and nitrogen transformation rates in sediments of Pojo Bay, Finland

Coastal environments are considered to be nitrogen (N) removal hot spots, regulating the amount of land-derived N reaching the open sea. However, gradual mixing of fresh and saline water masses alters the availability nitrate and bioavailable carbon from near-shore to offshore, which may alter the function of \"coastal filter\". Here, we combine stable isotope analysis, N2O measurements and organic carbon characterization, and show that a significant amount of the N removed in estuary is released as greenhouse gas nitrous oxide (N2O), while archipelago areas are characterized by efficient recycling of N in coastal Baltic Sea, challenging the current understanding on role of these regions as filters of land-to-sea transfer of N.海岸带环境被视为氮（N）移除热点区域，能够调控陆地来源氮素向开阔海域输送的总量。然而，淡水与盐水团的逐步混合，会改变近岸至远岸海域的硝酸盐与生物可利用碳的可获得性，进而可能改变“海岸过滤器”的功能。本研究整合稳定同位素分析、N₂O（一氧化二氮）测定与有机碳表征技术，结果表明：河口区域中被移除的大量氮素会以温室气体一氧化二氮（N₂O）的形式释放；而波罗的海沿岸的群岛区域则呈现出高效的氮循环特征，这对当前“此类区域作为氮素陆-海传输过滤器”的主流认知构成了挑战。