Coastal environments are nitrogen (N) removal hot spots, which regulate the amount of land-derived N reaching the open sea. However, mixing between freshwater and seawater creates gradients of inorganic N and bioavailable organic matter, which affect N cycling. In this study, we compare nitrate reduction processes between estuary and offshore archipelago environments in the coastal Baltic Sea. Denitrification rates were similar in both environments, despite lower nitrate and carbon concentrations in the offshore archipelago. However, DNRA (dissimilatory nitrate reduction to ammonium) rates were higher at the offshore archipelago stations, with a higher proportion of autochthonous carbon. The production rate and concentrations of the greenhouse gas nitrous oxide (N₂O) were higher in the estuary, where nitrate concentrations and allochthonous carbon inputs are higher. These results indicate that the ratio between nitrate and autochthonous organic carbon governs the balance between N-removing denitrification and N-recycling DNRA, as well as the end-product of denitrification. As a result, a significant amount of the N removed in the estuary is released as N₂O, while the offshore archipelago areas are characterized by efficient internal recycling of N. Our results challenge the current understanding of the role of these regions as filters of land-to-sea transfer of N.

沿海环境是除氮（N）的热点，这些热点调节着到达公海的陆源N的数量。但是，淡水和海水之间的混合会产生无机氮和可生物利用的有机物的梯度，从而影响氮的循环。在这项研究中，我们比较了波罗的海沿岸河口和离岸群岛环境之间的硝酸盐还原过程。尽管离岸群岛的硝酸盐和碳浓度较低，但两种环境的反硝化率均相似。但是，离岸群岛群岛站的DNRA（将硝酸盐异化还原为铵）的比率较高，而自生碳的比例较高。温室气体一氧化二氮（N _2）的产生速率和浓度O）在河口较高，硝酸盐浓度和异源碳输入较高。这些结果表明，硝酸盐和原生有机碳之间的比例决定着脱氮反硝化和氮再循环DNRA之间的平衡，以及反硝化的最终产物。结果，河口中去除的大量N以N ₂ O的形式释放，而近海群岛地区则以有效的N内部循环为特征。我们的研究结果挑战了当前对这些区域作为过滤器的作用的认识。陆地向海的转移