Research on the impact of seawater intrusion on nitrogen (N) cycling in coastal estuarine ecosystems is crucial; however, there is still a lack of relevant research conducted under in-situ field conditions. The effects of elevated salinity on N cycling processes and microbiomes were examined in situ seawater intrusion experiments conducted from 2019 to 2021 in the Nakdong River Estuary (South Korea), where an estuarine dam regulates tidal hydrodynamics. After the opening of the Nakdong Estuary Dam (seawater intrusion event), the density difference between seawater and freshwater resulted in varying degrees of seawater trapping at topographically deep stations. Bottom-water oxygen conditions had been altered in normoxia, hypoxia, and weak hypoxia due to the different degrees of seawater trapping in 2019, 2020, and 2021, respectively. Denitrification mostly dominated the nitrate (NO3-) reduction process, except in 2020 after seawater intrusion. However, denitrification rates decreased because of reduced coupled nitrification after seawater intrusion due to the dissolved oxygen limitation in 2020. Dissimilatory nitrate reduction to ammonium (DNRA) rates immediately increased after seawater intrusion in 2020, replacing denitrification as the dominant pathway in the NO3- reduction process. The enhanced DNRA rate was mainly due to the abundant organic matter associated with seawater invasion and more reducing environment (maybe sulfide enhancement effects) under high seawater-trapping conditions. Denitrification increased in 2021 after seawater intrusion during weak hypoxia; however, DNRA did not change. Small seawater intrusion in 2019 caused no seawater trapping and overall normoxic condition, though a slight shift from denitrification to DNRA was observed. Metagenomic analysis revealed a decrease in overall denitrification-associated genes in response to seawater intrusion in 2019 and 2020, while DNRA-associated gene abundance increased. In 2021 after seawater intrusion, microbial gene abundance associated with denitrification increased, while that of DNRA did not change significantly. These changes in gene abundance align mostly with alterations in nitrogen transformation rates. In summary, ecological change effects in N cycling after the dam opening (N retention or release, that is, eutrophication deterioration or mitigation) depend on the degree of seawater intrusion and the underlying freshwater conditions, which constitute the extent of seawater-trapping.

中文翻译：

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韩国洛东江口海水入侵对氮循环的影响

研究海水入侵对沿海河口生态系统氮（N）循环的影响至关重要；但目前仍缺乏相关研究原位现场条件。研究了盐度升高对氮循环过程和微生物组的影响就地2019 年至 2021 年在洛东江口（韩国）进行了海水入侵实验，该河口大坝调节潮汐水动力。洛东河口大坝启用（海水入侵事件）后，海水与淡水的密度差导致不同程度的海水滞留在地形深部站。由于2019年、2020年和2021年不同程度的海水滞留，底层水氧条件分别发生了常氧、缺氧和弱缺氧的变化。反硝化作用主要以硝酸盐（NO3-）减少过程，2020年海水入侵后除外。然而，由于2020年的溶解氧限制，海水入侵后耦合硝化作用减少，反硝化率下降。2020年海水入侵后，异化硝酸盐还原成铵（DNRA）率立即增加，取代反硝化成为NO的主要途径3-还原过程。 DNRA率的提高主要是由于与海水入侵相关的丰富的有机物以及在高海水捕集条件下更多的还原环境（可能是硫化物增强效应）。 2021年弱缺氧海水入侵后反硝化增加；然而，DNRA 并没有改变。 2019 年的小规模海水入侵没有造成海水滞留，总体含氧量正常，但观察到从反硝化到 DNRA 的轻微转变。宏基因组分析显示，2019 年和 2020 年，随着海水入侵，反硝化相关基因整体减少，而 DNRA 相关基因丰度增加。 2021年海水入侵后，与反硝化相关的微生物基因丰度增加，而DNRA的丰度没有显着变化。基因丰度的这些变化主要与氮转化率的变化一致。综上所述，开坝后氮循环的生态变化效应（氮滞留或释放，即富营养化恶化或减轻）取决于海水入侵程度和底层淡水条件，构成海水滞留程度。