The reactive nitrogen (N) emitted from continents significantly perturbs the pristine N cycle around the land-ocean boundary resulting in eutrophication and hypoxia. As nutrients are transported downstream through an estuary, various types of biological processes co-occur to modulate nitrogen speciation to influence the biogeochemical habitats for downstream microorganisms. We surveyed the Pearl River Estuary to examine the N transfer dynamics among nitrogen species with considering process-specific oxygen production and consumption. By using ¹⁵N pulse-tracing techniques, we measured ammonia oxidation and uptakes of ammonium, nitrite, and nitrate simultaneously under dark and light conditions in parallel. Light strongly inhibited nitrification but enhanced N uptake, and such light effect was further considered in the calculation for nitrogen transformation rates over a diel cycle. We found both oxidation and uptake of ammonium decreased seaward as substrate decreased. The nitrifier and phytoplankton work in antiphase to draw down incoming ammonium rapidly. Contrary to ammonium uptake, uptake of nitrite and nitrate showed a seaward increasing pattern. Such an inverse spatial pattern implies a shift in N preference for phytoplankton. Such high ammonium preference inhibits nitrate/nitrite uptake allowing them to behave conservatively in the estuary and to travel farther to outer estuary. By integrating oxygen consumption and production induced by N transformation processes over the diel cycle, oxygen was produced although allochthonous ammonium input is high (∼250 μM). For most stations, ammonium was completely consumed within 2 days, some stations even less than 0.5 days, implying that although the water residence time is short (2-15 days), tremendous input of ammonium N from upstream was transformed into particulate organic or nitrate forms during traveling to modulate the biogeochemical niche, including substrate, organics and oxygen, of coastal microbes in water column and sediments.

大陆释放的活性氮 (N) 极大地扰乱了陆地-海洋边界周围的原始 N 循环，导致富营养化和缺氧。随着营养物质通过河口向下游运输，各种类型的生物过程共同发生以调节氮物种形成，从而影响下游微生物的生物地球化学栖息地。我们调查了珠江口，在考虑特定过程的氧气生产和消耗的情况下检查氮物种之间的氮转移动态。通过使用¹⁵N 脉冲跟踪技术，我们在黑暗和光照条件下同时测量氨氧化和铵、亚硝酸盐和硝酸盐的吸收。光强烈抑制硝化作用但增强了N的吸收，并且在计算昼夜循环中的氮转化率时进一步考虑了这种光效应。我们发现随着底物的减少，铵的氧化和吸收都减少了。硝化菌和浮游植物反相工作以迅速吸收进入的铵。与铵吸收相反，亚硝酸盐和硝酸盐的吸收呈现出向海增加的模式。这种反向空间模式意味着对浮游植物的 N 偏好发生了变化。如此高的铵偏好抑制了硝酸盐/亚硝酸盐的吸收，使它们在河口表现得较为保守，并能更远地到达外河口。通过整合整个昼夜循环过程中由 N 转化过程引起的耗氧量和产生量，尽管异地铵输入量很高（~250 μM），但仍产生了氧气。大部分站点在 2 天内完全消耗铵态氮，有些站点甚至不到 0.5 天，这意味着虽然水停留时间短（2-15 天），但上游输入的大量铵态 N 转化为颗粒有机物或硝酸盐在旅行过程中形成以调节水体和沉积物中沿海微生物的生物地球化学生态位，包括底物、有机物和氧气。