南海北部沉积物中氮循环的关键过程研究

Nitrogen cycling is closely related to marine primary production, carbon cycling and global climate change. Nitrification, denitrification, anaerobic oxidation of ammonium (anammox), and dissimilatory nitrate reduction to ammonium (DNRA) are key nitrogen cycling processes, which may affect primary production, phytoplankton species composition and abundance in coastal ecosystems. The fixed nitrogen in marine ecosystem can be removed by nitrate reduction through denitrification which has ever been considered the exclusive pathway to achieve the N2 loss until the discovery of anammox process. Denitrification is a nitrate reduction process that microbially facilitated under anaerobic environments and produce free N2 gas as the ultimate product. Anammox is a recently confirmed nitrate reduction pathway that can also form N2 gas. But the novel point of this process is that two species of fixed nitrogen is removed simultaneously. Additionally, DNRA is an alternative pathway that carries out the nitrate reduction, instead of N2 gas, bio-available ammonium is the final product, thus, there is no fixed nitrogen loss via DNRA. Nitrification and DNRA can adjust nitrogen composition, through which the fixed nitrogen is not lost but reserved in the system which will prolong the residence time of fixed nitrogen and increase the risk of eutrophication or hypoxia in the estuarine and coastal water. Denitrification and anammox was recognized as the quantitatively important process that converts fixed nitrogen to atmospheric nitrogen gas, N2. The South China Sea is one of the ecologically interesting and economically important marine areas, but frequent harmful algal blooms and near-bottom layer low contents of oxygen are observed in coastal areas. Primary production in the South China Sea is higher in winter than in summer, and is limited by both N and P in summer and limited by N in winter. Nitrogen cycle plays an important role in sustainable development and environment health of the investigation region. The current project is to quantify the rates of nitrification, denitrification, anammox and DNRA using 15N isotope pairing technique, to determine the oxygen consumption in sediments, and to understand the relationship between nitrification and development of low oxygen areas in the northern region of the South China Sea; to identify nitrogen loss via denitrification and anammox processes, so as to study key processes of nitrogen cycling in the South China Sea and their roles in the evolvement of eutrophication, and the effects of changes in nutrient concentrations and their ratios on photic bioproduction.

氮循环与海洋初级生产力、碳循环和全球气候变化息息相关。硝化、反硝化、缺氧铵氧化和NO3-异化还原为NH4+作用是氮循环的关键过程，可引起营养盐的组成和结构变动，影响浮游植物的种类组成和数量，并由此影响整个生态系统。硝化与NO3-异化还原为NH4+作用调节氮的组成，反硝化和缺氧铵氧化作用是将固定的氮转变为气态氮的重要过程。本项目充分采用15N同位素示踪技术在氮循环研究方面的优势，探讨南海北部沉积物中氮循环的关键过程，揭示南海北部沉积物中硝化速率、反硝化速率、缺氧铵氧化速率和NO3-异化还原为NH4+的速率及其分布特征与季节变化，定量给出研究区域的耗氧量及其与硝化作用的关联；定量反硝化作用和缺氧铵氧化作用去除N2的量，深入认识南海北部反硝化和缺氧铵氧化等过程对氮循环的调节机制，揭示其在富营养化演变过程中的作用，对生态系统低营养级的影响及其反馈机制，为南海生态环境的健康发展提供理论基础。

氮循环与海洋初级生产力、碳循环和全球气候变化息息相关。系统研究了南海沉积物中氮循环的关键过程。在南海进行了5个航次涵盖三个季节的观测与受控培养实验，发表论文7篇，其中SCI论文5篇。取得的主要认识有：.1 南海北部沉积物中的氮循环. 沉积物耗氧速率为0.55-8.14 mmol O2 m-2 d-1。受控实验获得的底界面氮与磷的交换通量小于根据耗氧速率计算的理论值，与沉积物中发生的转化过程有关。. 国际上原有的定量氮转化过程的方法只考虑到了一种或两种氮的转化过程，当多种硝酸盐还原过程同时存在时则面临着挑战。我们改进的定量氮转化过程速率的方法，则解决了这一棘手问题。进一步拓展了15N同位素对技术在海洋氮循环研究中的应用。. 沉积物中存在显著的异化硝酸盐还原过程，且厌氧铵氧化过程对氮移除的贡献在陆架和陆坡区分别为3-41%和60-66%，反硝化过程对氮移除的贡献随水深增加逐渐降低。厌氧铵氧化和反硝化速率在间隙水中硝酸盐渗透深度以浅，随深度增加而增加，而硝酸盐被耗尽的下层却随深度增加逐渐降低；DNRA速率随沉积物深度增加而逐渐升高。此外，沉积物中的真核生物对于硝酸盐的转化也起着非常重要的作用。沉积物中厌氧铵氧化、反硝化和DNRA速率分别为0.6-16，0.4-116，1.1-46 μmol N m-2 d-1。.2 南海北部沉积物与水体中营养元素的动态变化. 有机碳、总氮与有机磷间的摩尔比值指出沉积在南海北部陆架区的有机质主要归因于陆源输入为主导。碎屑磷和自生磷是沉积物中两个主要的磷赋存形态。粒径和有机质的降解等是影响磷地球化学特征的重要因素。沉积物中铁氧化物大都有较高的磷吸附能力。潜在生物可利用性的磷约占TP的17.9-55.7%。. 营养盐分析时醋酸纤维滤膜（0.45μm，国家海洋局杭州水处理技术开发中心）使用前需要酸处理，酸处理后未干燥的醋酸纤维滤膜和聚碳酸酯滤膜（0.4μm，Whatman）结果较为一致。叶绿素a分析时聚碳酸酯膜更适合，而醋酸纤维膜的截留效率明显偏低。南海北部海域的营养盐有明显的季节变化，受到冲淡水、上升流、中尺度涡和黑潮水入侵的多种物理过程影响。