黄海沉积物氮气移除速率的时空变异与调控机制—基于改进的15N同位素对技术的研究

15N isotope pairing technique (15N IPT) is the key method for studying the mechanism of marine benthic N2 loss and quantifying the benthic N2 loss rate. However, the early study in the Yellow Sea by the applicant indicated that the current 15N IPT-intact sediment core incubation can significantly underestimate (by 90-790%) the benthic N2 loss rate, and thus is very likely to deviate the relative contribution of marine sediment in the global ocean nitrogen loss. In this project, we will first try to improve the current 15N IPT-intact sediment core incubation by introducing the side injection of 15NO3-, based on the theoretical derivation and field incubation experiment, to measure the benthic N2 loss rate accurately. Then the improved 15N IPT, combined with sediment slurry incubation technique, dissolved oxygen micro-electrode measuring technique, Rhizon pore water sampler-VCl3 reduction-chemiluminescence measurement for nitrate, as well as the benthic nutrient flux measurement technique for biogenic elements, will be applied in the Yellow Sea for studying the benthic N2 loss. The purpose of this project is, (1) quantifying the benthic N2 loss rate in the Yellow Sea, (2) elucidating the roles of anammox, denitrification and nitrification on the benthic N2 loss, and (3) characterizing the temporal and spatial variation and regulation mechanism of the benthic N2 loss rate in the Yellow Sea. This project will provide an improved method for measuring the benthic N2 loss rate, and will promote our understandings on marine nitrogen cycle.

15N同位素对技术是目前研究海洋沉积物氮气移除机制和量化氮气移除速率的关键技术。申请人在黄海的预研数据表明现行15N同位素对技术的整柱培养严重低估(90-790%)了沉积物中的N2移除速率，进而有可能使得沉积物在全球海洋N2移除中的贡献被弱化。本项目首先通过理论推导与整柱培养，以侧向注射15NO3-的方式改进现行的15N同位素对技术，以获取准确的N2移除速率；其次，利用改进后的15N同位素对技术，结合泥浆培养、溶解氧微电极测量、Rhizon间隙水取样-VCl3还原-化学发光现场检测硝酸盐、沉积物-水界面生源要素通量测量等技术研究黄海沉积物的N2移除机制，旨在准确量化黄海沉积物的N2移除速率；厘清厌氧铵氧化、反硝化、硝化等过程在N2移除中的作用；探讨N2移除速率的时空变化规律及调控机制。本项目将为准确测量沉积物N2移除速率提供一种改进方法，并有望对海洋沉积物氮循环研究取得新的认识。

陆架边缘海沉积物是活性氮移除的主要场所，对于评估全球海洋氮收支至关重要。然而对于沉积物活性氮移除通量的评估存在着很大不确定性，导致了海洋氮收支是否处于平衡的争议。这主要与实测的沉积物氮气移除速率的技术存在缺陷以及复杂的氮气移除机制有关。为了能更加深入的了解陆架边缘海沉积物活性氮移除机制以及量化N2移除速率，本项目将黄海沉积物作为研究对象，利用15N技术研究了沉积物中异化硝酸盐还原的关键过程，取得的主要研究结果如下：(1) 建立了利用自行组装低成本膜进样质谱系统测定核心参数29N2和30N2的方法，测试29N2/28N2和30N2/28N2的相对标准偏差分别小于0.1%和1%，能够满足15N加富样品的测试。还建立了基于Rhizon取样-三氯化钒还原小体积快速测定沉积物间隙水NOx−的方法，为海上现场进行沉积物15N加富柱培养提供了有效的技术支撑。(2) 利用15N同位素对技术的泥浆培养实验在黄海测定了3个航次的反硝化、厌氧铵氧化和DNRA的潜在速率，并量化了厌氧铵氧化在N2移除中的贡献(ra%)及DNRA在异化硝酸盐还原中的贡献。结果表明，黄海沉积物中异化硝酸盐主要以DNRA为主，平均能贡献50%的异化硝酸盐还原，其余由反硝化和厌氧铵氧化承担。在氮气移除过程中，反硝化为主导过程，但是在黄海冷水团所覆盖的沉积物中厌氧铵氧化的贡献也处于比较高的水平，平均33%，最高可达77.4%，基本厘清了厌氧铵氧化和反硝化在黄海沉积物N2移除机制中所起的作用；通过不同季节和不同站位沉积物N2移除速率与基本环境参数的关系，基本揭示了黄海沉积物N2移除速率的时空变化特征及环境因素的调控机制；(3) 通过3个航次的实验结果，证实了现行的基于15N同位素对技术的整柱培养测定的沉积物N2移除速率存在低估，低估范围48%-95%，平均高达74%，并改进了现行的基于15N同位素对技术的整柱培养操作，测定了黄海沉积物N2移除速率，黄海沉积物N2移除速率处于8.4-192 μmol N m−2 d−1，平均98.8 μmol N m−2 d−1，季节变化不显著。