海洋最小含氧带氮流失过程与机制

Process and mechanism of nitrogen loss in the ocean oxygen minimum zone

对全球大洋氮循环的研究发现,大洋输入和输出的氮存在严重的不平衡,所固定的氮中有相当一部分被还原为N_2或N_2O从大洋中流失,而海洋最小含氧带(OMZ)被认为是发生氮流失的最主要区域,通过反硝化作用和厌氧氨氧化作用,固定氮在OMZ海区内损失量可达40～450 Tg·a^(-1).对不同海区OMZ内固定氮损失的两种主要作用总结发现,异养反硝化作用在热带太平洋东部、阿拉伯海的OMZ内以及海洋沉积物内占有显著优势,在智利、秘鲁沿岸海域及阿拉伯海域也已发现自养反硝化作用的存在;而在黑海、非洲西南部的本格拉上升流、智利北部沿岸等地,厌氧氨氧化作用强烈,且其在陆架区的作用强度和面积要大于大洋区.OMZ氮的流失除受氮流失过程自身影响外,固氮作用、硝化作用、硝酸盐异化还原作用等都可能对OMZ海区内氮收支不平衡造成影响.其中固氮作用的影响最不能忽视,其在全球OMZ内固定的氮的总量可达15～40 Tg·a^(-1),是对OMZ氮流失量的重要补充.区分反硝化作用和厌氧氨氧化作用对OMZ氮流失的相对贡献,明确氮流失的另一产物N_2O的形成机制和定量评估方法是当前OMZ氮流失研究中存在的最主要问题.本文针对存在问题提出了相应的研究设想,以期为海洋最小含氧带的研究提供参考.

There is a big imbalance between the input and output of oceanic nitrogen in global ocean nitrogen cycles, because a part of the fixed nitrogen is reduced to N2 or N2O and then lost from the ocean. Oxygen minimum zone (OMZ) is the most important area for nitrogen loss, which could lose fixed nitrogen up to 40 to 450 Tg·a^-1 through the denitrification and anammox. A summary of the two main roles of nitrogen loss in the different OMZ sea areas reveals that heterotrophic denitrification dominates in eastern tropical Pacific, Arabian Sea, and marine sediments. The autotrophic denitrification has been found in Chile, Peru’s coastal waters, and Arabian waters. In the Black Sea, the Benguela upwelling in southwestern Africa, and the northern coast of Chile, anaerobic ammonia oxidation is strong, with greater effects on the continental shelf than that in the ocean. In addition to the loss of nitrogen, nitrogen fixation, nitrification, and dissimilatory nitrate reduction to ammonium may affect the imbalance of nitrogen budget in the OMZ. The effects of nitrogen fixation can’t be ignored. The total amount of nitrogen fixed in the global OMZ can reach 15-40 Tg·a^-1, which is an important supplement to the loss of nitrogen in OMZ. Disentangling the relative contribution of denitrification and anammox to the loss of nitrogen, ascertaining the formation mechanism and quantitative evaluation method of N2O (another product of nitrogen loss) are the most important challenges in the current study of OMZ. Focusing on the existing problems, we put forward corresponding research ideas with references for related studies of the OMZs in the ocean.