ABSTRACT
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.