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It is understood that the global mean ocean nitrate δ¹⁵N is set by the δ¹⁵N of the input of fixed nitrogen (N) to the ocean (mostly N₂ fixation) and the net isotopic discrimination of fixed N loss (mostly denitrification). Here, we demonstrate that, in addition to the fixed nitrogen input/output budget, the isotopic discrimination of nitrate assimilation in the Southern Ocean also plays a role in setting the δ¹⁵N of both deep ocean nitrate and global mean ocean nitrate. A prognostic model is used to simulate the global overturning circulation, focusing on the Southern Ocean overturning cell that ventilates the global pycnocline. N₂ fixation and denitrification occur mainly in the model's surface and pycnocline boxes, as is appropriate given the observations. Experiments with this model indicate that, at a steady state, pycnocline nitrate δ¹⁵N is mostly controlled by the ocean's fixed N budget. Simultaneously, partial nitrate …