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Abstract

Nitrogen (N) fixation is the primary way to supply biologically available nitrogen to the global ocean. A nitrate (NO3-) isotope budget has been used to estimate the rates and distribution of N fixation in the Atlantic Ocean, with a focus on the North Atlantic. To this point, this approach has not taken into account dissolved organic nitrogen (DON), despite its pervasive presence in the upper ocean where nitrogen fixation occurs. Here we measured the 15N/14N isotopic ratio of total dissolved nitrogen (TDN), encompassing both nitrate and DON, along two meridional transects in the western North Atlantic. Our results reveal that the previously observed low 15N/14N feature in NO3- in the subtropical North Atlantic thermocline, inferred to indicate accumulated newly fixed nitrogen, is also present but slightly less pronounced in TDN. Overall, the TDN-based N fixation rate in the North Atlantic (> 11°S) is estimated as 23.0 Tg N yr-1, about 15% lower than estimates based solely on NO3- data. In addition, relative to the NO3--only calculations, the latitudinal band with the maximum N fixation rate is shifted northward, placing it north of the tropical Atlantic upwelling zone. Comparison with the dissolved phosphate distribution suggests that relatively faster recycling of dissolved organic phosphorus (DOP) than DON delays the onset of P limitation and thus of N fixation.