Large Uncertainty in the Rate of Global Marine N₂ Fixation: A reanalysis

Ya-Wei Luo^{*, 1, 2}, Yangchun Xu^{1, 2
1} State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China,
2 College of Ocean and Earth Sciences, Xiamen University, Xiamen, China

The role of marine diazotrophs in converting dinitrogen (N₂) into bioavailable nitrogen is essential for sustaining life in the global ocean. However, significant uncertainties remain in previous estimates of global marine N₂ fixation rates. A recently published new version of global diazotroph database has considerably expanded spatial coverage. In this study, over 10,000 in situ measurements of N₂ fixation from the updated database were used to reassess global marine N₂ fixation. First, using the arithmetic mean rates from different ocean basins, we estimated a global marine N₂ fixation rate of 223 Tg N yr⁻¹, which is higher than most previous estimates. However, applying a bootstrap method revealed a 95% confidence interval of 144 – 402 Tg N yr⁻¹. This large uncertainty results from the nature of log-normal distribution of the data. Second, acknowledging that the observed N₂ fixation rates remain spatially uneven, we reanalyzed the quantitative relationship between the observed rates and environmental factors. Unfortunately, we achieved only moderate predictability, limiting our ability to refine estimates of the global distribution of marine N₂ fixation. Our reanalysis highlights that, despite a significant increase in observations over the past decade, major challenges persist in accurately estimating global marine N₂ fixation, which remains a critical parameter for marine biogeochemical studies.