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Convergent estimates of marine nitrogen fixation

Abstract

Uncertainty in the global patterns of marine nitrogen fixation limits our understanding of the response of the ocean’s nitrogen and carbon cycles to environmental change. The geographical distribution of and ecological controls on nitrogen fixation are difficult to constrain with limited in situ measurements. Here we present convergent estimates of nitrogen fixation from an inverse biogeochemical and a prognostic ocean model. Our results demonstrate strong spatial variability in the nitrogen-to-phosphorus ratio of exported organic matter that greatly increases the global nitrogen-fixation rate (because phytoplankton manage with less phosphorus when it is in short supply). We find that the input of newly fixed nitrogen from microbial fixation and external inputs (atmospheric deposition and river fluxes) accounts for up to 50 per cent of carbon export in subtropical gyres. We also find that nitrogen fixation and denitrification are spatially decoupled but that nevertheless nitrogen sources and sinks appear to be balanced over the past few decades. Moreover, we propose a role for top-down zooplankton grazing control in shaping the global patterns of nitrogen fixation. Our findings suggest that biological carbon export in the ocean is higher than expected and that stabilizing nitrogen-cycle feedbacks are weaker than previously thought.

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Data availability

All other data used to constrain the inverse model are publicly available (see Supplementary Information). The particulate organic matter data used for the comparison between the N:P of suspended particulate organic matter with the inferred N:P of exported organic matter shown in Fig. 2 is from a previous publication59. The model output for generating all of the other figures is available upon request.

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Competing interests

The authors declare no competing interests.

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Acknowledgements

This work was supported by the National Science Foundation (grant OCE 1436922 awarded to F.W.P.). F.W.P., J.K.M. and W.-L.W. also acknowledge support from the US Department of Energy Office of Biological and Environmental Research (grants DE-SC0007206, DE-SC0012550 and DE-SC0016539) and the RUBISCO-SFA (grant PC13115 to J.K.M.) and A.C.M. acknowledges financial support from the National Science Foundation (grants OCE-1046297 and OCE-1559002). We also acknowledge support from the National Science Foundation (grant OCE-1848576).

Reviewer information

Nature thanks K. Casciotti, N. Gruber and C. Somes for their contribution to the peer review of this work.

Author information

All authors contributed to the design of the study. W.-L.W. and F.W.P. built the inverse model and analysed its results. J.K.M. performed the CESM simulations and analysed its output. F.W.P., W.-L.W. and J.K.M. wrote the manuscript. All authors participated in the discussion of the results and commented on the manuscript.

Competing interests

The authors declare no competing interests.

Correspondence to François W. Primeau.

Supplementary information

Supplementary Information

This file contains the Supplementary Materials, Supplementary Figures S1-S21 and Supplementary Tables S1-S8.

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Fig. 1: Schematic representation of the P-cycle and N-cycle models.
Fig. 2: N:P ratio of exported organic material.
Fig. 3: Maps of the column-integrated sources and sinks of fixed nitrogen.
Fig. 4: Contribution of newly fixed N to export production of carbon.
Fig. 5: Prognostic model simulations of diazotrophs and N2 fixation.

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