The role of nitrogen fixation in shaping the stoichiometry of marine particulate organic matter

The main source of bioavailable N in the ocean is N₂-fixation by diazotrophs. This study investigates the impact of nitrogen fixation on the elemental (carbon:nitrogen:phosphorus = C:N:P) composition of marine particulate organic matter (POM) in marine environments. Using a global ocean optimality-based eco-physiological model, we demonstrate the influence of nitrogen fixation through two scenarios: The Optimal Nitrogen Fixation Scenario (ONFS) represents optimal (realistic) nitrogen fixation, and the Reduced Nitrogen Fixation Scenario (RNFS) has half the nitrogen fixation of ONFS. The physiological model predicts that N₂-fixing diazotrophs have lower C:N (5.4:1) and higher N:P (19.3:1) ratios than non-N₂-fixing phytoplankton (C:N=7.1, N:P=16.3) and the Redfield ratio (C:N=6.6, N:P=16). With optimal nitrogen fixation, our model better reproduces the relatively low C:N ratios and high N:P ratio of observations, compared to our simulation with reduced nitrogen fixation, particularly in the subtropical zones, although our model still underestimates the observed deviations from the Redfield ratio. Despite representing less than 2% of the total POM, diazotrophs significantly influence the latitudinal patterns of particulate C:N:P ratios by altering their own stoichiometry as well as affecting the stoichiometry of non-N₂-fixing phytoplankton and detritus through ecological interactions. A sensitivity analysis reveals that phytoplankton subsistence quotas and grazing pressure are key traits determining global stoichiometry. These results highlight the importance of accurately representing the nitrogen cycle, specifically nitrogen fixation, in reproducing the stoichiometry of marine particulate organic matter in global ocean models.

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Primary Presenter: Na Li, GEOMAR Helmholtz Centre for Ocean Research Kiel (nli@geomar.de)

Authors:

Na Li, GEOMAR Helmholtz Centre for Ocean Research Kiel  (nli@geomar.de)

Christopher Somes, GEOMAR Helmholtz Centre for Ocean Research Kiel  (csomes@geomar.de)

Angela Landolfi, Institute of Marine Sciences, National Research Council of Italy  (angela.landolfi@cnr.it)

Markus Pahlow, GEOMAR Helmholtz Centre for Ocean Research Kiel  (mpahlow@geomar.de)

Chia-Te Chien, Institute of Oceanography, National Taiwan University  (cchien308@ntu.edu.tw)

Andreas Oschlies, GEOMAR Helmholtz Centre for Ocean Research Kiel  (aoschlies@geomar.de)