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General Session 2: Marine & estuarine biogeochemistry |
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Nitrate limitation as the primary driving force of dinitrogen fixation.
Wednesday 11th @ 1420-1440
Conference Hall
Weilei Wang* , Earth System Science, University of California Irvine, CA, USA, 92617
Francois W. Primeau, Earth System Science, University of California Irvine, CA, USA, 92617
Presenter Email: weileiw@uci.edu
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| N2 fixation, a process that converts dinitrogen (N2) gas to biologically available nitrogen forms (NH4+, NO3-) by diazotrophic plankton (nitrogen fixer), supports roughly half the ocean’s new production1, due to the vast existence of diazotrophic plankton in the oligotrophic tropical and subtropical oceans2. However, the dominant “force” driving N2 fixation, an energetically demanding process, is still unsettled with two main candidates: the relative deficit of nitrate compared to the Redfield nitrate-to-phosphate ratio (P* method), or the supply of iron by dust deposition needed to meet the high iron demand of diazotrophic plankton. Here we present a global inverse model for the marine nitrogen cycle, with which we infer large-scale patterns of N2 fixation. We find that N2 fixation is primarily controlled by low nitrate concentrations rather than by its deficit relative to the ambient phosphate concentration. The model predicts the highest N2 fixation rates are in the subtropical gyres and the lowest (essentially no fixation) are in high nutrient upwelling regions regardless of the N:P ratio in those waters. This result contradicts the inferences made with the P* method that there is a tight coupling in space between N2 fixation and denitrification, which tends to produce waters that upwell with N:P ratios significantly below the Redfield value. 1. Karl, D. et al. Dinitrogen fixation in the world’s oceans. Biogeochemistry 57, 47–+ (2002). 2. Capone, D. G. Trichodesmium, a Globally Significant Marine Cyanobacterium. Science (80-. ). 276, 1221–1229 (1997). |
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