Global impact of benthic denitrification on diazotroph physiology and N2 fixation rates

crossref(2022)

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摘要
<p>Nitrogen (N) is one of the crucial limiting nutrients for phytoplankton growth in the&#160;ocean.&#160;The main source of bioavailable N to the ocean is N<sub>2</sub>-fixing diazotrophs in the surface layer. Since the global coverage of N<sub>2</sub> fixation observations is sparse on temporal and spatial scales, the fundamental processes and mechanisms controlling N<sub>2</sub> fixation are not well understood nor constrained. We implement benthic denitrification in the optimality-based plankton ecosystem model (OPEM), which is incorporated into an Earth System Model of intermediate complexity (UVic-ESCM 2.9). Benthic denitrification occurs mostly in coastal upwelling regions and shallow continental shelves, and affects significantly the marine fixed-N budget since it is the largest N-loss process in the global ocean. We carry out model calibration and parameter selection based on observations of Chl, NO<sub>3</sub><sup>-</sup>, PO<sub>4</sub><sup>3-</sup>, O<sub>2</sub> and N*=NO<sub>3</sub><sup>-</sup>-16PO<sub>4</sub><sup>3-</sup>. Compared to considering water-column denitrification in suboxic zones as the only N-loss process in the ocean, our new model version simulates a more realistic distribution and higher global rates of N<sub>2</sub> fixation, which are supported by independent rate measurements. The optimized cellular N:P ratios of diazotrophs in the model version with benthic denitrification better correspond to independent culture estimates, and result in a closer reproduction of the particulate N:P ratios. Our model results indicate that benthic denitrification plays an important role shaping patterns, rates and even physiological aspects of N<sub>2</sub> fixation throughout the global ocean and should be accounted for when understanding and predicting changes to N<sub>2</sub> fixation.&#160;</p>
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