Year-end Sale % OFF 
Abstract
Abstract. A major impediment to understanding long-term changes in the marine nitrogen (N) cycle is the persistent uncertainty about the rates, distribution, and sensitivity of its largest fluxes in the modern ocean. We use a global ocean circulation model to obtain the first 3-D estimate of marine denitrification rates that is maximally consistent with available observations of nitrate deficits and the nitrogen isotopic ratio of oceanic nitrate. We find a global rate of marine denitrification in suboxic waters and sediments of 120–240 Tg N yr−1, which is lower than many other recent estimates. The difference stems from the ability to represent the 3-D spatial structure of suboxic zones, where denitrification rates of 50–77 Tg N yr−1 result in up to 50% depletion of nitrate. This depletion reduces the effect of local isotopic enrichment on the rest of the ocean, allowing the N isotope ratio of oceanic nitrate to be achieved with a sedimentary denitrification rate about 1.3–2.3 times that of suboxic zones. This balance of N losses between sediments and suboxic zones is shown to obey a simple relationship between isotope fractionation and the degree of nitrate consumption in the core of the suboxic zones. The global denitrification rates derived here suggest that the marine nitrogen budget is likely close to balanced.
Highlights
Introduction in these ratesOne solution to the challenge of deriving a global esti-Relative to the cycles of other biologically important nutrients, the marine nitrogen cycle is potentially highly dynamic, with large input and output rates and a relatively short turnover time
The parameters of the nitrogen cycle model include the critical oxygen threshold for water-column denitrification; the ratio of nitrate consumed to organic matter remineralized during water-column denitrification, and that during benthic denitrification; the oxygen and nitrate dependence of benthic denitrification; the isotopic enrichment factors for nitrogen fixation, water-column denitrification, benthic denitrification, and uptake of nitrate to form organic nitrogen; the maximum nitrogen fixation rate as well as its temperature, light, nitrate, iron, and depth dependence; the fraction of organic matter production routed to the dissolved organic nitrogen (DON) pool; and the decay timescale for DON
Water-column and benthic denitrification have distinct effects on the vertical distribution of N∗ in the ocean. Quantifying these effects can help to reveal the imprint of watercolumn and benthic denitrification on N∗, and allow us to judge the relative misfit between the modeled and observed N∗ depth profiles (Fig. 3)
Summary
The physical component of the nitrogen cycle model is based on the data-constrained model of DeVries and Primeau (2011), which has been extended to increase the resolution of the model to 2◦ in the horizontal, with 24 vertical levels. As in DeVries and Primeau (2011), the circulation of the model has been tuned to closely reproduce the observed temperature, salinity, and radiocarbon distributions in the ocean using an adjoint method. The internal cycling of N is driven by restoring surface nitrate toward observations, as done for PO4 This ensures that the model reproduces the observed nutrient stoichiometry (N∗) of surface waters. We model N2 fixation according to a simple dependence on light, temperature, and nutrient availability (see Appendix A), and use it primarily to close the N budget. Water-column denitrification in the model occurs where observed oxygen concentrations fall below a critical threshold O2,crit. Denitrification in the water column and sediments is balanced by nitrogen fixation, with a rate that depends on local surface NO3 concentrations, temperature, light levels, and iron supply. See Appendix A for a full description of the nitrogen cycle model
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
