title: Isotopic overprinting of nitrification on denitrification as a ubiquitous and unifying feature of environmental nitrogen cycling.
abstract: Natural abundance nitrogen and oxygen isotopes of nitrate (δ15NNO3and δ18ONO3) provide an important tool for evaluating sources and transformations of natural and contaminant nitrate (NO3-) in the environment. Nevertheless, conventional interpretations of NO3-isotope distributions appear at odds with patterns emerging from studies of nitrifying and denitrifying bacterial cultures. To resolve this conundrum, we present results from a numerical model of NO3-isotope dynamics, demonstrating that deviations in δ18ONO3vs. δ15NNO3from a trajectory of 1 expected for denitrification are explained by isotopic over-printing from coincident NO3-production by nitrification and/or anammox. The analysis highlights two driving parameters: (i) the δ18O of ambient water and (ii) the relative flux of NO3-production under net denitrifying conditions, whether catalyzed aerobically or anaerobically. In agreement with existing analyses, dual isotopic trajectories >1, characteristic of marine denitrifying systems, arise predominantly under elevated rates of NO2-reoxidation relative to NO3-reduction (>50%) and in association with the elevated δ18O of seawater. This result specifically implicates aerobic nitrification as the dominant NO3-producing term in marine denitrifying systems, as stoichiometric constraints indicate anammox-based NO3-production cannot account for trajectories >1. In contrast, trajectories <1 comprise the majority of model solutions, with those representative of aquifer conditions requiring lower NO2-reoxidation fluxes (<15%) and the influence of the lower δ18O of freshwater. Accordingly, we suggest that widely observed δ18ONO3vs. δ15NNO3trends in freshwater systems (<1) must result from concurrent NO3-production by anammox in anoxic aquifers, a process that has been largely overlooked.
authors.affiliation: Department of Marine Sciences, University of Connecticut, Groton, CT 06340; julie.granger@uconn.edu sdwankel@whoi.edu.
authors.forename: Julie
authors.initials: J
authors.lastname: Granger
authors.affiliation: Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02540 julie.granger@uconn.edu sdwankel@whoi.edu.
authors.forename: Scott D
authors.initials: SD
authors.lastname: Wankel
mesh_terms: D058440:Denitrification; D004784:Environmental Monitoring; D007554:Isotopes / Q000032:analysis*; D008962:Models, Theoretical; D009566:Nitrates / Q000032:analysis / Q000737:chemistry; D009569:Nitric Oxide / Q000032:analysis / Q000737:chemistry; D058465:Nitrification; D009584:Nitrogen / Q000032:analysis* / Q000737:chemistry; D058458:Nitrogen Cycle; D010084:Oxidation-Reduction; D010103:Oxygen Isotopes / Q000032:analysis / Q000737:chemistry; D014867:Water / Q000032:analysis / Q000737:chemistry
chemical_list: D007554:Isotopes; D009566:Nitrates; D010103:Oxygen Isotopes; D014867:Water; D009569:Nitric Oxide; D009584:Nitrogen
keywords: anammox; denitrification; isotopes; nitrate; nitrification