Isotopic overprinting of nitrification on denitrification as a ubiquitous and unifying feature of environmental nitrogen cycling

Natural abundance nitrogen and oxygen isotopes of nitrate (δ15NNO3 and 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 δ18ONO3 vs. δ15NNO3 from 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 δ18ONO3 vs. δ15NNO3 trends in freshwater systems (<1) must result from concurrent NO3− production by anammox in anoxic aquifers, a process that has been largely overlooked.