Natural abundance nitrogen and oxygen isotopes of nitrate (?15NNO3 and ?18ONO3) 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 ?18ONO3 vs. ?15NNO3 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 ?18O 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 ?18O 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 ?18O of freshwater. Accordingly, we suggest that widely observed ?18ONO3 vs. ?15NNO3 trends in freshwater systems (<1) must result from concurrent NO3- production by anammox in anoxic aquifers, a process that has been largely overlooked.