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