Abstract
Natural abundance nitrogen and oxygen isotopes of nitrate (δ(15)N(NO3) and δ(18)O(NO3)) provide an important tool for evaluating sources and transformations of natural and contaminant nitrate (NO(3)(-)) in the environment. Nevertheless, conventional interpretations of NO(3)(-) 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 NO(3)(-) isotope dynamics, demonstrating that deviations in δ(18)O(NO3) vs. δ(15)N(NO3) from a trajectory of 1 expected for denitrification are explained by isotopic over-printing from coincident NO(3)(-) production by nitrification and/or anammox. The analysis highlights two driving parameters: (i) the δ(18)O of ambient water and (ii) the relative flux of NO(3)(-) 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 NO(2)(-) reoxidation relative to NO(3)(-) reduction (>50%) and in association with the elevated δ(18)O of seawater. This result specifically implicates aerobic nitrification as the dominant NO(3)(-) producing term in marine denitrifying systems, as stoichiometric constraints indicate anammox-based NO(3)(-) production cannot account for trajectories >1. In contrast, trajectories <1 comprise the majority of model solutions, with those representative of aquifer conditions requiring lower NO(2)(-) reoxidation fluxes (<15%) and the influence of the lower δ(18)O of freshwater. Accordingly, we suggest that widely observed δ(18)O(NO3) vs. δ(15)N(NO3) trends in freshwater systems (<1) must result from concurrent NO(3)(-) production by anammox in anoxic aquifers, a process that has been largely overlooked.