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© 2018 American Chemical Society. A dual or multiple stable isotope relationship, for example, a trajectory in a δ-δ (or δ′-δ′) space, can be used to deduce the relationship of underlying diagnostic isotope fractionation factors (α) and therefore reveal the mechanism of a reaction process. While temporal data sampled from a closed-system can be treated by a Rayleigh distillation model, spatial data should be treated by a reaction-transport model. Owing to an apparent similarity between the temporal and spatial trajectories, the research community has often ignored this distinction and applied a Rayleigh distillation model to cases where a reaction-transport model should be applied. To examine the potential error of this practice, here we compare the results of a Rayleigh distillation model to a diffusional reaction-transport model by simulating the trajectories in nitrate's δ′ 18 O-δ′ 15 N space during a simple denitrification process. We found that an incorrect application of a Rayleigh distillation model can underestimate the degree of a diagnostic fractionation to 50% but results in an insignificant difference in the regression slope of a δ′-δ′ trajectory when α ≠1.0. The regression slope predicted by a Rayleigh distillation model can, however, be 0.03-0.3 greater than predicted by a reaction-transport model when NO 3- is involved in complex nitrogen cycling. Our reaction-transport model rarely predicts a δ′ 18 O-δ′ 15 N regression slope > 1 for reasonable Earth surface conditions. We found that for those published cases of regression slopes > 1, many can be attributed to the grouping of multiple NO 3- sources from independent origins. Our results highlight the importance of linking the underlying physical model to the plotted data points before interpreting their high-dimensional isotope relationships.

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ACS Earth and Space Chemistry

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