This manuscript represents a valuable contribution to N₂O stable isotope science. It provides a suitable approach for including appropriate mass balance considerations in applying the Rayleigh model to position-specific N isotope effects, and points towards a more comprehensive framework for position-specific isotope effects in a broader array of molecules. The level of clarity and detail with which the assumptions of this model are explained, including how they extend and differ from those of the conventional Rayleigh model, is very welcome. Also, the creation and evaluation of the simulated datasets provides great value in showing the strengths and limitations of the Rayleigh model at fitting natural data with error and variations, even beyond the specific application to the Extended Rayleigh model. However, given its complexity it is hard to follow in places. Any way that the calculations described in the Methods could be better connected with their outcomes in the Results would be welcome, as would better integration between figures illustrating various datasets and the outcome of calculations based on them. Furthermore, perhaps details regarding plotting and calculations that apply throughout could be consolidated and mentioned only once. Sections of the methods, especially Section 2.8 could also be streamlined without eliminating any information, which would improve readability. 

The inclusion of both KIE and ε throughout the manuscript also introduces some potential for confusion. This is mitigated somewhat by inclusion of ‘normal’ and ‘inverse’ in reference to various isotope effects in the text, but perhaps it would be clearer to note the alternative definitions but choose a single parameter to report throughout the text. Relatedly, I think that the definition of a given at line 60 would be better referenced to Mariotti et al. (1981) or another source, and the cited reference (Bigeleisen and Wolfsberg, 1958) would be more suitable for the definition of KIE (and the overall concept).

Regarding the fungal P450 NOR case study, it was not entirely clear to me why the variation in fractionation over the course of the experiment requires that the Expanded Rayleigh model was applied only to subsets of the dataset. Doing so also limits the comparability of these outcomes to the results of the Standard Rayleigh model. For the NH₂OH oxidation case study, even after looking in the supporting information and the original Sutka et al. (2006) paper, it was not clear to me what the initial value of substrate was, or exactly how the extent of reaction f  was calculated from the information provided.

This preprint already sparked an excellent discussion in our reading group, and I look forward to seeing it published soon, and discovering what past and future measurements this approach can be applied to, thereby improving our understanding of N₂O generations mechanisms in natural settings.