|The revised manuscript by Wong and colleagues is much improved; laying out their arguments more clearly for the none expert reader and I thank the authors for taking the time to address both mine and the other reviewers concerns. I have a few relatively minor suggestions prior to publication, mainly asking for a number of newly added sentences to be clarified. |
Scetion 2.1 Study area: please refer to Figure S2 throughout this section, to support your description.
Page 5, Line 22: Please provide the precision / reproducibility of the elemental analyzer measurements.
Page 7, Lines 3 to 6: ‘minimal isotopic fractionation of both processes’ is this really the case for nitrification. Based on the literature ammonia oxidation / nitrification can be associated with a large isotope effect (e.g. Mariotti et al, 1981; Casciotti et al, 2003 and Santoro and Casciotti, 2011). Surely it is the tight coupling of processes that is resulting in no isotope effect being expressed and hence the 15N of organic matter / ammonium and nitrate being similar.
Page 7, Lines 12 to 15: Here the authors are discussing how nitrate produced from nitrification can be fractionated by subsequent processes such as denitrification and this is of course correct. However, I am confused how this fits into this section about calculating the 18O of newly produced nitrate from nitrification. Put your use of equation one in context, tight coupling of the steps of nitrification, little exchange or nitrite accumulation and why not to use the equation presented by Buchwald and Casciotti, 2010. Most of these points the authors already highlight, but the text needs to reformulated to make this clearer, and I would suggest to remove or clarify the discussion of denitrification / a subsequent fractionation process.
Page 10, Lines 5 to 16
- Until now in the manuscript you have discussed that denitrification results in a 2:1 pattern. Now you have switched to phrases such as ‘trajectory of 1’, put this in context for the reader, 2:1 versus 1:1 and how Granger and Wankel, 2016 are trying to reconcile this.
- ‘anammox is still disputable’ what are the authors referring to here, that anammox has not been observed in your system?
Page 11, Lines 5 to 7 and Figure 9: Here you are also using a 1:1 line and this needs to be explained for the reader and also highlighted in the caption of Figure 9 (the whole caption of Figure needs to be looked at, as currently information is missing, it is correct in the reviewer response). Surely you can also exclude denitrification here, due to the high levels of DO?
Page 11, Lines 22 to 24: Should this be Figure 9? Otherwise I do not understand this sentence as currently written.
Figure 5: Reference is missing from caption, but is present in the reviewer response, please make sure it is in the final version. I missed this in my last review, but I can see that in this figure you are showing negative isotope effects, whereas in the text they are positive (e.g. volatilization, Page 7, Line 23). I know the use of positive versus negative varies in the literature, but please be consistent with your use throughout the manuscript.