Dear Authors,

Thank you for providing detailed responses to the comments and suggestions offered by two reviewers.

While both reviewers recognized the significance of your research, they also raised concerns about the inadequate discussion of some key findings. Based on the overall positive evaluations of the two reviewers and your thoughtful responses to the relatively small number of major comments, I am pleased to recommend ‘Publish subject to minor revisions’.

I would like to ask you to make all the changes easily identifiable in a marked-up manuscript based on your point-by-point responses to the reviewer comments. If possible, please specify the line numbers of the revised parts in your responses accompanying the revised manuscript.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences

Dear Authors,

Thank you for your careful revision of the manuscript considering all reviewer comments and suggestions.

I am pleased to let you know that your manuscript can be published after technical corrections of the following minor points:

- Title: First, “decomposition”, not “decomposition rate”, is rapid, so it would be more appropriate if the title is corrected to “Rapid soil organic carbon decomposition in river systems…” Second, “priming” might have played a role in the enhanced decomposition of SOC, but you have not introduced the concept at all, either in the abstract or the introduction. You can use a more relevant term you used, such as “aquatic microbial communities”.
- Line 8 (track-change version) “which is often old”: As the first reviewer commented (and you responded to the comment), you need to make clear that soil organic carbon is a mixture or continuum of materials varying in age. This specific sentence is confusing because it is not clear whether “which” refers to “a significant fraction” or “SOC”. As your 14C data showed, the significant fraction that rapidly decomposed might be a rather young, labile fraction. Therefore, you need to clarify the sentence.
- Line 29 “storage time”: Do you mean “retention time”? I would place “among rivers” before “depending…..”.
- Line 99 “4 ml unfiltered river water was added”: Please provide the ratio of inoculum to the incubated water sample.
- Line 104 “POC concentration was controlled at 10–12 mg L−1”: here or somewhere later in the discussion, please provide the rationale for using this rather high (but often observed during intense rainfall events) range of POC concentrations, as you described in your response to the first reviewer’s comment. In addition, please provide the water sample volume to which 60 mg or 160 mg soil was added.
- Line 203: As mentioned earlier, only “some fraction of SOC”, not all SOC components, can be mineralized quickly.
- Line 205 “83–139 μg C L−1 d−1μg C−1 d−1.”: This rate depends on how much POC you put in your water samples, so it would be more appropriate if you provide the values in the unit of ug C/g soil C/d.
- Lines 182-187: Please indicate significant differences in the corresponding tables and figures. For instance, in Figs. 6 and 7 you can easily show this by putting asterisks on top of the corresponding values.
- Line 238 “priming”: Please define this concept at its first use and provide a relevant reference.
- Line 252 “systematic”: Difficult to understand; please replace the word or elaborate what you meant.
- Line 255 “which is consistent with its higher 14C 255 age (Table 3)”: This is not always the case, because aged SOC tends to have lower C/N ratios given N accumulation during decomposition.
- Line 316 “have a weak structure”: Is this a confirmed fact? You can be more cautious in wording, like using “may”.
- Tables 3 & 4: “A”rable land, “F”orest
- Fig. 8: For “high”, please use consistently the uppercase initial letter (H).

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences