Dear Authors,

Thank you for providing detailed responses to the two referee reports.

Given the large number of issues raised during the review process, a major revision would be required to reconsider your manuscript for publication in Biogeosciences. I would like to ask you to make all the changes easily identifiable in a marked-up manuscript and a point-by-point reply to the reviewers’ and my own comments to facilitate editor decision. I would suggest you to specify the line numbers of the revised parts when you respond to the reviewer’s and my own comments.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences

Dear Authors,

Thank you for providing your detailed responses to reviewer comments and accordingly revising your manuscript.

Although you have provided detailed responses to the reviewer comments, I have some serious concerns about many points that have not been addressed properly in the first revision, as detailed below. Please provide (and indicate clearly) your explanations and subsequent changes in the main text in a point-by-point reply and a marked-up manuscript, respectively. I would suggest you to specify the page and line numbers of the revised parts in your responses.

Please keep in mind that I had to provide my own detailed referee report and here add still many suggestions to expedite the review process. However, I might need to ask another reviewer if your second revision would still have many uncertainties. I would thank you if you thoroughly revise the manuscript and double check any editorial errors because only some are indicated below.

Sincerely,

Ji-Hyung Park
Associate Editor, Biogeosciences


1. Inadequate author response
“…Nevertheless, higher basal respiration in sediments compared to the source soils only occurred at the surface layer of the deposits (e.g., alluvial wedges and reservoir; please see Table 4), where at the same time aggregate formation is occurring (as explained before). On the contrary, at the deep layers of the alluvial wedges and the reservoir, where the carbon is being stored and stabilized, much lower basal respiration rates than at the upper layers, and even than in the source soils are observed.”

Another related review comment and response:
P 13 L 17-21: Please rewrite (better split) this long and vague sentence. This and the following sentences are logically conflicting, because you are arguing that OC in the deposited sediments is more stabilized than the source soils, even though more labile materials, as evidenced by higher BR rates, exist in the same sediments. Please clarify this.
“Please note that our statements are not contradictory at all. On one hand, higher basal respiration is a surrogate of microbial activity and therefore can be used as an index of the lability of the material being degraded by the microorganisms. Microorganisms are activated by the presence of labile (easily decomposable) organic matter and then respire more. But this labile organic matter is further transformed in other bioproducts which can be more or less recalcitrant, can be chemically attached to other molecules and even physically protected by aggregates, as it has been pointed out by many previous studies (Six et al., 2000; 2002; Cotrufo et al 2013; 2015, Denef et al 2014, etc). Atthe end organic matter can be protected (and preserved) by their inherent chemical composition, by physical protection mechanisms (intra-POM), or both. See Schmidt et al 2011 for futher details.

Editor comment: As you noted, the surface and deep layers of both alluvial wedges and reservoir sediments exhibited different degrees of “microbial degradation” and “protection”, which should be described and interpreted with caution. Please change ambiguous descriptions(e.g., L 33-35 on P 1: “Aggregate formation and OC accumulation, mainly associated with macroaggregates and occluded microaggregates within macroaggregates, were predominant in depositional areas”.) throughout the manuscript and make it clear that your argument would be valid for the deeper layers in depositional areas. Of course you can cite some supporting papers in your discussion, but above all you need to base your argument on your own findings. For instance, protected OC and microbial degradation index values for surficial sediments (Table 4) don’t support what you are saying here.

2. Inadequate author response
“…We agree with the comment made by the editor that soil and sediment respiration rates under field conditions may differ from those under laboratory controlled conditions.”

Editor comment: Please provide this discussion in the main text and indicate clearly the new discussion if you had already done it.

3. Inadequate response to a specific comment
P 2, L 1-2: Please be more specific in providing your major conclusion about the relative importance of “temporary and permanent deposits. Do you mean that both sources are equally important?
“According to the results from our work we consider that both, temporary and permanent deposits are very important from the point of view of the physico-chemical mechanisms of OC protection and stabilization that are occurring across the catchment. In addition, it is not our purpose to determine if one of them is more important than the other. We consider that both temporary and permanent deposits should be preserved, although not beyond the natural fluvial dynamics specially for the natural transitory deposits (short-term residence times), due to their high potential as C sinks.”

Editor comment: Again, please pay attention to differences in basal respiration and microbial degradation index among different sediment categories - i.e., the suspended/bed sediments, and the surface and deep layers of depositional sediments. At least you need to specify what role is important and how different it is among the sediment categories.

4. Inadequate author response
P 7 L 10: Why don’t you use simply “OC-M” as denominator? In addition, it is assumed here that OC in free microaggregates and mineral fractions is older than OC in macroaggregates. Do you have any data supporting this assumption? If not, you need to reformulate relevant sentences throughout the manuscript.
“We agree with the editor that the total OC in M could have been used in the denominator (the sum of each sub-fraction is equal to the total OC in macroaggregates). However, we feel that displaying the different sub-fractions contained within the macroaggregates is a clearer way to present this index.”

Editor comment: Please clarify why you felt so, or the index makes sense. Based on what?

5. Inadequate author response
P 7 L 14: There must have been significant reductions in soil moisture given the high incubation temperature and 32 days of incubation. Please clarify this.
“As stated in the previous MS version (see lines 13-16 in page 17) the moisture content of the samples was regularly checked for potential water losses by evaporation by weighting the bottles, but there was not water losses and therefore it was not necessary to add any water". Please note that 32 days of incubation is not a long period for water losses to occur.

Editor comment: This would be very difficult to believe for any research who has ever done a similar soil incubation experiment. Where would evaporated water go during the ‘long’ period of 32 days? Did you keep the bottles closed or open to the air?

6. Minor correction needs:
- Please check carefully editorial errors such as “100 cc volume” (P 5, L 13)

- P3 L 5 - P4 L2: Please check and split paragraph if required.

- P 8 L 1: Please describe why you opted for the nonparametric test. You might need to mention any prior test for normal distribution.
“We used a non-parametric test because our sampling design was not balanced. That is, we did not have the same amount of representative samples (nº of replicates) across eroding, transport and depositional areas (see Table 1). Please note that prior normality distribution tests are not required when non-parametric tests are performed.”
Editor comment: Please describe this in section 2.5.

- “P 11 L 14: Please rephrase “macroaggregates are the nucleus for microaggregate formation”. How can larger macroaggregates function as the nucleus?”
This sentence is based in Oades (1984) theory who postulated that “the roots and hyphae holding together the macroaggregate form the nucleus for microaggregate formation in the center of the macroaggregates.
Editor comment: Please provide the reference, whenever you cite a specific paper.

- Table 4: “Protected OC” is still in use in the table though the caption uses the new term.

- Figs. 3-4: Please double check different letters indicating significant differences. For example, I doubt whether free mineral is really not different between bedload and surface reservoir.