This study investigates the seasonal variations in dissolved organic matter fluxes and concentrations from the organic layer of boreal forests. Sites that have been logged ten years ago are compared with unharvested controls in a replicated experimental design.

The study design is sound, and the information generated is original and pertinent. DOM originating from the organic layer of boreal forests represent an important and poorly known flux of carbon that may be affected by disturbances and climate change. The factors that control this flux as well as those that control the quality of DOM are poorly known. This study brings new knowledge on how disturbances and climate change may affect DOM originating from boreal forest organic horizons. The results, showing a similar response of DOM  to climate for both harvested and control plots as well as an important effect of the season are pertinent to better understanding the C cycling of boreal forests.

Comments:

To better understand the results, more information is needed concerning the experimental design. Specifically, what is the status of regeneration on the harvested plots? Is it bare soil?  What is the status of tree regeneration, 10 years after harvesting? Has the site been recolonized by trees? What is the vegetation height, % cover… This information would be useful to understand the results. It is interesting that the differences in water and DOC fluxes are still present 10 years after harvesting. Another study conducted in somewhat similar conditions found coherent results with this one. However, it was conducted one year after harvest: warmer and wetter conditions in harvested treatment led to greater soil humidity, higher soil temperatures, and greater ammonification and DON production of harvested plots in the fall season (Coulombe et al. 2016). Can the author comment on the reason for a 10-year effect?

Minor comments

l.54: fate of DOM: there is abundant literature on the importance of mineral soil properties on the fate of DON that could be mentioned here.

l.74: recalcitrance of coniferous litter, how about that of bryophytes, are they not present?

l.104: 10 years may not be considered long-term in a rotation that lasts more than 50 years;  preferably use 10 years, – also line 535 and elsewhere.

l.381; l. 535 and elsewhere: the use of Sr for slope could be misleading (Strontium), use another notation.

Discussion/conclusions

Winter rain/thaw events are increasing. The results of this study suggest that these events may bring more transformed DOM into streams. Is it possible to support this claim by referring to studies in warmer ecosystems, such as coastal Maine or New England?

Reference:

https://doi.org/10.1139/cjfr-2016-0301 Coulombe et al. 2016. Effect of harvest gap formation and thinning on soil nitrogen cycling at the boreal–temperate interface

This study follows up on an earlier case study evaluating fluxes of DOC from harvested (10 years post harvest) and unharvested black spruce forest plots (n=4), in evaluating the composition of the dissolved organic matter through the season (n=3).  A suite of solutes and DOM composition indices (absorbance spectrometry) were analyzed through four seasons.  The harvest treatment effects were quite muted in comparison to seasonal changes, which is perhaps surprising given the purportedly different litter inputs between a 10 year-old aggrading forest and a mature forest.  There was higher DON and DOC exported from organic horizons in the harvest treatment, but the effect size of this was small compared with seasonal changes.  In agreement with other work, this work also demonstrated the diagenesis of DOM in surface soils through the winter, owing to snow buffering soil temperatures.  The conclusions are, for the most part, supported by the work and the writing is clear and should be of broad interest. I offer minor comments by line number, and hope they are helpful in publishing this work. 

Line 35 (abstract): How does this study inform on climate change effects? 

Line 90-95 (and throughout): There is a bit of text explaining the consequences of DOM quality from the O horizons for stabilization in the mineral soil, which seems like conjecture for the present study, which did not characterize organo-metallic complexation or stabilization in the mineral soil.  I recommend tempering this.  Moreover, soil type (for example, Podzols vs. Cambisols) is very important in mediating these processes.

Line 125: Some detail on the type of soil (taxonomy would be great) is needed.  Even mean depths of L, F, H... type of parent material would be helpful.

Line 144: I thought there were four plots?

Line 170: Here, I was hoping to see some detail on typical snowpack for the region. 

Line 196: What's a typical peak snow water equivalent at the freshet? 

Line 213: It is too bad that a conservative ion wasn't measured to act as a "tracer" (like Cl, or Br), as flux is likely to go down in H, yet solute concentrations are likely to go up, detracting from nailing down the mechanism for any changes (or not) in export in response to harvest treatment that account for concentration or dilution effects. 

Line 362: It is interesting that summer fluxes were so high- as high as fluxes during the freshet!

Line 378: Interesting. Why would the snow be different by treatment? Is this owing to differences in throughfall?

Figure 3: Do you think part of the autumnal differences could be owing to changes in litterfall between the harvest treatments? 

Line 346: What is the mechanism for “inconsistent snowpack” increasing soil water fluxes?  Does this have to do with soil freezing, which has been shown to increase [DOC]?  I'm assuming that this is meant to prime the reader, and you'll get to this later?

Line 565: Could you please provide a reference for rhizodeposited DOM having LMW? 

Line 610: This is indeed very interesting, and has been observed in peatlands as well (a negative relationship between SUVA254 and DOM oxidation, as well as DOM molecular weight).  It could also be that relatively low molecular weight compounds are being polymerized (mainly by fungi). See for example, "polyphenol theory" described by F.J. Stevenson; "Humus Chemistry- genesis composition, reactions".  1994.  pp: 188-211.

Line 638: I think it is also relevant to mention the importance of texture and parent material type for DOM adsorption and infiltration.