Insights on Dissolved Organic Matter Production Revealed by Removal of Charge-Transfer Interactions in Senescent Leaf Leachates

Round 1

Reviewer 1 Report

This study examined the effect of borohydride reduction on the absorbance and fluorescence properties of leaf leachates, which represent source material for dissolved organic matter. Overall this is a well-conceived straightforward study that identified that fresh leaf litter leachates responded similarly to a strong reductant (borohydride), in terms of both absorbance losses and fluorescence increases, as monitored by optical indices, apparent quantum yields, and spectral changes. While the overall sample set is limited, the authors analyzed several aspects of their optical properties to fully characterize their optical changes upon reduction, and that they are sensitive to redox environments. This work adds to the body of literature on the evolution of DOM from source to downstream character. Some minor critiques are presented below.

Minor Comments:

"Apparent quantum yield" should be represented by Φ_app instead of simply Φ to highlight that what is being reported are apparent quantum yields.

Line 123-124: Since the leaves were fresh and filtering of leachate was only to 0.45 um (non-sterile), the authors should briefly address how further microbial processing during leaching or after filtration was prevented.

Line 145-148: should be rephrased, currently reads as if introducing formula for spectra slopes but instead introduces E2:E3 ratio formula in equation 2.

Line 174-175: There is not a strong explanation here as to why a variation >10% for n=3 demonstrates the need to eliminate the third data point in all experiments. Either a more standard outlier test (e.g., Grubbs) or more discussion as to the rationale for this choice needs to be present. 

Line 319-332: It may be useful in this section to include reference to the study by Murphy et al. 2014 (DOI: 10.1039/c3ay41935e), who reported that pure sodium salicylate overlaps extremely similarly to a "protein-like" PARAFAC component, highlighting that tryptophan is not the only compound that fluorescences in this region, and suggests that forest-derived phenolics may have similar characteristics.

Lines 334-337: Suggesting that degradation of leaf litter could degrade into a common signal regardless of material may be too bold of a claim for the data presented, given that only deciduous broadleaf tree leachates were sampled, and not also coniferous trees or shrubs. The limits of the study with respect to leaf sources should be acknowledged.

Refs: Check fonts refs 32,33

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors brought up an interesting topic to understand the origin of organic matter using optical measurements. The approached concept is a conventional technology for a fraction of electronic charge transfer between the molecular. However, they focused on freshly introduced dissolved organic matter. The approached method was relatively simple, but the interpretation was constructive and well organized for readers, in my opinion.  

 

Justify to use IHSS reference standards for comparing with your samples; the reference materials are humic acid and fulvic acid, and those are different with fresh leaf leachates.

 

Does UVB mean the ultraviolet and visible band?

 

L122-124: the description is unclear: homogenized fresh leaves?  Was it cut a specific size or grinding powder?

 

L179-182: a loss of absorption in the UV Why?

The authors brought up an interesting topic to understand the origin of organic matter using optical measurements. The approached concept is a conventional technology for a fraction of electronic charge transfer between the molecular. However, they focused on freshly introduced dissolved organic matter. The approached method was relatively simple, but the interpretation was constructive and well organized for readers, in my opinion.  

 

Justify to use IHSS reference standards for comparing with your samples; the reference materials are humic acid and fulvic acid, and those are different with fresh leaf leachates.

Does UVB mean the ultraviolet and visible band?

L122-124: the description is unclear: homogenized fresh leaves?  Was it cut a specific size or grinding powder?

L179-182: a loss of absorption in the UV Why?

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf