The Role of Iron Minerals in the Preservation of Organic Carbon in Mangrove Sediments

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors of the manuscript study binding of iron minerals to organic matter in different horizons of mangrove sediments associated with four plant communities in Zhanjiang Mangrove National nature reserve (Guangdong, China). Several physicochemical analysis methods were applied to estimate the mineral composition, organic matter, carbon isotope and iron contents in sediments, and fluxes of iron-bound organic matter were assessed. The study is interesting and contributes to the understanding of the carbon and iron cycles in mangrove estuaries.

However, I have several questions and comments that should be addressed by the authors before the manuscript could be recommended for publication.

Introduction/Methodology

The authors associate the sediments with certain plant communities, but take no notice of possible vegetation succession over time. The sampled sediment layer thickness (1 m) implies that this layer could be accumulated for hundreds of years, when the plant communities could have been different from the current ones. Any evidence against this supposition is welcome.

The authors use the term "iron-bound organic carbon (Fe-OC)" throughout the manuscript. However, it is much more likely that iron is bound to other elements in the organic matter, primarily, oxygen. Some reservations should be made in the manuscript regarding this.

Materials and Methods

Which elemental analyzer model was used for the OC quantitation?

What was the reference material for the δ¹³C calculation?

Were any measures taken to control the accuracy and quality of the measurements, especially of organic carbon and iron (intercalibration/external quality control, use of certified reference material, use of standard additions)?

Results and Discussion

In Figure 2 and caption to it, "geothite" should be substituted for "goethite"

In Figure 4, the y-axis label is not relevant to the histograms.

In L.168-170 the authors state the existence of significant differences between δ¹³C_Fe-OC and δ¹³C_bulk. However, no results of statistical tests for differences are provided although the whiskers are shown (by the way, it is not clear what they mean). The same applies to the corresponding bars in Figure 4.

In Section 3.5 the authors refer the molar ratio of Fe-OC/Fe to Figure 3f (the same ratio is mentioned in Section 4.2). However, it is the ratio Fe-OC/OC that is displayed in Figure 3f. The value of 0.8±0.02 is not readily apparent from this panel (in particular, the dispesion appears to be much higher). Furthermore, it is not clear what Fe means in the Fe-OC/Fe ratio, nor is the related discussion regarding it comprehensible. In particular, if Fe is fancied as the total iron concentration in sediments, then it is not clear why the ratio Fe-OC/Fe can be above 1.

L.197: What "mineral peaks" are in question?

L.198-201 (also L.289-291): "Aliphatic carbon was enriched at 1600~1750cm^-1 and 2800cm^-1~3000cm^-1 after extracting iron in the Kandelia obvolata plus Bruguiera gymnorrhiza sediments and Aegiceras corniculatum sediments, which may indicate that iron preferentially combined with aromatic carbon." - 1) Are these bands of uncomplexed aliphatic carbon or Fe-OC aliphatic carbon bands? 2) Do the authors know how aliphatic carbon bands are affected by combination with iron? 3) There is some logic failure in this statement. If these are Fe-OC aliphatic carbon bands, then one would expect that iron extraction will attenuate them (while the opposite pattern is seen in Figure 7); otherwise, if these are unbound aliphatic carbon bands, iron extraction would not affect them at all. 4) Unlike Results, Discussion (L.282-284) attributes the peak at 1600~1750cm^-1 to "amide, carboxylic acid and aromatic functional groups."

The next sentence in L. 201-204 should be revisited for the same reason.

In Results (Section 3.6) the authors mention Avicennia marina in connection with low tide. However, no references to the sediment sampling in relation to high or low tides were made in Materials and Methods.

L. 216: "tephra" is the correct spelling

It is necessary that the authors double check English language of the manuscript since it contains numerous flaws.

Comments on the Quality of English Language

Moderate corrections are required.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The text contains a number of mistakes and missing blank spaces (e. g. between numerical value and unit). There are also some sentences and phrases that are not comprehensible. In addition, the following issues should be discussed and added:

·      Page 4, line 122: The authors should provide additional information on the FTIR analysis (e. g. Which device was used? Which settings were selected?).

·      Page 9 (4. Discussion) line 240: The statement “insoluble Fe³⁺-organic ligand” has to be clarified/explained. Do the authors mean a complex compound (OC as ligand and Fe³⁺ as centre atom)?

·      The authors took a few random samples for each of the four mangrove species/sediments studied. It should therefore be discussed to what extent general statements can be derived from the results obtained.

 

The specific comments are summarized in the attached pdf file “Specific comments_water-2890877”.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

In some cases the use of English language could be improved through restructuring of sentences by a native English speaker.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have answered my questions and addressed my suggestions to a satisfactory extent. However, English of the manuscript still requires correction, and moreover, some language modifications introduced by the authors have even deteriorated its quality. I suggest that the manuscript be proofread by an English-speaking specialist.

Comments on the Quality of English Language

As noted, moderate corrections of the English are still required.

Author Response

Thanks. The English language of this manuscript has been polished by professional institutions