the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon
Bo Zhao
Amin Dou
Zhiwei Zhang
Zhenyu Chen
Wenbo Sun
Yanli Feng
Xiaojuan Wang
Abstract. Reactive iron (Fe) oxides are vital for long-term soil/sediment organic carbon (SOC) storage. However, the patterns and drivers of Fe‐associated organic carbon (Fe-OC) over global geographic scales under various ecosystem types remain largely controversial. Here, we provided for the first time a systematic assessment of the distribution patterns and determinants of Fe-OC content and its contribution (ƒFe-OC) by assembling a global dataset comprising 862 observations from 325 sites in distinct ecosystems. We found that Fe-OC content across global ecosystems ranged from 0 to 83.3 g/kg (ƒFe-OC ranged from 0 to 82.4 %), reflecting the high variability of the Fe-OC pool. Fe-OC contents varied with ecosystem type, being greater in wetlands with a high molar ratio of Fe-OC/dithionite-extractable Fe (Fed) compared with marine and continental ecosystems. Furthermore, ƒFe-OC in wetlands was significantly lower than that in other ecosystems due to rich OC. In contrast with climate variables and soil pH, the random forest modelling and multivariate analysis showed that the Fe-OC:Fed and SOC were the predominant predictors of Fe-OC content and ƒFe-OC in wetlands and continents, whereas Fed content was a primary driver in marine ecosystems. Based on upper estimates of global SOC storage in various ecosystem types, we further estimated that 83.84 ± 3.86 Pg, 172.45 ± 8.74 Pg, and 24.48 ± 0.87 Pg of SOC were preserved by association with Fe oxides in wetlands, continental and marine ecosystems, respectively. Taken together, our findings highlighted the importance of reactive Fe oxides in global SOC preservation, and their controlling factors were ecosystem-specific.
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Bo Zhao et al.
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RC1: 'Comment on bg-2023-59', Anonymous Referee #1, 20 Jul 2023
The manuscript entitled “Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon” discussed the spatial variability and characteristics of Fe-OC among continental, wetland and marine ecosystems and its governing factors globally. In this study, they highlighted the importance of reactive Fe oxides in global SOC preservation, and their controlling factors were ecosystem-specific. It is an interesting topic that should fit the scope of the journal. However, minor revision would be needed before a possible consideration of acceptance.
Major comments:
1. The author suggested that Fe-OC/Fed molar ratios less than 1 indicating an Fe-OC bonding form of monolayer surface sorption, and greater than 6 indicating a bonding mechanism dominated by coprecipitation in Line 98-101. So what does the Fe-OC/Fed molar ratio between 1 and 6 mean? Please provide references.
2. Line 153-155: Is the mean annual air temperature extracted from the WordClim database was taken in the same year as the sample collection? Please add something here.
3. Line 164-167: Why did you represent “treatment” and “control” in this way in the manuscript?
4. There are some ambiguous sentences in the manuscript. For example, 1) Since marine ecosystems do not have soil pH, the descriptions may be ambiguous in Line 208-210; 2) “The molar ratio of Fe-OC/Fed was positively correlated with Fe-OC and ƒFe-OC in three ecosystem types, except for ƒFe-OC in wetlands (Figs. 3e, m).” in Line 224-226. Check other places as well.
5. fFe-OC was significant differences between wetlands and marine (p < 0.001) and continent (p < 0.05) in Fig. 2b. Why were no significant difference observed in Line 197? However, the authors concluded that significant difference in ƒFe-OC was observed among different ecosystem types in discussion in Line 281-282. Please specify.
6. How did you estimate the data for SOC storage was bound to Fe oxides in different ecosystems in Line 288-292? Could you provide some calculation process to support this point of view?
7. Line 345-348: To further clarify the complexation of Fe with OC under reducing conditions. Please add the following references:
Patzner M S, Kainz N, Lundin E, et al. Seasonal fluctuations in iron cycling in thawing permafrost peatlands[J]. Environmental science & technology, 2022, 56(7): 4620-4631.
8. Line 338-339: “……, with <1 suggesting that the OC-Fe bonding form is dominated by simple mono-layer adsorption, while higher ratios indicating coprecipitation.” This sentence is confusing. Was the higher ratio >1 or > 6 in this sentence? However, you showed that the molar ratio >6, indicating a bonding mechanism dominated by coprecipitation in the intrdouction. Please specify.
9. Line418-428: It would have been more intuitive to have a table with comparisons to previous studies.
10. Figure7: 1) What do the different coloured triangles and squares represent? 2) There are some symbols and labelling errors. For example, “Sea” should be “marine”; “fF-OC” should be “fFe-OC”; 3) Wetlands are not a transitional phase between continent and marine, they are more like a terrestrial ecosystem, so why show them in the middle of continent and marine? Please check other places as well.
Minor comments:
1. Line 15: Fe‐associated organic carbon should be abbreviated as OC-Fe not Fe-OC. It should be changed throughout the paper.
2. Please standardize the unit format. For example, “g/kg” (Line 21) should be changed to “g kg-1”, and the formatting of the units in the article (e.g., mg g-1) and in the pictures (e.g., mg/g) was inconsistent.
3. Please check the format of “Fe-OC/dithionite-extractable Fe (Fed)” (e.g., Line 23) and “Fe-OC:Fed” (e.g., Line 26).
4. Line 26: Abbreviation for Random Forest.
5. The fFe-OC was repeated in Lines 60 and 75. In addition, the explanation of fFe-OC should be advanced in Line 75.
6. Line 86. Mean annual temperature modified to mean annual air temperature. MAT modified to MAAT. It should be changed throughout the paper.
7. Line 151, Table S1 is not in your manuscript.
8. Line 155: “version” should be “v”. In addition, the web address should be added. Check other places as well.
9. Line 222: “Fig. 3O” should be “Fig. 3o”.
10. There are many formatting errors, such as the space between a number and a symbol on page 15, Line 287. Please check other places as well.
11. Line 338: Here, it’s OC-Fe. Please standardise usage in the manuscript.
12. The letter case of figures in the manuscript should be resized.
13. Figure 2: 1) What does the legends (e.g., a, b, and c) mean? 2) Panels (a), (b), (c), (d), (e), and (f) were shown on different pages. Check other places as well.
14. Figure 3 needs to be re-optimized. For example, R, p and the image overlap. Check other places as well.-
AC1: 'Reply on RC1', Qiang Wang, 07 Sep 2023
Response of Authors on Manuscript Entitled: “Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon (bg-2023-59)”
Dear Editors and Reviewers,
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon (bg-2023-59)”. Those comments are all valuable and very helpful for revising and improving our paper and of important guiding significance to our research. We have studied comments carefully and have made corrections which we hope meet with approval. Revised portion is marked in yellow in the paper. The main corrections in the paper and the responds to the reviewers’ comments are as following:
Response to the First Referee (Reviewer #1):
(With brown characters as the original comments and suggestions)
----------------------------------------------------------------------------------
- The manuscript entitled “Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon” discussed the spatial variability and characteristics of Fe-OC among continental, wetland and marine ecosystems and its governing factors globally. In this study, they highlighted the importance of reactive Fe oxides in global SOC preservation, and their controlling factors were ecosystem-specific. It is an interesting topic that should fit the scope of the journal. However, minor revision would be needed before a possible consideration of acceptance.
{Response} Thank you to the reviewers for their positive comments and feedback. We have individually responded to each comment.
- The author suggested that Fe-OC/Fed molar ratios less than 1 indicating an Fe-OC bonding form of monolayer surface sorption, and greater than 6 indicating a bonding mechanism dominated by coprecipitation in Line 98-101. So what does the Fe-OC/Fed molar ratio between 1 and 6 mean? Please provide references.
{Response} Thank you for your review. We have provided the corresponding reference. Wagai et al. (2007) first pointed out that the maximum adsorption capacity of FeOx phase on OC is 0.22 g-OC g-Fe-1 (approximately equal to 1 Fe-OC/Fed molar ratios). The mass ratio of OC:Fe ranges from 1.3 to 2.2 (approximately 6-10 Fe-OC/Fed molar ratios), forming organic metal complexes (precipitated complexes). Similarly, Lalonde et al. (2012) further supported and confirmed that the simple sorption of organic matter on reactive iron oxide surfaces results in a maximum molar ratio of organic carbon to iron (OC:Fe) of 1.0 for the co-extracted species, based on the maximal sorption capacity of reactive iron oxides for natural organic matter. However, co-precipitation and/or chelation of organic compounds with iron generates low-density, organic-rich structures with OC:Fe ratios between 6 and 10. Recent study by Wang et al. (2017) suggested that the molar ratio of Fe-bound SOC:Fed was used as an indicator of Fe–OC interaction type, which is <1.0 for sorption and >6.0 for co-precipitation. It is generally accepted that adsorption and coprecipitation coexist when the Fe-OC/Fed molar ratio between 1 and 6.
References:
Lalonde, K., Mucci, A., Ouellet, A., Gelinas, Y., 2012. Preservation of organic matter in sediments promoted by iron. Nature 483, 198-200.
Wagai, R., Mayer, L.M., 2007. Sorptive stabilization of organic matter in soils by hydrous iron oxides. Geochimica et Cosmochimica Acta 71, 25-35.
Wang, Y., Wang, H., He, J.S., Feng, X.J., 2017. Iron-mediated soil carbon response to water-table decline in an alpine wetland. Nature Communications 8, 1-9.
- Line 153-155: Is the mean annual air temperature extracted from the WordClim database was taken in the same year as the sample collection? Please add something here.
{Response} Thank you for your valuable and thoughtful comments. The data obtained from the WorldClim database is the average climate data from 1970 to 2000, which has been supplemented in the article. (see Page 9 Lines 155-156 in the revised manuscript). Furthermore, most articles have reported annual average temperature and precipitation data, with only a small number of literature sources collecting from the WorldClim database.
- Line 164-167: Why did you represent “treatment” and “control” in this way in the manuscript?
{Response} Thank you for your valuable comments. We will compare the average values of Fe-OC in all articles as a “control”, and the average values of individual cases as a “treatments”. By calculating the effect size, we can know the difference and contribution between a single case and the overall mean. (see Fig. S2 in the Supporting Materials)
- There are some ambiguous sentences in the manuscript. For example, 1) Since marine ecosystems do not have soil pH, the descriptions may be ambiguous in Line 208-210; 2) “The molar ratio of Fe-OC/Fed was positively correlated with Fe-OC and ƒFe-OC in three ecosystem types, except for ƒFe-OC in wetlands (Figs. 3e, m).” in Line 224-226. Check other places as well.
{Response} Thank you for your valuable comments. We have reworded these sentences according to your suggestion. We have changed sentence “Taken together, the Fe-OC, SOC, Fe-OC/Fed molar ratio, and soil pH were significantly higher in wetlands, with the lowest values in marine ecosystems across global ecosystem types.” to “Taken together, the Fe-OC, SOC, and Fe-OC/Fed molar ratio were significantly higher in wetlands, with the lowest values in marine ecosystems across global ecosystem types.”. “The molar ratio of Fe-OC/Fed was positively correlated with Fe-OC and ƒFe-OC in three ecosystem types, except for ƒFe-OC in wetlands” to “The molar ratio of Fe-OC/Fed was positively correlated with Fe-OC and ƒFe-OC in other ecosystems, except for ƒFe-OC which not correlated with the molar ratios in wetlands.”. (see Page 12 Lines 213-214; 229-231 in the revised manuscript)
- fFe-OC was significant differences between wetlands and marine (p < 0.001) and continent (p < 0.05) in Fig. 2b. Why were no significant difference observed in Line 197? However, the authors concluded that significant difference in ƒFe-OC was observed among different ecosystem types in discussion in Line 281-282. Please specify
{Response} Thank you for your valuable and thoughtful comments. We have modified it according to your suggestion as follows: “Correspondingly, the contribution of Fe-OC to SOC (fFe-OC) was significantly different among different ecosystem types (p < 0.05; Fig. 2b).” (see Page 11 Lines 200-201 in the revised manuscript)
- How did you estimate the data for SOC storage was bound to Fe oxides in different
ecosystems in Line 288-292? Could you provide some calculation process to support this point of view?
{Response} Thank you for your valuable comments. We estimated Fe-OC stocks by multiplying the SOC stock of each ecosystem by the fFe-OC in the corresponding ecosystem. We have added the relevant calculation methods in the Materials and Methods section: “We also collected global data on SOC stocks in terrestrial, wetland and marine ecosystems, respectively, which will allow us to further estimate Fe-OC stocks in different ecosystems”. (see Pages 8-9 Lines 145-147 in the revised manuscript)
- Line 345-348: To further clarify the complexation of Fe with OC under reducing conditions. Please add the following references:
Patzner M S, Kainz N, Lundin E, et al. Seasonal fluctuations in iron cycling in thawing permafrost peatlands[J]. Environmental science & technology, 2022, 56(7): 4620-4631.
{Response} We agree with your comments and have cited this reference. We added references: Patzner M S, Kainz N, Lundin E, et al. Seasonal fluctuations in iron cycling in thawing permafrost peatlands. Environmental Science & Technology, 2022, 56(7): 4620-4631. (see Page 18 Line 351 in the revised manuscript)
- Line 338-339: “……, with <1 suggesting that the OC-Fe bonding form is dominated by simple mono-layer adsorption, while higher ratios indicating coprecipitation.” This sentence is confusing. Was the higher ratio >1 or > 6 in this sentence? However, you showed that the molar ratio >6, indicating a bonding mechanism dominated by coprecipitation in the introduction. Please specify.
{Response} Thank you for your valuable comments. We have improved the expression according to your suggestion. “Numerous studies have shown that the Fe-OC:Fed acts as an indicator of Fe/OC interaction types (Lalonde et al., 2012; Wang et al., 2017), with <1 suggesting that the Fe-OC bonding form is dominated by simple mono-layer adsorption, while higher molar ratios (>6) indicating coprecipitation (Wagai and Mayer, 2007; Faust et al., 2021).” (see Pages 17-18 Lines 340-344 in the revised manuscript)
References:
Huang, X., Liu, X., Liu, J., Chen, H., 2021. Iron-bound organic carbon and their determinants in peatlands of China. Geoderma 391.
Lalonde, K., Mucci, A., Ouellet, A., Gelinas, Y., 2012. Preservation of organic matter in sediments promoted by iron. Nature 483, 198-200.
Wang, Y., Wang, H., He, J.S., Feng, X., 2017. Iron-mediated soil carbon response to water-table decline in an alpine wetland. Nature Communications 8, 15972.
- Line 418-428: It would have been more intuitive to have a table with comparisons to previous studies.
{Response} Thank you for your valuable and thoughtful comments. In accordance with your suggestion, we have added a table about molar ratios of Fe-OC:Fed for comparison with previous studies. (see Supporting Data Set in the revised manuscript)
Table 1. The molar ratios of Fe-OC:Fed across marine sediments and wetland ecosystems.
Location
Fe-OC：Fed
References
East China Sea
1.53 ± 1.28
Ma et al., 2018
South Yellow Sea, China
1.68 ± 1.80
Ma et al., 2018
Bohai Sea, China
1.59 ± 1.37
Wang et al., 2018
Barents Sea
2.56 ± 1.76
Faust et al., 2020, 2021
Arctic shelf
3.04 ± 1.73
Salvadó et al., 2015
Intermediate/old river delta
5.02 ± 5.85
Shields et al., 2016
Changjiang Estuary/East China Sea shelf
0.23 ± 0.14
Zhao et al., 2018
Margin sea, China
2.75 ± 3.07
Sun et al., 2017
Mississippi River
2.76 ± 1.50
Ghaisas et al., 2021
Global oceans (n = 42)
6.10 ± 7.5
Lalonde et al., 2012
Global oceans (n = 320)
2.56 ± 0.19
This study
Martinique Beach, Canada
0.4 ± 0.7
Sirois et al., 2018
Min River Estuary, China
11.0 ± 4.5
Bai et al., 2021
Petland, China
6.23 ± 3.34
Huang et al., 2021
Freshwater wetland of Sanjiang Plain, China
2.24 ± 1.52
Duan et al., 2020
Boreal lake sediment
5.92 ± 3.34
Peter et al., 2018
Permafrost peatland
0.26 ± 0.09
Wang D et al., 2021
Large-Scale wetlands (19.96˚–52.04˚N, 87.44˚E–132.33˚E)
12.62 ± 11.46
Wang S et al., 2021
Drained thaw lake basins near Utqiaġvik
7.40 ± 5.18
Joss et al., 2022
Delmarva Peninsula in the eastern U.S.A
20.69 ± 27.89
Kottkamp et al., 2022
Mt. Shen Nong Jia, China
84.75 ± 111.95
Zhao et al., 2019
Global wetlands (n = 251)
13.47 ± 1.81
This study
- Figure7: 1) What do the different coloured triangles and squares represent? 2) There are some symbols and labelling errors. For example, “Sea” should be “marine”; “fF-OC” should be “fFe-OC”; 3) Wetlands are not a transitional phase between continent and marine, they are more like a terrestrial ecosystem, so why show them in the middle of continent and marine? Please check other places as well.
{Response} Thank you for your valuable and thoughtful comments. We have corrected and improved the Figure7. Different colored triangles and squares represent SOC molecular diversity. We have corrected the symbol and labeling errors as you suggested. Wetlands encompass a broad range of ecosystems ranging from submerged coastal grass beds to salt marshes, swamp forests, and boggy meadows. In this study, the term “wetlands” refers to transition zones between terrestrial and aquatic systems with soils water saturated for at least part of the year or covered by shallow water. In this study, wetland ecosystem included coastal wetlands (for instance, mangrove wetland and tidal wetland) and inland wetlands (for instance, alpine wetland and peatland); Aquatic ecosystem mainly refers to marine and freshwater ecosystems, and the data of freshwater systems in this manuscript are scarce and dominated by marine systems.
Fig. 7 Schematic representation of drivers, dynamic and patterns of Fe-OC associations in different ecosystem types on global scale. The wetland ecosystem included coastal wetlands and inland wetlands; aquatic ecosystem mainly refers to marine and freshwater ecosystems, but the data of freshwater systems in this study are scarce and dominated by marine systems. Data are averages of different ecosystem types. Different coloured triangles and squares represent SOC molecular diversity. A lower SOC molecular diversity and concomitant lower contents of Fe-OC (e.g., terrestrial and aquatic ecosystems), whereas higher diversity increases the Fe-OC contents (e.g., wetlands). Meanwhile, there was a significant difference in the proportion of Fe-OC in total SOC (fFe-OC). The asterisk (*) indicates significant differences.
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AC1: 'Reply on RC1', Qiang Wang, 07 Sep 2023
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RC2: 'Comment on bg-2023-59', Anonymous Referee #2, 05 Aug 2023
The Fe-associated organic carbon is a key component to long-term soil organic carbon (SOC) stocks, making it essential to gain a global understanding of its patterns and drivers. However, there are certain limitations in the current analysis approach. The classification of global ecosystems into continental, wetland, and marine ecosystems lacks precise definitions. Here, "continental" is commonly defined on a geographic scale (e.g., Guerin et al., 2020). Does the term "continental ecosystem" refer to the terrestrial ecosystem? Additionally, the term "wetland ecosystem" in this article includes both freshwater and coastal or estuarine wetland ecosystems. It is important to note that coastal or estuarine wetlands have unique characteristics of salinity. While it is generally believed that Fe complexes may be less affected by increasing salinity, it is dependent on particle size actually (Simon, 2018). Also, salinity can affect Fe transport capacity (Herzog et al., 2019). As such, it is recommended to separate the analysis into freshwater and saline wetlands for more accurate characterization.
Citation: https://doi.org/10.5194/bg-2023-59-RC2 -
AC2: 'Reply on RC2', Qiang Wang, 07 Sep 2023
Response of Authors on Manuscript Entitled: “Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon (bg-2023-59)”
Dear Editors and Reviewers,
Thank you for your letter and for the reviewers’ comments concerning our manuscript entitled “Ecosystem-specific patterns and drivers of global reactive iron mineral-associated organic carbon (bg-2023-59)”. Those comments are all valuable and very helpful for revising and improving our paper and of important guiding significance to our research. We have studied comments carefully and have made corrections which we hope meet with approval. Revised portion is marked in yellow in the paper. The main corrections in the paper and the responds to the reviewers’ comments are as following:
Response to the Second Referee (Reviewer #2)
(With brown characters as the original comments and suggestions)
----------------------------------------------------------------------------------
1. The Fe-associated organic carbon is a key component to long-term soil organic carbon (SOC) stocks, making it essential to gain a global understanding of its patterns and drivers. However, there are certain limitations in the current analysis approach. The classification of global ecosystems into continental, wetland, and marine ecosystems lacks precise definitions. Here, "continental" is commonly defined on a geographic scale (e.g., Guerin et al., 2020). Does the term "continental ecosystem" refer to the terrestrial ecosystem?
{Response} Thank you for your valuable and thoughtful comments. According to your suggestion, we have revised the “continental ecosystem” to “terrestrial ecosystem” in the whole manuscript. We agree that “continental” is usually defined in terms of geographical scale, so the expression “continental scale” in the article remains.2. Additionally, the term "wetland ecosystem" in this article includes both freshwater and coastal or estuarine wetland ecosystems. It is important to note that coastal or estuarine wetlands have unique characteristics of salinity. While it is generally believed that Fe complexes may be less affected by increasing salinity, it is dependent on particle size actually (Simon, 2018). Also, salinity can affect Fe transport capacity (Herzog et al., 2019). As such, it is recommended to separate the analysis into freshwater and saline wetlands for more accurate characterization.
{Response} Thank you for your valuable and thoughtful comments. We agree that salinity can affect the Fe-OC and ƒFe-OC. The higher the salt concentration in coastal wetlands, the higher the Fe-OC and ƒFe-OC (Bai et al., 2021). However, there are few research cases on Fe-OC and ƒFe-OC in coastal wetlands. Among the 17 wetland articles collected in this article, only four involve coastal wetland (the rest are freshwater wetlands), and the sample size is small. With the increasing number of research articles on Fe-OC in coastal wetlands, it is possible to separate coastal wetlands from freshwater wetlands in the future to further discuss the effects of salinity on Fe-OC.
References:
Bai, J., Luo, M., Yang, Y., Xiao, S., Zhai, Z., Huang, J., 2021. Iron-bound carbon increases along a freshwater-oligohaline gradient in a subtropical tidal wetland. Soil Biology and Biochemistry 154.
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AC2: 'Reply on RC2', Qiang Wang, 07 Sep 2023
Bo Zhao et al.
Bo Zhao et al.
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