the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Aug 2021
Prediction of the vertical scaling of soil organic carbon in temperate forest soils using percolation theory
Abstract. Forest soil stores a large portion of soil organic carbon (SOC), making it one of the essential components of global carbon cycling. There is apparent spatial variability of SOC in forest soils, but the mechanism that regulates the vertical pattern of SOC is still not clear. Understanding the vertical distribution as well as the transport process of SOC can be of importance in developing comprehensive SOC models in forest soils, as well as in better estimating terrestrial carbon cycling. We propose a theoretical scaling derived from percolation theory to predict the vertical scaling of SOC with soil depth in temperate forest soils, with the hypothesis that the content of SOC along soil profile is limited by the transport of solute. The powers of the vertical scaling of 5 published datasets across different regions of the world are −0.920, −1.097, −1.196, −1.062, and −1.038, comparing with the theoretical value of −1.149. Field data from Changbai Mountain region, Jilin, China, with spatial variation of SOC correlating strongly to temperature, precipitation, and sampling slope is constrained well by theoretical boundaries predicted from percolation theory, indicating that the vertical transport so as the content of SOC along soil profile is limited by solute transport, which can be described by percolation theory in both small and large scales. Prediction of SOC content in Changbai Mountain region based on an estimated SOC content at 0.15 m from available data demonstrates a good agreement with field observation, suggesting the potential of collaborating the presented model with other surface soil models to predict SOC storage and carbon cycling in temperate forest soils.
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RC1: 'Comment on soil-2021-84', Anonymous Referee #1, 06 Oct 2021
The authors of this paper attempted to predict the distribution of soil organic carbon (SOC) in temperate forest soil profiles using percolation theory, a theory commonly used to describe material transport in disordered porous media. Soil is an inhomogeneous medium composed of complex components. From the perspective of medium, it is feasible to apply this theory. However, SOC is not a single substance and has very complex forms: particulate, dissolved and adsorbed. Therefore, if the authors want to achieve the goal in the title, they should study SOC in different forms separately. In this paper, the soil profile of some typical forestlands in Changbai Mountain was measured by stratified soil sampling, and SOC was studied as a solute index.The study design is unreasonable. The measurement index is simple and single, and the workload is insufficient. The core viewpoint of the study is seriously inconsistent with the vertical distribution mechanism of SOC on profiles. Therefore, it is recommended not to be published.Specific comments are as follows:
1) The vertical distribution of soil organic carbon (SOC) in forest ecosystems has two meanings: one is vertical distribution along elevation and the other is vertical distribution in a profile. In order to avoid ambiguity, I suggested that the author emphasize vertical distribution on profile in the title and article (L1, 8...)..
2) L26-32 talks about the influence of vertical factors on soil organic carbon. Vertical factors here refer to the influence of altitude rather than the vertical depth of the profile studied in this paper.Should be deleted!
3) L76-80 mentioned that the vertical transport of SOC is mainly in the form of DOC leaching, so why not take DOC as the research index in this paper?
4) The full names of the six stand types of L100 should appear before giving abbreviations.
5) L115 there were only two replicates for each forest at the standard standscale, which was not enough for field experiments.
6) L124-125 soil samples were only screened through 10 mesh (1.7mm), while the standard method for determining SOC needed soils been through 100 mesh (0.15mm).
7) L130-137 This paper only selected the data of 5 published articles for fitting analysis, which could not constitute a reasonable meta-analysis.
8) L207-212 Here, spatial variability, environmental factors and other unrelated topics are discussed.
9) L215-216 There is no need to emphasis the dominant species in stands.
10) The goodness of fit of curves in the results of L219-220 is not high, and the tolerance range in Table 3 is very wide, so almost indicators of the forest soils reduced along the profile depth can appear this curve, such as fine root biomass and microbial biomass, which are not distributed according to the principle of percolation theory.Therefore, the method of this paper is not appropriate.
11) 226-227 The shallow soil was more disturbed by the external environment, while the SOC in the deep soil was mainly from the roots. The influence range of litter was generally considered to be in the Leaching - deposition (B) layer. The deeper soil has more adsorption sites, so the vertical transport of DOC in the soil is also affected by soil adsorption.
To sum up, the design of this study is unreasonable; the measurement index is single; and the workload is insufficient. It is suggested that the author increase the workload and select reasonable indicators such as DOC and mineral nitrogen for the study.
Citation: https://doi.org/10.5194/soil-2021-84-RC1 -
AC1: 'Reply on RC1', Chunnan Fan, 05 Feb 2022
Dear reviewer,
Thanks for the comments. In response, we will make several changes according to the suggestions and criticisms，Our point-by-point response to the reviewers’ comments is given below. In most points we agree with the reviewer but not for all. Our point to point responds are in italic type.
1) The vertical distribution of soil organic carbon (SOC) in forest ecosystems has two meanings: one is vertical distribution along elevation and the other is vertical distribution in a profile. In order to avoid ambiguity, I suggested that the author emphasize vertical distribution on profile in the title and article (L1, 8...)
This will be addressesd.
2) L26-32 talks about the influence of vertical factors on soil organic carbon. Vertical factors here refer to the influence of altitude rather than the vertical depth of the profile studied in this paper. Should be deleted!
We are confused here, since L26-32 didn't mention any point relating to the influence of altitude...
3) L76-80 mentioned that the vertical transport of SOC is mainly in the form of DOC leaching, so why not take DOC as the research index in this paper?
The main hypothesis of the study is that the vertical transport of the percolating water (mainly in the form of DOC) redistributes SOC. DOC could be transformed in other forms and such as taken up by microbes, which we don't think is a stable and good indicator.
4) The full names of the six stand types of L100 should appear before giving abbreviations.
This will be addressed.
5) L115 there were only two replicates for each forest at the standard standscale, which was not enough for field experiments.
There were 3 replicates for each subsite. Here P1 and P2 are two subsites locating in one forest but with different sampling slopes. We should've mentioned the number of replicates here. We apologize for the confusion.
6) L124-125 soil samples were only screened through 10 mesh (1.7mm), while the standard method for determining SOC needed soils been through 100 mesh (0.15mm).
This is a typo, it was screened through 100 mesh. We apologize for the careless.
7) L130-137 This paper only selected the data of 5 published articles for fitting analysis, which could not constitute a reasonable meta-analysis.
One of the dataset referenced from Jobbágy and Jackson (2000) are averaged values from 3 global databases of soil profile in temperate forest soils (National Soil Characterization Database produced by the U.S. Department of Agriculture, World Inventory of Soil Emission Potential Database, and the Canadian Forest Service) which cover 60 samples for the temperate deciduous forest, and 123 for the temperate evergreen forest across the world.
8) L207-212 Here, spatial variability, environmental factors and other unrelated topics are discussed.
The purpose of this paragraph is to demonstrate that the surface SOC strongly correlates to environmental factors in our sampling sites, and that the vertical distribution of SOC on profile is constrained by the transport of water and it is decoupled with the environmental factors. But we do agree with the reviewer, this paragraph can be shortened into 1-2 sentences.
9) L215-216 There is no need to emphasis the dominant species in stands.
This will be addressed.
10) The goodness of fit of curves in the results of L219-220 is not high, and the tolerance range in Table 3 is very wide, so almost indicators of the forest soils reduced along the profile depth can appear this curve, such as fine root biomass and microbial biomass, which are not distributed according to the principle of percolation theory. Therefore, the method of this paper is not appropriate.
The results mentioned in L219-230 covers results from different regions of the world, including 3 global dataset with 60 samples from the temperate deciduous forest, and 123 from the temperate evergreen forest, and 3 sites in China and 1 in Germany. The deviations are 20%, 4.5%, 4%, 7.6% and 9.7%. There is no range mentioned in Table 3.We think what the reviewer meant here is the range in Figure 3. L174-179 explained how we determined the boundary. And Figure 5 demonstrates the agreement of prediction and the averaged values from Figure 3 in each layer of soil. There are many indicators in forest soils that accumulate more in the surface and show curving trend as soil goes deeper, but the exponents of the curving can be various. However, here we demonstrate that the exponent of the SOC trend agrees with the proposed value from percolation theory, which is -1.149.
11) 226-227 The shallow soil was more disturbed by the external environment, while the SOC in the deep soil was mainly from the roots. The influence range of litter was generally considered to be in the Leaching - deposition (B) layer. The deeper soil has more adsorption sites, so the vertical transport of DOC in the soil is also affected by soil adsorption.
We agree with the reviewer's point. The hypothesis of the study is that the downword movement of water, with DOC as the solute, redistributes SOC on soil profile. root is a source of carbon input, there are studies show that the SOC profile is deeper than root profile, and that the downward transport of DOC along the soil profile is the potential driver of the redistribution of carbon in the soil. Previous studies have shown that percolation theory can describe the chemical weathering rate becasue the reaction rate is limited by solute transport , which means that reaction is in equilibrium along the track, and the reaction rate is porpotional to solute velocity. Similar to chemical weathering, adsorption and desorption can be limited by solute transport as well if the solute travels too slow. However, here the reviewer pointed a good point which we should dig deeper to see the adsorption and desorption has the same situation as chemical weathering in the subsurface.
Citation: https://doi.org/10.5194/soil-2021-84-AC1
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AC1: 'Reply on RC1', Chunnan Fan, 05 Feb 2022
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RC2: 'Comment on soil-2021-84', Anonymous Referee #2, 14 Jan 2022
GENERAL COMMENTS
The manuscript deals with an important topic to highlight the role of soil in the global C budget. As a matter of fact, the soil organic carbon (SOC) in forest ecosystems could suppose 704 Pg C, up to the 43% of the total SOC stock, which is considered the largest pool of the terrestrial C (Lal, 2005). The mathematical background It’s suitable and well explained, the results are properly presented and consistent with the hypothesis and the conclusions.
However, my main concern is related with the true scientific scope of this work. Beyond the metadata, the dataset for an original research paper is rather poor (60 composite samples with only a standard analytical routinely procedure –potassium dichromate oxidation-). In my opinion, it would be much more relevant to advance the knowledge of the distribution at depths > 1 meter, since there are evidences this could represent a significant additional stock and it’s poor described in forests. Did the authors consider take samples deeper in the soil? Moreover, I have some troubles with the hypothesis put forward by the authors of the leaching of C as the main driver of C vertical pattern (see specific comments).
SPECIFIC COMMENTS
Introduction
L48: “coarse”
L49: Please enter the citation of “Trumbore et al., 2006” in references.
L50: As far as I read, neither Gill nor Joslin study the total SOC in the subsurface layer of soil.
L53-57: I’m not as optimistic as the authors about the vital importance of C leaching as a potential driver for the redistribution of C in the soil. For example, Jobbagy and Jackson (2000, p. 433) consider the decrease of SOC turnover with depth a much more plausible explanation (in fact, they find an inverse trend between precipitation and SOC depth). I would recommend that the authors consider other possible explanations in the manuscript (see also comment to L221 – 224).
Material and Methods:
L81: Doesn't the percolation theory also depend on soil texture? The physical process of water infiltration is highly dependent on it (eg. hydraulic saturated conductivity).
L82. The study of Sheppard et al. does not seem refer to the transport of SOC in temperate forest. Please, modify the sentence or move the reference elsewhere.
Table 2: Please, indicate here the soil type (WRD or Soil Survey Staff classifications). This could be interesting to understand the vertical distribution of SOC.
Table 2: Are these temperature the annual average temperatures? I find these (< 5ºC) too much low for a broad-leaved deciduous forest. Please, indicate the Koppen climate classification.
L122: Why was the C horizon not sampled?
Results
Figure 1b: I think the point “TS-P1” is really CS-P1.
L163 – 169 and Figure 2: The effect of comparing a 12% slope (CS-P) vs. 17% (LX-P2) does not seem very convincing… In my opinion, it is very difficult to assess the relative influence of both factors (slope and precipitation) on the SOC.
Table 4: It would be useful to show the mean SOC value for the first meter.
L195: “temperate”
Discussion and conclusions
L221 – 224. It is true that the empirical data fit well as predicted by the percolation theory. However, I’m not clear about the relationship between this theory (beyond its name, it is a statistical theory that can be applied to many non-hydraulic phenomena, as traffic jams!) and the soil infiltration process. Could it not be also explained by other "power-law" soil processes such as roots decomposition rate or microbial activity? (see, eg., the vertical distribution of Soil Microbial Biomass Carbon showed by Sun et al., 2020). In my opinion, it would be needed to provide some additional evidence linking percolation theory with the physical process of water infiltration in soils.
Cited reference:
Lal, R., 2005. Forest soils and carbon sequestration. Forest Ecology and Management 220, 242 – 258.
Sun, T., Wang, Y., Hui, D., Jing, X., Feng, W., 2020. Soil properties rather than climate and ecosystem type control the vertical variations of soil organic carbon, microbial carbon, and microbial quotient. Soil Biology and Biogeochemistry 148, 107905.
Citation: https://doi.org/10.5194/soil-2021-84-RC2 -
AC2: 'Reply on RC2', Chunnan Fan, 05 Feb 2022
Dear reviewer,
Thanks for the comments. In response, we will make several changes according to the suggestions and criticisms. Our point-by-point response to the reviewers’ comments is given below. In most points we agree with the reviewer but not for all.
1) L48: “coarse”.
This will be addressed. We apologized for the typo.
2) L49: Please enter the citation of “Trumbore et al., 2006” in references.
This will be addressed.
3) L50: As far as I read, neither Gill nor Joslin study the total SOC in the subsurface layer of soil.
We presented incorrectly here. What we attempted to present here is that studies from Gill and Joslin suggest that higher root litter inputs generally occur in the upper soil layers.
4) L53-57: I’m not as optimistic as the authors about the vital importance of C leaching as a potential driver for the redistribution of C in the soil. For example, Jobbagy and Jackson (2000, p. 433) consider the decrease of SOC turnover with depth a much more plausible explanation (in fact, they find an inverse trend between precipitation and SOC depth). I would recommend that the authors consider other possible explanations in the manuscript (see also comment to L221 – 224).
Root growth is limited by nutrients and water, there are evidences that root growth also follows the percolation theoretical scaling (Figure below, referenced from Fig.5 in Hunt, 2016). The turnover of SOC affected by microbe activities is certainly a factor, and we agree with the reviewer that it should be considered in the manuscript.
Reference: Hunt (2016). Spatio-temporal scaling of vegetation growth and soil formation from percolation theory. Vadose zone journal VZJ.
5) L81: Doesn't the percolation theory also depend on soil texture? The physical process of water infiltration is highly dependent on it (eg. hydraulic saturated conductivity).
Correct. The more useful form of describing solute transport in soil is (t/t0)(x/x0)Db, where x0 is the pore size, which depends on the soil texture, and t0 is the time it takes for solute to pass a single pore, which equals to x0/v0, and v0 is the infiltration rate. That means that water infiltrates soils with different textures (with same saturation condition) will show parallel lines on the log-log plot (time versus distance) but with different intercepts. Here in the proposed paper, we demonstrates the power-law form of SOC distribution along soil profile, and the determination of the intercept still needs more investigation.
6) L82. The study of Sheppard et al. does not seem refer to the transport of SOC in temperate forest. Please, modify the sentence or move the reference elsewhere.
Good point. We referenced the value of Db in 3D saturated conditions from Sheppard et al. But the original presentation was confusing. This will be addressed.
7) Table 2: Please, indicate here the soil type (WRD or Soil Survey Staff classifications). This could be interesting to understand the vertical distribution of SOC.
The main soil types are sandy loam and loamy clay in the region.
8) Table 2: Are these temperature the annual average temperatures? I find these (< 5ºC) too much low for a broad-leaved deciduous forest. Please, indicate the Koppen climate classification.
Correct. Temperature showed in Table 2 is the annual average values. All study sites are temperate continental climate.
9) L122: Why was the C horizon not sampled?
There have been conflicts of the definition of "soil depth".
From previous work applying percolation theory to soil formation, "soil depth" refers to a particular weathering horizon like the Bw (e.g. Hunt, 2015). Here, to apply the same theoretical framework, we follow the same criteria.
Reference: Hunt, A. G.: Soil depth and soil production. Complexity, doi: 10.1002/cplx.21664, 2015a.
10) Figure 1b: I think the point “TS-P1” is really CS-P1.
The main point here in Figure 1b is to present effects of precipitation on SOC content. CS-P1 and LX-P1 have similar temperatures and slopes, but different precipitations. TS-P1 is not appropriate to present here because except for different temperatures, the sampling slopes are not close neither.
11) L163 – 169 and Figure 2: The effect of comparing a 12% slope (CS-P) vs. 17% (LX-P2) does not seem very convincing… In my opinion, it is very difficult to assess the relative influence of both factors (slope and precipitation) on the SOC.
The sampling slope is in "degree". We attempted to explain the effect of topography on SOC content, and more precipitation and lower sampling slope seemed favor the conservation of water, which has a positive feedback on SOC production. When comparing JL-P1 and HG-P1, more precipitation in JL-P1 didn't result in higher SOC since the sampling slope is steeper.
12) Table 4: It would be useful to show the mean SOC value for the first meter.
This will be addressed
13) L195: “temperate”
This will be addressed. We apologized for the typo.
14) L221 – 224. It is true that the empirical data fit well as predicted by the percolation theory. However, I’m not clear about the relationship between this theory (beyond its name, it is a statistical theory that can be applied to many non-hydraulic phenomena, as traffic jams!) and the soil infiltration process. Could it not be also explained by other "power-law" soil processes such as roots decomposition rate or microbial activity? (see, eg., the vertical distribution of Soil Microbial Biomass Carbon showed by Sun et al., 2020). In my opinion, it would be needed to provide some additional evidence linking percolation theory with the physical process of water infiltration in soils.
The basic theoretical form of solute transport described by percolation theory is x is propotional to tDb, where x is the transport distance, t is the travel time, and Db is the fractional dimensionality of percolation backbone, which is a known value that only dependents on dimensionality and the saturation condition. First figure below (from Figure f, Hunt et al., 2020) show the fitness of theoretical scaling of percolation theory and chemical weathering rate data observed either from the field or from laboratory, though it is not the direct comparison of water velocity and the theory, chemical weathering are water limited in subsurface, and it is proportional to flow velocity (as shown in the second figure below(referenced from Figure1 in the same paper) ) . We think it is convinced evidence that percolation theory can describe solute transport in soil.
Reference: Hunt, Faybishenko, Ghanbarian, Egli, & Yu. (2020). Predicting water cycle characteristics from percolation theory and observational data. International Journal of Environmental Research and Public Health, 17(3), 734.
Citation: https://doi.org/10.5194/soil-2021-84-AC2
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AC2: 'Reply on RC2', Chunnan Fan, 05 Feb 2022
Status: closed
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RC1: 'Comment on soil-2021-84', Anonymous Referee #1, 06 Oct 2021
The authors of this paper attempted to predict the distribution of soil organic carbon (SOC) in temperate forest soil profiles using percolation theory, a theory commonly used to describe material transport in disordered porous media. Soil is an inhomogeneous medium composed of complex components. From the perspective of medium, it is feasible to apply this theory. However, SOC is not a single substance and has very complex forms: particulate, dissolved and adsorbed. Therefore, if the authors want to achieve the goal in the title, they should study SOC in different forms separately. In this paper, the soil profile of some typical forestlands in Changbai Mountain was measured by stratified soil sampling, and SOC was studied as a solute index.The study design is unreasonable. The measurement index is simple and single, and the workload is insufficient. The core viewpoint of the study is seriously inconsistent with the vertical distribution mechanism of SOC on profiles. Therefore, it is recommended not to be published.Specific comments are as follows:
1) The vertical distribution of soil organic carbon (SOC) in forest ecosystems has two meanings: one is vertical distribution along elevation and the other is vertical distribution in a profile. In order to avoid ambiguity, I suggested that the author emphasize vertical distribution on profile in the title and article (L1, 8...)..
2) L26-32 talks about the influence of vertical factors on soil organic carbon. Vertical factors here refer to the influence of altitude rather than the vertical depth of the profile studied in this paper.Should be deleted!
3) L76-80 mentioned that the vertical transport of SOC is mainly in the form of DOC leaching, so why not take DOC as the research index in this paper?
4) The full names of the six stand types of L100 should appear before giving abbreviations.
5) L115 there were only two replicates for each forest at the standard standscale, which was not enough for field experiments.
6) L124-125 soil samples were only screened through 10 mesh (1.7mm), while the standard method for determining SOC needed soils been through 100 mesh (0.15mm).
7) L130-137 This paper only selected the data of 5 published articles for fitting analysis, which could not constitute a reasonable meta-analysis.
8) L207-212 Here, spatial variability, environmental factors and other unrelated topics are discussed.
9) L215-216 There is no need to emphasis the dominant species in stands.
10) The goodness of fit of curves in the results of L219-220 is not high, and the tolerance range in Table 3 is very wide, so almost indicators of the forest soils reduced along the profile depth can appear this curve, such as fine root biomass and microbial biomass, which are not distributed according to the principle of percolation theory.Therefore, the method of this paper is not appropriate.
11) 226-227 The shallow soil was more disturbed by the external environment, while the SOC in the deep soil was mainly from the roots. The influence range of litter was generally considered to be in the Leaching - deposition (B) layer. The deeper soil has more adsorption sites, so the vertical transport of DOC in the soil is also affected by soil adsorption.
To sum up, the design of this study is unreasonable; the measurement index is single; and the workload is insufficient. It is suggested that the author increase the workload and select reasonable indicators such as DOC and mineral nitrogen for the study.
Citation: https://doi.org/10.5194/soil-2021-84-RC1 -
AC1: 'Reply on RC1', Chunnan Fan, 05 Feb 2022
Dear reviewer,
Thanks for the comments. In response, we will make several changes according to the suggestions and criticisms，Our point-by-point response to the reviewers’ comments is given below. In most points we agree with the reviewer but not for all. Our point to point responds are in italic type.
1) The vertical distribution of soil organic carbon (SOC) in forest ecosystems has two meanings: one is vertical distribution along elevation and the other is vertical distribution in a profile. In order to avoid ambiguity, I suggested that the author emphasize vertical distribution on profile in the title and article (L1, 8...)
This will be addressesd.
2) L26-32 talks about the influence of vertical factors on soil organic carbon. Vertical factors here refer to the influence of altitude rather than the vertical depth of the profile studied in this paper. Should be deleted!
We are confused here, since L26-32 didn't mention any point relating to the influence of altitude...
3) L76-80 mentioned that the vertical transport of SOC is mainly in the form of DOC leaching, so why not take DOC as the research index in this paper?
The main hypothesis of the study is that the vertical transport of the percolating water (mainly in the form of DOC) redistributes SOC. DOC could be transformed in other forms and such as taken up by microbes, which we don't think is a stable and good indicator.
4) The full names of the six stand types of L100 should appear before giving abbreviations.
This will be addressed.
5) L115 there were only two replicates for each forest at the standard standscale, which was not enough for field experiments.
There were 3 replicates for each subsite. Here P1 and P2 are two subsites locating in one forest but with different sampling slopes. We should've mentioned the number of replicates here. We apologize for the confusion.
6) L124-125 soil samples were only screened through 10 mesh (1.7mm), while the standard method for determining SOC needed soils been through 100 mesh (0.15mm).
This is a typo, it was screened through 100 mesh. We apologize for the careless.
7) L130-137 This paper only selected the data of 5 published articles for fitting analysis, which could not constitute a reasonable meta-analysis.
One of the dataset referenced from Jobbágy and Jackson (2000) are averaged values from 3 global databases of soil profile in temperate forest soils (National Soil Characterization Database produced by the U.S. Department of Agriculture, World Inventory of Soil Emission Potential Database, and the Canadian Forest Service) which cover 60 samples for the temperate deciduous forest, and 123 for the temperate evergreen forest across the world.
8) L207-212 Here, spatial variability, environmental factors and other unrelated topics are discussed.
The purpose of this paragraph is to demonstrate that the surface SOC strongly correlates to environmental factors in our sampling sites, and that the vertical distribution of SOC on profile is constrained by the transport of water and it is decoupled with the environmental factors. But we do agree with the reviewer, this paragraph can be shortened into 1-2 sentences.
9) L215-216 There is no need to emphasis the dominant species in stands.
This will be addressed.
10) The goodness of fit of curves in the results of L219-220 is not high, and the tolerance range in Table 3 is very wide, so almost indicators of the forest soils reduced along the profile depth can appear this curve, such as fine root biomass and microbial biomass, which are not distributed according to the principle of percolation theory. Therefore, the method of this paper is not appropriate.
The results mentioned in L219-230 covers results from different regions of the world, including 3 global dataset with 60 samples from the temperate deciduous forest, and 123 from the temperate evergreen forest, and 3 sites in China and 1 in Germany. The deviations are 20%, 4.5%, 4%, 7.6% and 9.7%. There is no range mentioned in Table 3.We think what the reviewer meant here is the range in Figure 3. L174-179 explained how we determined the boundary. And Figure 5 demonstrates the agreement of prediction and the averaged values from Figure 3 in each layer of soil. There are many indicators in forest soils that accumulate more in the surface and show curving trend as soil goes deeper, but the exponents of the curving can be various. However, here we demonstrate that the exponent of the SOC trend agrees with the proposed value from percolation theory, which is -1.149.
11) 226-227 The shallow soil was more disturbed by the external environment, while the SOC in the deep soil was mainly from the roots. The influence range of litter was generally considered to be in the Leaching - deposition (B) layer. The deeper soil has more adsorption sites, so the vertical transport of DOC in the soil is also affected by soil adsorption.
We agree with the reviewer's point. The hypothesis of the study is that the downword movement of water, with DOC as the solute, redistributes SOC on soil profile. root is a source of carbon input, there are studies show that the SOC profile is deeper than root profile, and that the downward transport of DOC along the soil profile is the potential driver of the redistribution of carbon in the soil. Previous studies have shown that percolation theory can describe the chemical weathering rate becasue the reaction rate is limited by solute transport , which means that reaction is in equilibrium along the track, and the reaction rate is porpotional to solute velocity. Similar to chemical weathering, adsorption and desorption can be limited by solute transport as well if the solute travels too slow. However, here the reviewer pointed a good point which we should dig deeper to see the adsorption and desorption has the same situation as chemical weathering in the subsurface.
Citation: https://doi.org/10.5194/soil-2021-84-AC1
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AC1: 'Reply on RC1', Chunnan Fan, 05 Feb 2022
-
RC2: 'Comment on soil-2021-84', Anonymous Referee #2, 14 Jan 2022
GENERAL COMMENTS
The manuscript deals with an important topic to highlight the role of soil in the global C budget. As a matter of fact, the soil organic carbon (SOC) in forest ecosystems could suppose 704 Pg C, up to the 43% of the total SOC stock, which is considered the largest pool of the terrestrial C (Lal, 2005). The mathematical background It’s suitable and well explained, the results are properly presented and consistent with the hypothesis and the conclusions.
However, my main concern is related with the true scientific scope of this work. Beyond the metadata, the dataset for an original research paper is rather poor (60 composite samples with only a standard analytical routinely procedure –potassium dichromate oxidation-). In my opinion, it would be much more relevant to advance the knowledge of the distribution at depths > 1 meter, since there are evidences this could represent a significant additional stock and it’s poor described in forests. Did the authors consider take samples deeper in the soil? Moreover, I have some troubles with the hypothesis put forward by the authors of the leaching of C as the main driver of C vertical pattern (see specific comments).
SPECIFIC COMMENTS
Introduction
L48: “coarse”
L49: Please enter the citation of “Trumbore et al., 2006” in references.
L50: As far as I read, neither Gill nor Joslin study the total SOC in the subsurface layer of soil.
L53-57: I’m not as optimistic as the authors about the vital importance of C leaching as a potential driver for the redistribution of C in the soil. For example, Jobbagy and Jackson (2000, p. 433) consider the decrease of SOC turnover with depth a much more plausible explanation (in fact, they find an inverse trend between precipitation and SOC depth). I would recommend that the authors consider other possible explanations in the manuscript (see also comment to L221 – 224).
Material and Methods:
L81: Doesn't the percolation theory also depend on soil texture? The physical process of water infiltration is highly dependent on it (eg. hydraulic saturated conductivity).
L82. The study of Sheppard et al. does not seem refer to the transport of SOC in temperate forest. Please, modify the sentence or move the reference elsewhere.
Table 2: Please, indicate here the soil type (WRD or Soil Survey Staff classifications). This could be interesting to understand the vertical distribution of SOC.
Table 2: Are these temperature the annual average temperatures? I find these (< 5ºC) too much low for a broad-leaved deciduous forest. Please, indicate the Koppen climate classification.
L122: Why was the C horizon not sampled?
Results
Figure 1b: I think the point “TS-P1” is really CS-P1.
L163 – 169 and Figure 2: The effect of comparing a 12% slope (CS-P) vs. 17% (LX-P2) does not seem very convincing… In my opinion, it is very difficult to assess the relative influence of both factors (slope and precipitation) on the SOC.
Table 4: It would be useful to show the mean SOC value for the first meter.
L195: “temperate”
Discussion and conclusions
L221 – 224. It is true that the empirical data fit well as predicted by the percolation theory. However, I’m not clear about the relationship between this theory (beyond its name, it is a statistical theory that can be applied to many non-hydraulic phenomena, as traffic jams!) and the soil infiltration process. Could it not be also explained by other "power-law" soil processes such as roots decomposition rate or microbial activity? (see, eg., the vertical distribution of Soil Microbial Biomass Carbon showed by Sun et al., 2020). In my opinion, it would be needed to provide some additional evidence linking percolation theory with the physical process of water infiltration in soils.
Cited reference:
Lal, R., 2005. Forest soils and carbon sequestration. Forest Ecology and Management 220, 242 – 258.
Sun, T., Wang, Y., Hui, D., Jing, X., Feng, W., 2020. Soil properties rather than climate and ecosystem type control the vertical variations of soil organic carbon, microbial carbon, and microbial quotient. Soil Biology and Biogeochemistry 148, 107905.
Citation: https://doi.org/10.5194/soil-2021-84-RC2 -
AC2: 'Reply on RC2', Chunnan Fan, 05 Feb 2022
Dear reviewer,
Thanks for the comments. In response, we will make several changes according to the suggestions and criticisms. Our point-by-point response to the reviewers’ comments is given below. In most points we agree with the reviewer but not for all.
1) L48: “coarse”.
This will be addressed. We apologized for the typo.
2) L49: Please enter the citation of “Trumbore et al., 2006” in references.
This will be addressed.
3) L50: As far as I read, neither Gill nor Joslin study the total SOC in the subsurface layer of soil.
We presented incorrectly here. What we attempted to present here is that studies from Gill and Joslin suggest that higher root litter inputs generally occur in the upper soil layers.
4) L53-57: I’m not as optimistic as the authors about the vital importance of C leaching as a potential driver for the redistribution of C in the soil. For example, Jobbagy and Jackson (2000, p. 433) consider the decrease of SOC turnover with depth a much more plausible explanation (in fact, they find an inverse trend between precipitation and SOC depth). I would recommend that the authors consider other possible explanations in the manuscript (see also comment to L221 – 224).
Root growth is limited by nutrients and water, there are evidences that root growth also follows the percolation theoretical scaling (Figure below, referenced from Fig.5 in Hunt, 2016). The turnover of SOC affected by microbe activities is certainly a factor, and we agree with the reviewer that it should be considered in the manuscript.
Reference: Hunt (2016). Spatio-temporal scaling of vegetation growth and soil formation from percolation theory. Vadose zone journal VZJ.
5) L81: Doesn't the percolation theory also depend on soil texture? The physical process of water infiltration is highly dependent on it (eg. hydraulic saturated conductivity).
Correct. The more useful form of describing solute transport in soil is (t/t0)(x/x0)Db, where x0 is the pore size, which depends on the soil texture, and t0 is the time it takes for solute to pass a single pore, which equals to x0/v0, and v0 is the infiltration rate. That means that water infiltrates soils with different textures (with same saturation condition) will show parallel lines on the log-log plot (time versus distance) but with different intercepts. Here in the proposed paper, we demonstrates the power-law form of SOC distribution along soil profile, and the determination of the intercept still needs more investigation.
6) L82. The study of Sheppard et al. does not seem refer to the transport of SOC in temperate forest. Please, modify the sentence or move the reference elsewhere.
Good point. We referenced the value of Db in 3D saturated conditions from Sheppard et al. But the original presentation was confusing. This will be addressed.
7) Table 2: Please, indicate here the soil type (WRD or Soil Survey Staff classifications). This could be interesting to understand the vertical distribution of SOC.
The main soil types are sandy loam and loamy clay in the region.
8) Table 2: Are these temperature the annual average temperatures? I find these (< 5ºC) too much low for a broad-leaved deciduous forest. Please, indicate the Koppen climate classification.
Correct. Temperature showed in Table 2 is the annual average values. All study sites are temperate continental climate.
9) L122: Why was the C horizon not sampled?
There have been conflicts of the definition of "soil depth".
From previous work applying percolation theory to soil formation, "soil depth" refers to a particular weathering horizon like the Bw (e.g. Hunt, 2015). Here, to apply the same theoretical framework, we follow the same criteria.
Reference: Hunt, A. G.: Soil depth and soil production. Complexity, doi: 10.1002/cplx.21664, 2015a.
10) Figure 1b: I think the point “TS-P1” is really CS-P1.
The main point here in Figure 1b is to present effects of precipitation on SOC content. CS-P1 and LX-P1 have similar temperatures and slopes, but different precipitations. TS-P1 is not appropriate to present here because except for different temperatures, the sampling slopes are not close neither.
11) L163 – 169 and Figure 2: The effect of comparing a 12% slope (CS-P) vs. 17% (LX-P2) does not seem very convincing… In my opinion, it is very difficult to assess the relative influence of both factors (slope and precipitation) on the SOC.
The sampling slope is in "degree". We attempted to explain the effect of topography on SOC content, and more precipitation and lower sampling slope seemed favor the conservation of water, which has a positive feedback on SOC production. When comparing JL-P1 and HG-P1, more precipitation in JL-P1 didn't result in higher SOC since the sampling slope is steeper.
12) Table 4: It would be useful to show the mean SOC value for the first meter.
This will be addressed
13) L195: “temperate”
This will be addressed. We apologized for the typo.
14) L221 – 224. It is true that the empirical data fit well as predicted by the percolation theory. However, I’m not clear about the relationship between this theory (beyond its name, it is a statistical theory that can be applied to many non-hydraulic phenomena, as traffic jams!) and the soil infiltration process. Could it not be also explained by other "power-law" soil processes such as roots decomposition rate or microbial activity? (see, eg., the vertical distribution of Soil Microbial Biomass Carbon showed by Sun et al., 2020). In my opinion, it would be needed to provide some additional evidence linking percolation theory with the physical process of water infiltration in soils.
The basic theoretical form of solute transport described by percolation theory is x is propotional to tDb, where x is the transport distance, t is the travel time, and Db is the fractional dimensionality of percolation backbone, which is a known value that only dependents on dimensionality and the saturation condition. First figure below (from Figure f, Hunt et al., 2020) show the fitness of theoretical scaling of percolation theory and chemical weathering rate data observed either from the field or from laboratory, though it is not the direct comparison of water velocity and the theory, chemical weathering are water limited in subsurface, and it is proportional to flow velocity (as shown in the second figure below(referenced from Figure1 in the same paper) ) . We think it is convinced evidence that percolation theory can describe solute transport in soil.
Reference: Hunt, Faybishenko, Ghanbarian, Egli, & Yu. (2020). Predicting water cycle characteristics from percolation theory and observational data. International Journal of Environmental Research and Public Health, 17(3), 734.
Citation: https://doi.org/10.5194/soil-2021-84-AC2
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AC2: 'Reply on RC2', Chunnan Fan, 05 Feb 2022
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