Evaluation of the Influence of Individual Clay Minerals on Biochar Carbon Mineralization in Soils
Round 1
Reviewer 1 Report
A valuable contribution to the literature, exploring the relationship between clay minerals, biochar application and soil organic matter mineralisation/stabilisation. A straightforward experiment has been conducted, with reasonable results. The interpretation is perhaps lacking in nuance, maybe due to unfamiliarity with local conditions. The soil sampled from Wongan Hills in WA is not “sand” (as suggested at line 66) but definitely soil, and it has a clay content not unusually low for the WA wheat belt. The native clay should be characterised by XRF in the same manner as the added clays have been, as this is pertinent to the interpretation of the study. The clay addition of 10% is large, in comparison with agricultural practices in the region. This should be acknowledged and the following references referred to, to set this incubation study more firmly in it’s local context (Hall et al., 2010; Roper et al., 2015). The variability of biochars due to their source material could be stated more strongly: the switchgrass seems an odd choice for this study, however I appreciate that isotopically labelled material is not simple to produce. The manuscript is littered with grammatical errors; careful, native-English editing is required. Detailed suggestions are included below.
Line 21 add “The clay” before minerals
Line 23 and throughout replace “biochar added soil”- grammatically incorrect- with “biochar amended soil”
Line 26 “oxide” not “oxides”
Line 32 replace “on” with “for”
Line 34 replace “of” with “to”, “reduce” not reducing
Line 35 comma after emissions, “per year” not on an annual basis
Line 38 long time not long-time
Hall, D.J.M., Jones, H.R., Crabtree, W.L. and Daniels, T.L., 2010. Claying and deep ripping can increase crop yields and profits on water repellent sands with marginal fertility in southern Western Australia. Aust. J. Soil Res., 48: 178-187.
Roper, M.M. et al., 2015. Management options for water-repellent soils in Australian dryland agriculture. Soil Research, 53(7): 786-806.
Comments for author File: 
Comments.pdf
Author Response
Review of “Evaluation of the influence of individual clay minerals on biochar carbon mineralisation in soils”
A valuable contribution to the literature, exploring the relationship between clay minerals, biochar application and soil organic matter mineralisation/stabilisation. A straightforward experiment has been conducted, with reasonable results. The interpretation is perhaps lacking in nuance, maybe due to unfamiliarity with local conditions. The soil sampled from Wongan Hills in WA is not “sand” (as suggested at line 66) but definitely soil, and it has a clay content not unusually low for the WA wheat belt. The native clay should be characterised by XRF in the same manner as the added clays have been, as this is pertinent to the interpretation of the study. The clay addition of 10% is large, in comparison with agricultural practices in the region. This should be acknowledged and the following references referred to, to set this incubation study more firmly in it’s local context (Hall et al., 2010; Roper et al., 2015). The variability of biochars due to their source material could be stated more strongly: the switchgrass seems an odd choice for this study, however I appreciate that isotopically labelled material is not simple to produce. The manuscript is littered with grammatical errors; careful, native-English editing is required. Detailed suggestions are included below.
Response: We agree with the reviewer and changed the text to ‘soil’, the sand was used merely to emphasise the very low clay content in the soil. Since the clay content in the soil is so low (1.3%) the influence of native clay minerals (which is kaolinite) in the experiment is likely to be little and it will be the same for all artificial soils constituted by adding different clay minerals.
We constituted the artificial soils by adding large amount (10%) of different clay minerals to observe effects of clay minerals on biochar and native soil carbon mineralisation. Our aim was to compare the effect of different clay minerals on the mineralisation of biochar and native soil carbon, and not to evaluate the effect of deep ripping as the reviewer has mentioned. Therefore, the two references mentioned by the reviewer are not relevant in the context of our experiment.
As acknowledged by the reviewer, the choice of the biochar was based on a contrasting δ13C value from the soil carbon that allowed us to quantify the mineralisation of carbon from the two pools (i.e. biochar and native soil carbon) and hence we selected the switch grass biochar.
We had made English corrections in the revised manuscript.
Line 21 add “The clay” before minerals
Response: We have made this change.
Line 23 and throughout replace “biochar added soil”- grammatically incorrect- with “biochar amended soil”
Response: We revised as suggested by the reviewer, however, in the context of our incubation experiment, it is grammatically correct.
Line 26 “oxide” not “oxides”
Response: We meant ‘oxides’(in plural sense!) here and it is correct. I quote from ‘Iron oxides’ chapter in the ‘Minerals in Soil Environment’ book – “The iron oxides (embracing oxides, oxyhydroxides, and hydrated oxides) are most abundant of the metallic oxides in soils”.
Line 32 replace “on” with “for”
Response: Revised as suggested.
Line 34 replace “of” with “to”, “reduce” not reducing
Response: We revised accordingly.
Line 35 comma after emissions, “per year” not on an annual basis
Response: Revised as suggested.
Line 38 long time not long-time
Response: Revised as suggested.
Reviewer 2 Report
Dear authors,
The research and the layout of the presented manuscript is very interesting and easy to follow. I do not have many comments regarding to your study. Here are my suggestion that might me considered by the authors team before the acceptance:
the scientific name of switch grass should be also added in the beginning of the study. in the discussion part (line 229) - correct the citation Bruun (also the name spelling is incorrect) that not correspond with the reference number.After minor revision the paper can be accepted for the publication in Soil System journal.
Author Response
Reviewer 2
The research and the layout of the presented manuscript is very interesting and easy to follow. I do not have many comments regarding to your study. Here are my suggestion that might me considered by the authors team before the acceptance:
the scientific name of switch grass should be also added in the beginning of the study.
Response: We added the Latin name of switch grass.
in the discussion part (line 229) - correct the citation Bruun (also the name spelling is incorrect) that not correspond with the reference number.
Response: We checked the reference and revised the citation Bruun et al., in the discussion section.
After minor revision the paper can be accepted for the publication in Soil System journal.
Reviewer 3 Report
Overall: Fairly good manuscript. Data interpretation could be better organized
- The work presented seem carefully done and a significant contribution. However, the discussion is too general and rambling. More work needs to be done in analyzing the results, and thinking through the discussion section to bring out the mechanism taking place in effecting C mineralization or protection
-Must be made more explicit, through whole manuscript but especially in the abstract, whether the C mineralization measures is biochar-C, total C, or native SOC. It appears that SOC is often used which is not clear. I suggest always using the term ‘native SOC’
-The hypothesis regarding the effects of the minerals on C mineralizaton needs to better posed, and then addressed with the data.
Abstract
-Justification for the study could be better than given in the first sentence
-Some detail should be added to the abstract with some specific number on C min%
-Rather confusing to say both that biochar had no effect AND “Native SOC mineralization was significantly reduced in the presence of biochar “
Intro.
-Ln46-48: Unclear sentences
-Paragraph starting on Ln44: Need to outline proposed mechanisms
-Ln58: Hypothesis is unclear. What specific relationship between clay mineral type, charge characteristic and specific surface area are you proposing?
-Ln59-60: Unclear sentence: : “determine biochar-C mineralization in soils in the presence of three clay minerals” Deterine what? This is not testing any hypothesis. What about native SOC?
Methods
-How much soil was incubated? Did you keep the total C incubated constant? If the weight ws kept the same then it would not be true that “native organic carbon concentration was the same in all treatments” as stated in line 133. Explain.
-Ln94: explain ‘accounting for headspace’
-Ln98: More detail needed on microbial inoculum. Why was this added at all instead of using the native soil population?
-More detail on how was CO2 and 13C measured?
-How was equation 2 solved?
-Why was 20 C chosen? That is rather cold
Results
-Ln 144: If SOC was really the same in all treatments, then per C and per g presentations would be exactly the same and it would not be true that “total C mineralized data on soil mass basis better reflect the effects resulting from different mineral compositions” as state in Ln133. I am confused. If they are actually different, then per C basis should at least be presented in the SI section.
-Need to have a systematic approach to use of symbols in graph. For example, could use only closed symbols for treatments that include biochar. You have mostly don’t this but not in figure 3. Also use same symbols throughout the figures.
-How could it be true that “minerals significantly reduced the amount of total C mineralized” (Ln 140) and “effects of minerals on the cumulative SOC mineralisation during the incubation period was not significant (Ln173)? Total C and SOC is the same in treatments with only soil and minerals.
Discussion
-Discussion is rambling and difficult to follow. Discussion should be divided into topic subheadings. Results of modeling should be put into a separate section. Also, perhaps address the effects (or non-effect) of mineral and biochar on SOC mineralization separate from the effects of minerals on biochar mineralization.
-Discussion on modelling should include information (such as in the table) on how well the data fit the model and what fraction of the C was represented in the labile and refractory portions. Also, be more careful in discussion. For example, it is not correct to say “MRT of native SOC was 9.5 years in the control” as this is only for the refractory portion.
-Ln226: It is incorrect to say that a sample has a “lower proportion of total OC as BPCA-C”. BPCA-C is produced from condensed aromatic C upon acidification.
-Ln287: “most beneficial” is not specific enough. Be clear you are only addressing C sequestration.
- I suggest looking at mineralization rates (and differences in these between treatments) in the early and late portions of the incubations. It appears that the main effects only appeared early. What does this say about mechanisms at play? For example, there may have been positive priming in the early portion of the experiment and negative priming in the later part, but only looking at total mineralization removes the ability to see this.
-Also the properties of the various minerals need to be more clearly presented, i.e. such as with a table of surface area and charge, to address the hypothesis posed (though it was posed rather badly).
Author Response
Reviewer 3
Overall: Fairly good manuscript. Data interpretation could be better organized
- The work presented seem carefully done and a significant contribution. However, the discussion is too general and rambling. More work needs to be done in analyzing the results, and thinking through the discussion section to bring out the mechanism taking place in effecting C mineralization or protection
-Must be made more explicit, through whole manuscript but especially in the abstract, whether the C mineralization measures is biochar-C, total C, or native SOC. It appears that SOC is often used which is not clear. I suggest always using the term ‘native SOC’
Response: We have made some changes in the discussion and also have changed “SOC” to “native SOC” in the revised manuscript.
-The hypothesis regarding the effects of the minerals on C mineralizaton needs to better posed, and then addressed with the data.
Response: The hypothesis sentence has been revised.
Abstract
-Justification for the study could be better than given in the first sentence
Response: We have revised the sentence to give a clearer justification.
-Some detail should be added to the abstract with some specific number on C min%
Response: We have added a quantitative statement, however, the journal’s word limit for the abstract section restricts us adding too much details in this section.
-Rather confusing to say both that biochar had no effect AND “Native SOC mineralization was significantly reduced in the presence of biochar “
Response: The statement is now clearer, with the addition of ‘native’ for SOC and total carbon mineralized include the carbon mineralized from native SOC and biochar-C.
Intro.
-Ln46-48: Unclear sentences
Response: We have revised the sentence. “There is some evidence that clay minerals in soils decrease the mineralization of biochar-C, for example, the mineralization of barley root biochar-C decreased with increasing clay content in three Danish soils [20].”
-Paragraph starting on Ln44: Need to outline proposed mechanisms
Response: Agree and we have added a statement “Chemical interactions of soluble organic compounds with mineral surfaces and incorporation of soil organic matter into soil aggregates are the main mechanisms for the preservation of organic matter in soils” based on the references.
-Ln58: Hypothesis is unclear. What specific relationship between clay mineral type, charge characteristic and specific surface area are you proposing?
Response: Revised as “We hypothesized that clay minerals will have varying effects on biochar-C mineralization depending upon their charge characteristics, i.e. variable and permanent charge, and the mineralization of biochar-C will decrease with increasing specific surface area of the mineral.”
-Ln59-60: Unclear sentence: : “determine biochar-C mineralization in soils in the presence of three clay minerals” Determine what? This is not testing any hypothesis. What about native SOC?
Response: Revised as “To test our hypothesis, we measured the mineralization of biochar-C in artificial soils in the presence of three clay minerals, i.e. kaolinite, smectite and goethite, using an incubation experiment. Additionally, we measured the mineralization of native soil carbon to evaluate the effects of clay minerals and biochar on the stability of native soil carbon.”
Methods
-How much soil was incubated? Did you keep the total C incubated constant? If the weight ws kept the same then it would not be true that “native organic carbon concentration was the same in all treatments” as stated in line 133. Explain.
Response: We have added the soil weight (100 g) etc in the revised manuscript.
-Ln94: explain ‘accounting for headspace’
Response: In all containers there is some CO2 in the empty space (head space) and to account for this we included three chambers with no soil. The CO2 captured from the empty chambers was deducted from all treatments. This is a common practice in such experiments, and we have avoided including such trivial details.
-Ln98: More detail needed on microbial inoculum. Why was this added at all instead of using the native soil population?
Response: The soil used in this experiment was air-dried some time ago and included microbial inoculum to drive the microbial processes and a reference is provided for details.
-More detail on how was CO2 and 13C measured?
Response: The detailed procedures for CO2 and 13C measurements are given in the two references provided, i.e. Keith et al. (2011) and Fang et al. (2014), in the manuscript.
-How was equation 2 solved?
Response: the equation can be solved in Excel or other statistical packages. We solved this equation by regression process with SPSS, according to a series of data measured at different time.
-Why was 20 C chosen? That is rather cold
Response: Our main aim was to determine the effects of different clay minerals on the mineralization of biochar and native soil carbon at a given temperature. We could have conducted the experiment at the laboratory temperatures (varied between 20 and 26 deg C) but we decided to use the constant temperature room (20 deg C) that was available in our laboratory. We understand the mineralization rate will be greater at higher temperatures and actually we have conducted a temperature dependent study on the biochar-C mineralisation (Fang, Y., Singh, B., Singh, B.P. and Krull, E. (2015). Effect of temperature on biochar priming effects and its stability in soils. Soil Biology & Biochemistry, 80, 136–145.).
Results
-Ln 144: If SOC was really the same in all treatments, then per C and per g presentations would be exactly the same and it would not be true that “total C mineralized data on soil mass basis better reflect the effects resulting from different mineral compositions” as state in Ln133. I am confused. If they are actually different, then per C basis should at least be presented in the SI section.
Response: Yes, the referee is right the native SOC mineralization will appear the same, but it will be different with the biochar treatment, therefore we have chosen to display the data this way.
-Need to have a systematic approach to use of symbols in graph. For example, could use only closed symbols for treatments that include biochar. You have mostly don’t this but not in figure 3. Also use same symbols throughout the figures.
Response: Thank you and we have revised the figures (2 and 3) to make the symbols consistent in all figures.
-How could it be true that “minerals significantly reduced the amount of total C mineralized” (Ln 140) and “effects of minerals on the cumulative SOC mineralisation during the incubation period was not significant (Ln173)? Total C and SOC is the same in treatments with only soil and minerals.
Response: We agree with your observation, the statistical analysis presented in the data used the three minerals as one factor, that caused the confusion. To avoid this, we have excluded this table in the revised manuscript and the text has been revised accordingly. Since the minerals have variable effects on biochar-C and native SOC mineralization, the description is now based on old Table 3 (in the revised manuscript Table 2) and the figures.
Discussion
-Discussion is rambling and difficult to follow. Discussion should be divided into topic subheadings. Results of modeling should be put into a separate section. Also, perhaps address the effects (or non-effect) of mineral and biochar on SOC mineralization separate from the effects of minerals on biochar mineralization.
Response: We added topic subheading in discussion section. Results of modelling show the labile nature of native SOC and biochar-C, which indicates our data and results are believable. So we added a subheading “4.3 Labile nature of native SOC and biochar-C”.
-Discussion on modelling should include information (such as in the table) on how well the data fit the model and what fraction of the C was represented in the labile and refractory portions. Also, be more careful in discussion. For example, it is not correct to say “MRT of native SOC was 9.5 years in the control” as this is only for the refractory portion.
Response: We added a subheading “4.3 Labile nature of native SOC and biochar-C”. we added the information on fraction of labile carbon in result section. “Estimation of the proportion of labile C in native SOC was 1.6% for S treatment, 0.3-0.5% for biochar amended treatments, and 1.5-1.9% for only mineral amended treatments. Estimation of the proportion of labile C in biochar-C was 12.0% for S+B treatment, and 8.6-15.7% for mineral amended treatments.”
We checked the expression of MRT and revised. The data were all fitted very well (R2> 0.98).
-Ln226: It is incorrect to say that a sample has a “lower proportion of total OC as BPCA-C”. BPCA-C is produced from condensed aromatic C upon acidification.
Response: While we agree with the reviewer in terms of the process of BPCA-C but only a portion of the total carbon in a biochar is converted into B3, B4, B5 or B6 monomers of BPCA. Anyway, we have revised the statement slightly.
In the chapter ‘Analysis of biochars using benzene polycarboxylic acids’ by M.W.I. Schmidt, M.D. Hilf and G.L.B. Wiesenberg in the book ‘Biochar – A guide to analytical methods’ by Singh et al. (2017, CRC Press) have used the heading ‘BPCA-carbon as proportion of the OC’ to estimate the proportion of aromatic carbon in different biochars.
-Ln287: “most beneficial” is not specific enough. Be clear you are only addressing C sequestration.
Response: We have changed to ‘may be more beneficial…’.
- I suggest looking at mineralization rates (and differences in these between treatments) in the early and late portions of the incubations. It appears that the main effects only appeared early. What does this say about mechanisms at play? For example, there may have been positive priming in the early portion of the experiment and negative priming in the later part, but only looking at total mineralization removes the ability to see this.
Response: Yes, we agree the mineralization rates are much faster at the beginning of the experiments, which is similar to many other experiments including our own observations. However, the incubation time in our experiment was rather short and based on our earlier results the priming effects direction and magnitude is noisy in first several months of incubation experiments (Fang et al. 2015. Effect of temperature on biochar priming effects and its stability in soils. Soil Biol Biochem 80, 136-145. We added a sentence about priming in the result section.
-Also the properties of the various minerals need to be more clearly presented, i.e. such as with a table of surface area and charge, to address the hypothesis posed (though it was posed rather badly).
Response: We have added more details about the mineral properties in the revised manuscript.
Round 2
Reviewer 3 Report
I still think that the paper should be edited to improve the English grammar/usage. Nevertheless, it is good enough to publish in its present form.
I also think that not all the comments made in my first review were addressed completely. Nevertheless, enough of my comments were addressed to publish in its present form.
