Pyrolysis Temperature Affects Dissolved Phosphorus and Carbon Levels in Alkali-Enhanced Biochar and Its Soil Applications
Round 1
Reviewer 1 Report
The paper by Wang et al. “Pyrolysis temperature affects dissolved phosphorus and carbon levels …” deals with an interesting aspect of biochar application. Modifications of biochar are considered as potent mean to adjust the properties of biochar to the specific requirements of the intended deployment. Therefore, the effects of alkali-modification of biochar on phosphorus and carbon availability is a relevant and useful topic to be studied.
· The experimental design has its merits; it appears as a logical combination of the studied main factors “alkalizing agents”, their concentrations and the pyrolysis temperatures. The complex task of presenting the effect of three factors for two different feedstocks has been resolved decently but still there is a certain challenge to understand the graphs correctly. The description of the methodological procedures has some shortcomings that I suggest to supplement:
· In the description of the alkali-modification, it is not clear what the authors mean with the “equilibration with 100 mL …”. Has the material been dried before pyrolysis or have the crucibles been put with the water in the muffle furnace? The subsequent drying (?) for 30 min followed by a 60 min pyrolysis steps might not have been enough for a complete pyrolysis, especially considering the low carbon and high nitrogen concentrations in the biochars (in comparison to the feedstock) that are not typical for biochar. Or are the main elements not given on an ash-free basis? Thus, it should be made clear if this was a proximate or ultimate analysis. The ultimate analyses could be used to calculate the O:C and H:C atomic ratios, an important characterization parameter of biochar.
· In the description of the rice greenhouse study, there suddenly appears an Si-P-source. This has not been described before. What is it made from, and what was the SI- and P-concentration? Or is this an alternative name for the biochars? This should be made clear. Si concentrations are nowhere to find, also not in the supplementary tables.
· In the rice greenhouse study a phosphate fertilizer was added to the soil. If the authors wanted to study P release from the biochars, why did they do this? How can the reader distinguish in Fig. 5 from which source the P has originated? The same problem applies to K and Fig. 6. Besides, it was unfortunate that the authors used a different soil for the pot experiment than for the incubation experiment.
· It is a pity that the authors did not use two-way or three-way ANOVA for the statistical evaluation. Much more information could have been derived than by only applying one-way ANOVAs to small groups of treatments.
· The authors did not specify which multiple range test they have used in the Figures 1-6.
· The captions of Figures 1-6 contain a lot of repetitive information. It would be sufficient to list the “Note: …” only once and then refer to it.
· Tables S1 and S2 list valuable information on the biochar that is painfully missing in the main paper. If it is not possible to include the whole tables, at least some basic parameters should be included in the main paper and the rest remain for the supplementary information.
Author Response
The paper by Wang et al. “Pyrolysis temperature affects dissolved phosphorus and carbon levels …” deals with an interesting aspect of biochar application. Modifications of biochar are considered as potent mean to adjust the properties of biochar to the specific requirements of the intended deployment. Therefore, the effects of alkali-modification of biochar on phosphorus and carbon availability is a relevant and useful topic to be studied.
- The experimental design has its merits; it appears as a logical combination of the studied main factors “alkalizing agents”, their concentrations and the pyrolysis temperatures. The complex task of presenting the effect of three factors for two different feedstocks has been resolved decently but still there is a certain challenge to understand the graphs correctly. The description of the methodological procedures has some shortcomings that I suggest to supplement:
- In the description of the alkali-modification, it is not clear what the authors mean with the “equilibration with 100 mL …”. Has the material been dried before pyrolysis or have the crucibles been put with the water in the muffle furnace? The subsequent drying (?) for 30 min followed by a 60 min pyrolysis steps might not have been enough for a complete pyrolysis, especially considering the low carbon and high nitrogen concentrations in the biochars (in comparison to the feedstock) that are not typical for biochar. Or are the main elements not given on an ash-free basis? Thus, it should be made clear if this was a proximate or ultimate analysis. The ultimate analyses could be used to calculate the O:C and H:C atomic ratios, an important characterization parameter of biochar.
Response: The paragraph has been revised as: “For biochar making, rice straw or husk were mixed with each of three alkali chemicals (KOH, K2CO3 and CaO), respectively, at alkali:biomass ratios (on weight basis) of 0:100, 5:100 and 10:100 in a porcelain crucible followed by adding 100 mL of ultra-pure water (based on 50 g of biomass) and allowed for dissolving alkali reagent for 90 minutes. The crucibles were then placed in muffle furnace under N2 flow at 400 mL min-1 for 30 min to remove air from the system. The mixtures were dried with muffle furnace temperature set at 180oC for 30 min, followed pyrolysis at 350, 450 and 550oC, respectively, for 60 min under N2 flow rate at 200 mL min-1 (Wang et al. 2018)”. (Please see page 5-6 Line 108-116). We did not measure total S, which will be required to calculate O:C of biochar. For H:C, the H and C content for the similar biochar has been given in our previous publication (Wang et al., 2018). For current study, we intend to focus primarily on nutrient availability with respect to dissolved P and C of produced biochar.
- In the description of the rice greenhouse study, there suddenly appears an Si-P-source. This has not been described before. What is it made from, and what was the SI- and P-concentration? Or is this an alternative name for the biochars? This should be made clear. Si concentrations are nowhere to find, also not in the supplementary tables.
Response: Si-P has been deleted in the text. Thank you for the comment.
- In the rice greenhouse study a phosphate fertilizer was added to the soil. If the authors wanted to study P release from the biochars, why did they do this? How can the reader distinguish in Fig. 5 from which source the P has originated? The same problem applies to K and Fig. 6. Besides, it was unfortunate that the authors used a different soil for the pot experiment than for the incubation experiment.
Response: Alkali-enhanced biochar has high phosphorus content, but it is not enough to support the whole growth period of rice and can partly replace the application of phosphorus fertilizer. As indicated in page 8 Line 161-163, all pots were added the same rates of blanket application of NPK, including those showed in Figs. 5 and 6 control (CK) treatments, therefore there should be no confusion with respect to the change in uptake of P as a result of different sources of biochar additions since all the comparisons are relevant to CK treatments.
In order to study the effect of alkali-enhanced biochars on the improvement of acid soil (Wang et al. 2018), Briley and Commerce soils were used to the incubation experiments due to their lower pH. For rice greenhouse experiment, Crowley, a typical rice soil for rice production in Louisiana, was used. We believe the use of different soils in this study would provide a more comprehensive picture as to the application effects of alkali-enhanced biochars.
- It is a pity that the authors did not use two-way or three-way ANOVA for the statistical evaluation. Much more information could have been derived than by only applying one-way ANOVAs to small groups of treatments.
Response: The results shown in figures are clearly visible presentations (temperature and alkali concentration levels) of treatment effects. The following has been added under 2.5 Statistical analysis: “Pyrolysis temperature and alkali level treatment effects on biochar and soil soluble P and C were analyzed using one-way ANOVA based on GLIMMIX procedure. When ANOVA test was significant, individual treatment level effects were assessed by Duncan’s multiple range test at a P < 0.05 level” (please see page 8 Line 171-177). We do not think that an additional ANOVA table would add more information. In fact, ANOVA without mean separations tends to overshadow the details of treatment effects at different individual treatment level.
The authors did not specify which multiple range test they have used in the Figures 1-6.
Response: Thank you for the suggestion. The test has been specified in the captions for 1-6 figures. (Please see each figure).
- The captions of Figures 1-6 contain a lot of repetitive information. It would be sufficient to list the “Note: …” only once and then refer to it.
Response: The caption below each figure is for the convenience of readers as it contains multiple designations.
- Tables S1 and S2 list valuable information on the biochar that is painfully missing in the main paper. If it is not possible to include the whole tables, at least some basic parameters should be included in the main paper and the rest remain for the supplementary information.
Response: Thank you for your comments. Most basic physical and chemical properties of similar alkali-enhanced biochars have been published elsewhere (Wang et al. 2021) as indicated in page 9, line 181-182. We feel it is appropriate to list some additional parameters in supplementary material rather in the main manuscript for this reason.
Reviewer 2 Report
Overall, the manuscript is well written.
In the introduction, hypothesis of the study is missing. I would suggest authors to include their hypothesis before the objectives.
In the materials and methods, I would suggest authors first to provide the site location and treatments, and then characterization of biochar followed by statistical analysis (under separate heading).
Statistical design for the experiment is missing. Generally, greenhouse experiments will be carried out under Completely Randomized Block design.
Results and discussion is ok.
Conclusion: Authors can improve this part by elaborating which biochar treatment would increase phosphorus and carbon in soil (including field level application, Mg h-1 scale).
Author Response
In the introduction, hypothesis of the study is missing. I would suggest authors to include their hypothesis before the objectives.
Response: Thank you for the comment. The following has been added into the introduction: “However, since alkaline solution is known to increase solubility of phosphorous and organic C (Turner et al., 2005; Ohno et al., 2019), we hypothesize that the alkali regent retreatment of biomass to produce Si-rich biochar could also influence the release of P and C from resulting biochar, which would subsequently affect dynamics of these elements in soil”. (Please see page 5 line 93-97).
In the materials and methods, I would suggest authors first to provide the site location and treatments, and then characterization of biochar followed by statistical analysis (under separate heading).
Response: This study contains multiple aspects of research work involving laboratory and potting experiments along with biochar source making with each using specific procedures. Therefore, we believe it is best served by the current arrangement of section headings rather than straight forward using site location and treatments. Nonetheless, based on the reviewer’s suggestion, the following has been added under 2.4 Rice greenhouse study section: “The soil used in potting study was Crowley silt loam (pH 7.6), collected from Crowley Rice Station, Crowley, USA” (Please see page 7, line 150-152). In addition, statistical analysis has been listed as a separate section 2.5 (Please see page 8, line 171-177).
Statistical design for the experiment is missing. Generally, greenhouse experiments will be carried out under Completely Randomized Block design.
Response: The following “The potting study was carried out using completely randomized block design” (Please see page 8 line 159-160).
Results and discussion is ok.
Conclusion: Authors can improve this part by elaborating which biochar treatment would increase phosphorus and carbon in soil (including field level application, Mg h-1 scale).
Response: Thank you for your recommendation. The following has been added to the conclusion: “Soil WSP and WSC contents of RS-10KB were 101~492% and 44~173% higher than RS-0B at 1% and 3% application rates in Commerce soil.” (Please see page 17 line 360-361). Also added to the conclusion: “ Based on the P release from water extraction of these biochar sources, it is estimated that application rate of 1 metric ton per hectare of RS-10KB and RH-10K2B produced by 550oC pyrolysis temperature has the potential to input 0.032 and 0.303 kg (or 4 and 35%) more WSP over RS-0B into a soil, while the same rate of amendment with RH-10KB and RH-K2B at 550oC could have 0.311 and 0.343 kg (or 115 and 126%) more WSP than RH-0B, respectively. The actual extent of the WSP increase requires the calibration for the specific soil type and alkali-biochar application rate”. (Please see page 17 line 365-372).
