the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Sep 2025
Amending solid carbon from methane cracking to arable soils: A sustainable approach to increase carbon storage and heavy metal immobilization?
Abstract. Carbon dioxide emissions from burning fossil fuels play a major role in driving global climate change. Reducing these emissions through innovative technologies is critical to achieve climate change mitigation goals. Methane pyrolysis, including catalytic and "plasmalytic" approaches, has attracted attention for its ability to produce so-called turquoise hydrogen alongside solid carbon as a by-product that could be reused as soil amendment. This study investigated the potential of solid carbon materials from catalytic pyrolysis and plasmalysis, alongside reference materials (biochar and graphite), to improve soil hydraulic properties and to reduce heavy metal mobility and whether ecotoxicological reactions affect soil organisms. In experiment 1, two arable soils of contrasting textures (sand and silty loam) were amended with these carbon materials at an application rate of 40 t ha-¹, followed by assessments of soil physical properties, soil respiration rate, microbial biomass, extractable organic carbon, nitrogen mineralization, the activity of soil macro- (earthworms) and mesofauna (springtails, Folsomia candida). In Experiment 2, we evaluated heavy metal mobility and availability in metal-contaminated soils. In uncontaminated soil, solid carbon from plasmalysis (SCplas) increased water retention of the silty loam, particularly in the range of pF 1.8–3.0, but had no effect in the sandy soil, likely due to its hydrophobic properties, which limited moisture retention. In the silty loam, SCplas reduced microbial activity and the abundance of springtails. In the sandy soil, it had a negative effect on soil macrofauna (earthworms). The solid carbon from catalytic pyrolysis (SCcat) had almost no effect on the biological properties studied. In soils contaminated with heavy metals, SCplas showed strong immobilisation of heavy metals particularly for Cd and Cu, across several sites, outperforming the reference materials. However, SCcat increased Cd mobility at some sites, indicating little or even adverse effects on heavy metal mobility. Our results highlight the promise of SCplas for site-specific soil improvement, while cautioning against its hydrophobic effects in sandy soils. In contrast, SCcat has more negative effects, especially ecotoxicological than positive ones depending on soil.
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Status: open (until 23 Jan 2026)
- RC1: 'Comment on egusphere-2025-3866', Jeroen Zethof, 18 Dec 2025 reply
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CC1: 'Comment on egusphere-2025-3866', Line Broszka, 19 Dec 2025
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Saki and Kostiuk (2025) studied the effects of two novel (CO2-emission free) solid carbon materials from methane pyrolysis, SCplas (plasmalytic pyrolysis) and SCcat (catalytic pyroly-sis), and reference materials (graphite and biochar) as soil amendment on soil biology, heavy metal immobilization and soil properties. Results across soil types (sandy and silty loam) and five polluted sites varied; SCplas successfully immobilized some heavy metals at some polluted sites but had ecotoxicological effects and negatively affected nutrients cycles in both soil types and decreased water retention in sandy soils. SCcat had little impact on soil biota, did not affect nutrient cycling in either soil type, but mobilized Cd at few sites. Conclusively, results suggested that site-specific physiochemical properties must be considered before applying the novel solid carbon materials as soil amendment.
The hypothesis and objectives are explicitly stated and methods thoroughly described, especially for experiment 1. The final discussion on how carbon materials interplay with soil hydraulic and chemical properties and organisms is strong. Lastly, the conclusion is clear. I find your paper interesting and of great relevance due to the novelty aspect of the solid carbon materials having the potential to be used as soil amendment whilst simultaneously being able to immobilize heavy metals, if found to be safe. Further, the materials might be able to act as carbon storage, a benefit only briefly touched upon in the manuscript.
However, I do have some major comments that I recommend being revised. This includes removing several figures as the large number of figures removes attention from important results, streamlining results shown in figures with text as there are multiple instances where results are misinterpreted or being oversold in the text, and lastly adding an in-depth point in the discussion on site-specific responses to heavy metal immobilization as e.g., it remains unclear why heavy metals were mobilized at some sites. Overall, these adjustments will increase clarity and scientific credibility and slightly affect the take-home messages. I perceive these adjustments necessary as the conclusions communicated in abstract are not consistent with the final conclusions.
Major concerns
- Too many figures
The manuscript presents 11 figures with 26 graphs, many of which are not all central to addressing the hypothesis. This makes it hard for the reader to identify key results. I recommend focusing on figures that clearly present findings and are central to answering the hypothesis. Further, error bars need to be defined as either standard errors or standard deviations. Additio-nally, the use of different lower-case letters indicating statistical differences should be consistent across all figures as the interpretation seems to differ throughout your manuscript.
For example, figures 2A and 2B (water retention curves) and 3A and 3B (hydraulic conductivity) are difficult to interpret visually and do not seem to greatly impact your conclusions. Consider leaving out these figures and presenting results on hydraulic conductivity in either Table 4 or a smaller table. Figures 5-7 (microbial biomass, EOC, N mineralization) present results as a function of incubation time. In the statistical analysis you did not include time as a factor in the ANOVA analysis. Since the study seems to focus on long-term effects of the amendments rather than when changes occur nor which mechanisms play a role, I suggest presenting cumulative results for EOC and N mineralization and net changes for microbial biomass between day 0 and 56, leaving out figures 5-7 and rather summarizing information in a table. Alternatively, current figures could be presented as time series rather than histograms to improve interpretation and comparison between treatments.
Across all figures, the letter annotation used to imply statistical differences is inconsistent. For example, in Figure 7C (nitrate ion concentration) SCcat (‘ab’) is not significantly different (p. 14, l. 343) from the control (‘a’), the same applies for Figure 11A (plant available heavy metals) where SCplas (‘ab’) is not significantly different (p. 17, l. 399) from control (‘a’), whereas in Figure 10A (Cd concentrations) SCcat ‘b’ is significantly different (p. 16, l. 378) than the control (‘ab’). These are just a few of many contradicting examples. Please make it consistent between figures whether single letters are statistically different from letter combinations containing the same letter to avoid confusion. Also, ensure that you specify whether significant difference refers to control or other treatments.
- Overselling results and conclusions inconsistent with figures
There are several inconsistencies throughout your manuscript between results in figures and conclusions. Conclusions in some instances directly contradict each other. This is a major concern, as it influences the scientific credibility of your results. The inconsistencies between abstract, results, discussion and conclusions reduce clarity. In the end it heavily influences take-home messages and could potentially influence agricultural policies that rely on scientific results, if policymakers only read the abstract, thus misinterpreting the effectiveness of the novel solid carbon materials. I recommend rephrasing all examples below to accurately reflect data and align abstract, results and discussion to avoid contradictions.
An overarching issue in your manuscript is repeated claims that SCplas “consistently” demonstrates “strong immobilization” (l. 28; 378; l. 397; l. 540; l. 618), however this is not reflected in figure 10 and 11. SCplas is misrepresented as better and more effective on heavy metal immobilization than results show, thus overselling results. Figure 10 shows that SCplas significantly reduced concentrations for Cd at two sites (although significance is unclear due to annotation as discussed in major comment above), for Cu at 3 sites, for Ni at two sites (at two sites below detection limit) and for Zn at no sites. SCplas significantly increased Zn concentration at one site. Figure 11 showed no significance of SCplas on Cd, significance for Cu at two sites (at two sites below detection limit), no significance for Ni (at two sites below detection limit) and neither for Zn. Altogether, these results do not convince me of the manuscript’s repeated claims that SCplas is consistent. I do agree that the effect is in some instances strong, however it seems to heavily depend on location and heavy metal. Therefore, the manuscript should be adjusted in the following examples where the word “consistent” is used. The list below also includes examples where other results are misinterpreted:
- 1, l. 28 (abstract): “In soils contaminated with heavy metals, SCplas showed strong immobilization particularly for Cd and Cu, across several sites, outperforming the reference materials”. Figure 10 shows that (1) biochar performs better than SCplas for Cd and Zn at two locations, (2) SCplas performs better than biochar for Cu, (3) effects are quite similar for Ni (SCplas more significant at one site, biochar more significant at the other). Graphite in no cases significantly increases concentration. Thus, I do not agree that biochar was outperformed by SCplas, only for Cu this is true. Please rephrase this.
- 1, l. 30 (abstract). You highlight a promise for SCplas for site-specific soil improvement and only caution against hydrophobic effects, thus failing to warn against ecotoxicological effects seen in figures 8 and 9. Springtails showed avoidance of SCplas in silty loam soil (Figure 9), whilst earthworms showed avoidance in sandy soils (Figure 8). Please add the ecotoxicological aspect to your promise for SCplas to nuance it.
- 1, l. 31 (abstract). You claim that SCcat has more negative ecotoxicological effects than positive ones depending on the soil. This is not in line with figures 8 and 9 showing that SCcat does not significantly influence avoidance behavior of earthworms and springtails in either of the soils. It is also not in line with conclusion in which you claim (P. 24, l. 625) that SCcat is a safe soil amendment. Please adjust this incorrect claim.
- 16, l. 367. You claim that springtail behavior in silty loam soil was only different for SCplas. This directly contradicts results in figure 9B and sentence that follows, both showing that springtails also significantly avoided biochar and graphite. Please correct to include biochar and graphite.
- 16, l. 378 (results):“(…) with SCplas consistently demonstrating a strong immobilizing effect” (referring to figure 10). Remove ‘consistently’.
- 16, l. 383 (results): “SCplas also reduced Ni concentrations (….), while SCcat and other amendments had negligible effects (Fig. 10C).” Figure 10C shows that biochar had a significant effect at both Braunschweig locations as well as SCplas. Please adjust, so that “other amendments” are not all said to have negligible effects.
- 17, l. 397 (results): “SCplas consistently demonstrated a strong immobilizing effect” (referring to figure 11). Remove ‘consistently’.
- 22, l. 540 (discussion): “SCplas consistently demonstrated a strong immobilizing effect across sites, particularly for Cd”. Remove ‘consistently’.
- P 24, l. 618 (conclusion): “Nevertheless, in soils contaminated with heavy metals, SCplas consistently strongly immobilized heavy metals, particularly Cd, Cu and Ni.” Remove ‘consistently’.
- Missing discussion on soil-specific responses to heavy metal immobilization
Your results clearly show that the soil amendments SCplas and SCcat cannot be safely applied across all sites and soil types. You note that this variability underscores the importance of considering soil properties such as soil texture, pH and organic matter in amendment selection. Other than the examples of limited Zn mobility in Neckarwestheim (p. 23, l. 571) and reduced Cd mobility in Gundelsheim and Neckarwestheim (p. 23, l. 581), where you provide strong explanations related to pH and organic matter contents, other site-specific responses remain unexplained. Failing to discuss all variable site-responses to soil amendment leaves the reader without possible explanations, thus further complicating applicability of results. I suggest ha-ving a more comprehensive discussion of how site-specific properties potentially influenced amendment performance to improve applicability of your results.
For example, biochar has significant positive effects on Cd, Ni and Zn in Braunschweig 1 and 2, but not on remaining sites. SCplas has significant (positive) effects on Cd, Ni and Cu at the same two sites. This indicates that Braunschweig sites might respond better to soil amendments than the remaining two sites, possibly due to differences in soil properties. Another example is that interestingly, in some cases heavy metal mobility is increased by soil amendments. This is only highlighted for SCcat (l. 29), but not for the other soil amendments. In Gundelsheim biochar statistically significantly increased Cd levels (Figure 10A), graphite increased Cu levels (Figure 10B) and all amendments, especially SCplas, significantly increased Zn levels compared to control (Figure 10C). These statistically significant observations were not addressed.
I recommend comparing site-specific pH, organic matter and texture (table 3) with the observed metal levels at each site for each soil amendment. Lastly, I recommend clarifying that the vari-ability in your results cannot be explained with your current knowledge to acknowledge research gaps.
Minor arguments
- Title: Indicates that the study deals with carbon storage, however the study only argues for increased carbon storage through microbial respiration of C which is not sufficient evi-dence. At least a measurement of total organic carbon would be needed to support this claim. I suggest changing your title to something like “Assessing the impact of solid carbon from methane cracking on soil hydraulic properties, soil biology, ecotoxicology and heavy metal mobility”.
- Missing background on methane pyrolysis: Readers of Biogeosciences might lack extensive chemical knowledge behind methane pyrolysis not enabling them to fully understand the chemistry of the novel solid carbon materials nor the effects on sorption of heavy metals or soil properties. I recommend elaborating on the materials section to point out differences between SCcat, SCplas and biochar (since you build your hypothesis on knowledge from biochar) and explain how the different physical and chemical properties (table 1) can possibly influence heavy metal sorption and microbial activity.
- Methodology: For the ecotoxicological tests on earthworms and springtails there are a few unclarities regarding which protocols were followed in each test.
- You reference Han et al (2021) when describing the earthworm avoidance test, thus implying having followed ISO 17512-1:2008 (2008) as this is the protocol followed by Han et al (2021) for their test on earthworms. You also refer to ISO (2008) for the avoidance rate threshold. Your experimental design is not consistent with ISO (2008) as the duration of 28 days breaks the standard of ISO (2008) being 2 days. Further, sample size, nor age and size of the earthworms are not consistent. Daily monitoring adds variability to the results and might pose stress to earthworms. This complicates the use of the avoidance rate threshold of 25 % stated in the protocol, as it might not be valid for a duration longer than recommended. It is unclear to me whether you have failed to meet the requirements for the correct protocol, used a modified version of the ISO protocol or an unknown protocol, which you fail to reference. I strongly recommend clarifying this, as it complicates cross-comparison of studies using avoidance tests and affects the credibility of your results. I also suggest referencing Han et al (2021) earlier when introducing the earthworm avoidance test.
- You explicitly state having conducted avoidance test with springtails in accordance with OECD guideline 232 (2009), a reproduction test used to test chemicals on collembolans (OECD, 2009), however, the experimental design is not consistent with this protocol. Rather, the design seems consistent with ISO 17512-2:2011 (2011), an avoidance test on collembolans, in terms of duration, replicates and number of springtails in each vessel. I suggest that you look into ISO (2011) and determine whether this is the protocol you used.
- Unclarities and conflicting statement regarding PAHs
- 3, L. 96: You state that SCplas is washed with dichloromethane to extract any residual PAHs, however later (p. 21, l. 501) you say that PAHs in SCplas might have inhibited microbial respiration due to the toxic effects. So, was the PAH not completely removed after all? Please clarify whether this is the case.
- 20, L. 481: Results for PAH levels in SCcat are mentioned. These results do not appear in result section nor does the method for determination appear in methods section. Please make sure this is corrected.
- 21, L. 501: Results for PAH levels in SCplas are mentioned. These were also not mentioned earlier. Please make sure this is corrected.
- Incorrect citation structure: There are numerous examples of incorrect citations, where names appear inside brackets, although in these cases they should be outside, whilst only years should be inside brackets. This list is not guaranteed to be conclusive:
- 8, l. 218: “(...) the study of (Ingwersen, 2001)”.
- 19, l. 435: “These findings align with (Bordoloi et al,. 2021).”
- 19, l. 436: (Bordoloi et al., 2021) investigated how (…)”.
- 20, l. 457: “The results are consistent with those of (Blanco-Canqui, 2017)”.
- 20, l. 471: “According to (Jing et al., 2022; Yao et al., 2014)”
- 20, l. 484: “In the experiment of (Van Zwieten et al., 2010)”
- 20, l. 515: “A study by (Wu et al, 2012) found…”
- 23, l. 566: “(…) previous research by (Edah et al., 2020) and (Villagra-Mendoza and Horn, 2018)”
- 23, l. 576: “with (Edeh et al, 2020)”
- 23, l. 590: ”(Wei et al, 2023) emphasized (…)”
- Faulty citations. You seem to cite things not explicitly mentioned in referenced papers.
- 9, l. 236: You cite Cao et al (2008) for ammonium-nitrate extractions yielding bio-available heavy metals, but the paper mentions only EDTA extractions, not ammonium-nitrate extractions. However, Meers et al., (2007) do mention ammonium-nitrate extractions. Perhaps you meant to cite this paper instead. Please check this.
- 19, l. 421: “A plausible explanation for this pattern is that SCplas aggregates within the soil matrix, creating stone-like clusters that block pore connectivity and hinder water retention (Ajayi and Horn, 2016; Hardie et al, 2024)”. The way you cite implies that the studies examined SCplas. Both studies were on biochar. The reference should be rewritten to include something like: “(…) as suggested by several studies (Ajayi and Horn, 2016; Hardie et al, 2024) studying biochar. Their findings might also apply to SCplas due to…”. When I investigate the articles I cannot find anything which backs up the claim that stone-like clusters decrease water retention. Please check this.
- 21, L. 521: “In the silty loam soil, the toxic effect of SCplas was likely to be neutralized by the high adsorption capacity of clay particles, which could bind PAHs and reduce their bioavailability (Yu et al., 2024)” You point out that clay is likely the neutralizer of PAHs, however it seems off, when the article you are referencing concluded that SOM is more dominant than clay in adsorption of PAHs. Why not use SOM as a possible explanation? Yu et al (2024) do not directly claim that clay can neutralize toxic effect by binding PAHs and reduce bioavailability. Rather they claim that dissolved organic matter does this, whilst acknowledging that clay minerals and texture also play a role in the binding of PAHs. Be careful of the phrasing in the above example and please rephrase. Further, I suggest noting that you assume that your silty loam soil contains the common clay particles of high adsorption capacity e.g., kaolinite, smectite and illite, as you did not determine the adsorption capacity of your clay minerals.
- 22, L. 547: “Conversely, SCcat may have introduced responsible organic components and has a lower surface area anyway, thus resulting in increased Cd solubility (Blanco-Canqui, 2017).” Your referencing implies that Blanco-Canqui (2017) investigated SCcat. They investigated biochar and do not mention cadmium, solubility nor surface areas in relation to solubility. I am unsure what exactly from the paper by Blanco-Canqui you aimed to reference. Please mention explicitly that Blanco-Canqui studied biochar. I recommend rephrasing this sentence and leaving out “anyway”.
- Suggested extended use of your own references
- Beesley et al (2010) could be used as a reference to compare similar results for biochar on Zn and Cd mobility and differing results for Cd.
- Ajayi (2016) could be useful to compare biochar effects on hydraulic conductivity and plant available water, as results are similar for the silty loam soils, although a different wood pyrolysis temperature was used.
- Han et al (2021) could be used to discuss effects of pH on earthworm behavior and genotoxic effects of PAHs on earthworm DNA, in the part where you discuss earthworm response to PAHs (P. 15 l. 522)
Minor issues
- 1, l. 31: I suggest not only cautioning against hydrophobic effects in sandy soils, but also ecotoxicological effects.
- 3, l. 72: Perhaps reference (TITAN Project) rather than inserting a link. The link should instead appear in the reference list.
- 4, l. 106: You chose the application rate based on previous experiments with biochar. I suggest explaining why you chose the rate – did the results of cited studies present evidence why this application rate is optimal?
- 9, l. 247: I do not think it is appropriate to say “their” sandy soils when referencing Streck and Richer (1997).
- 11, Table 4: The porosity is the same for all treatments and the whole column could be removed instead explaining in table text that the porosity is the same.
- 10, l. 285: An ‘s’ appears at the end of the line
- 12, l. 316: You mention that SCcat and SCplas reduced CO2 produced in silty loam soil. However, so did graphite and biochar. I think this should be included in the observation.
- 12, l. 317: You refer to Fig S1A and B, which do not exist in the paper. Do you mean Fig 4?
- 14, Figures 7A/7B: Missing a legend, since the total figure (7A-D) is cut in half.
- 15, l. 366: You have a space too much before “The avoidance”
- 17, Figure 10: Missing figure text on letter annotations indicating statistical differences.
Error bars are unclear on multiple bars or perhaps errors are so small, they are nearly invisible - if so, that should be clarified. Figure refers to “C: control”, which is unnecessary, as the legend labels it “Control” and not “C”. - 17, l. 403: You state that for Ni, SCplas, biochar and graphite reduced concentrations below detection limits at Neckarwestheim. This is inconsistent with figure 11C, which shows a detected level for biochar.
- 18, Figure 11: Refers to “C: control”, which is unnecessary, as the legend labels it “Control” and not “C”.
- 19, l. 440: Unnecessary to start the sentence by “Other than in the sandy soil, (…)”
- 20, l. 490: Claiming that high crystallinity and low reaction surface of SCcat likely ensure long-term stability as indicated by non-enhanced CO2 production seems a bit of a stretch. More evidence is required to determine that SCCat possibly ensure long-term stability.
- 21, l. 491 and l. 493: Do not include the title of Table 1 in the bracket.
- 22 l. 542: Do not say “other studies” when you only reference one single study.
- 22 l. 562: Do not use “studies” in plural when you only refer to one single study.
- 23, l. 589: Once again you state that for Ni, SCplas, biochar and graphite reduced concentrations below detection limits at Neckarwestheim. This is inconsistent with figure 11C, showing a detected level for biochar.
Citation: https://doi.org/10.5194/egusphere-2025-3866-CC1
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- 1
Dear authors,
I read with great interest your work on the use of solid carbon derived from methane pyrolysis as soil amendment and its effect on a wide range of soil quality indicators. You presented a large dataset, comparing them with graphite and biochar while giving many insights in the effect of these amendments. Although I encountered a few unclarities, given below, more importantly is the discussion, which is at this point more or less a summation of the results. Especially as you indicate in the introduction that you want to compare these novel materials with the more widely studied biochar and graphite, I had expected to find a more general discussion and conclusion on this point. Maybe it helps to create a two step comparison: (1) Do SCcat and SCplas differ from each other? (2) Does SC’s have an advantage/disadvantage over biochar or graphite?
I would also encourage you to include some form of summary statistics, like for example principle components analysis, as it will create a better connection between the two experiments and identify the most important differences between the studied soil amendments. Here below some minor comments, which hopefully encourage you to publish the interesting results you collected.
Minor points encountered:
Line 50-51: “The fertility of soils is known to be positively related to carbon content” as you’re discussing different carbon forms and inorganic carbon is not known to enhance soil fertility, I suggest to be more specific, i.e. “organic carbon content”. Furthermore, your study is more into soil functioning rather than productivity, therefore the term “soil quality” might fit better.
Line 69: “Building on the well-documented benefits of biochar,” Do I understand it right you assume the observations of biochar can be directly translated to solid carbon derived from methane pyrolysis? If so, why did you test SC against biochar and graphite if no difference was expected? This can be stated more clearly, otherwise indicate where the materials likely differ.
Line 73-75: Although you nicely state the focal points of your study, you don’t give an expected direction of effect. This could have easily been your hypotheses as well. Considering that you test the SCs against biochar and graphite, it is better to state what differences (advantages/disadvantages) you expected to find.
Line 88: “Fe@carbon catalyst,” Is this a typo or brand name?
Line 99: Do you have some data on the C and N content/ratio for the biochar? This would be good for further data interpretation.
Line 222: “4.4% TOC” -> According Table 3 it is 5.0%, please check.
Line 615: “, in contrast, had contrasting effects” Maybe change a “contrast” in a synonym or rewrite.
Table 2: How was Corg and Nt determined? Do you know if there are carbonates in the silty loam soil and if so, how much? Note that you also use Nt in the earthworm experiment, so please change the abbreviation.
Table 3: Do you have data on the carbonate content? Would be good to add, if available.