Amending solid carbon from methane cracking to arable soils: A sustainable approach to increase carbon storage and heavy metal immobilization?

Saki, Hermin; Kostiuk, Kateryna; Ingwersen, Joachim; Krauße, Galina; L’hospital, Valentin; Guilhaume, Nolven; Radoiu, Marilena; Farrusseng, David; Streck, Thilo; Kandeler, Ellen; Marhan, Sven

doi:10.5194/egusphere-2025-3866

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https://doi.org/10.5194/egusphere-2025-3866

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29 Sep 2025

| 29 Sep 2025

Amending solid carbon from methane cracking to arable soils: A sustainable approach to increase carbon storage and heavy metal immobilization?

Hermin Saki, Kateryna Kostiuk, Joachim Ingwersen, Galina Krauße, Valentin L’hospital, Nolven Guilhaume, Marilena Radoiu, David Farrusseng, Thilo Streck, Ellen Kandeler, and Sven Marhan

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 (SC_plas) 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, SC_plas 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 (SC_cat) had almost no effect on the biological properties studied. In soils contaminated with heavy metals, SC_plas showed strong immobilisation of heavy metals particularly for Cd and Cu, across several sites, outperforming the reference materials. However, SC_cat increased Cd mobility at some sites, indicating little or even adverse effects on heavy metal mobility. Our results highlight the promise of SC_plas for site-specific soil improvement, while cautioning against its hydrophobic effects in sandy soils. In contrast, SC_cat has more negative effects, especially ecotoxicological than positive ones depending on soil.

Received: 08 Aug 2025 – Discussion started: 29 Sep 2025

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Hermin Saki, Kateryna Kostiuk, Joachim Ingwersen, Galina Krauße, Valentin L’hospital, Nolven Guilhaume, Marilena Radoiu, David Farrusseng, Thilo Streck, Ellen Kandeler, and Sven Marhan

Status: final response (author comments only)

RC1: 'Comment on egusphere-2025-3866', Jeroen Zethof, 18 Dec 2025

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 SC_cat and SC_plas 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 C_org and N_t determined? Do you know if there are carbonates in the silty loam soil and if so, how much? Note that you also use N_t 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.

Citation: https://doi.org/10.5194/egusphere-2025-3866-RC1

AC1: 'Reply on RC1', Hermin Saki, 09 Jun 2026

Jeroen Zethof (Referee comment posted)

Dear authors,

Response: Thank you for this helpful general comment. We agree that the previous Discussion was too close to a summary of the Results and did not sufficiently compare the novel solid carbon materials with the reference materials. We therefore revised the Discussion to provide a clearer two-step comparison: first, whether SC_cat and SC_plas differ from each other, and second, whether they show advantages or disadvantages relative to biochar and graphite. We also added a general applicability section to synthesize the main benefits, limitations, and future research needs across soil physical properties, soil biology/ecotoxicology, and heavy metal mobility.

Regarding the suggestion to include additional summary statistics, we considered this carefully. However, the two experiments included different soils, endpoints, extraction methods, and response variables, which would make a combined statistical summary difficult to interpret and potentially misleading. Instead, we strengthened the synthesis in the Discussion to connect the two experiments and identify the most important material-specific differences.

Revision in manuscript: The Discussion was revised to provide a clearer comparison between SC_cat and SC_plas and their advantages and disadvantages relative to biochar and graphite. A new general applicability section was added to synthesize the results across the two experiments. Additional summary statistics were not included because the two experiments involved different soils, endpoints, and response variables, which would limit their interpretability.

Minor points encountered:

Response: Corrected. We revised the sentence to specify soil organic carbon rather than carbon in general and replaced “soil fertility” with “soil quality and functioning,” which better reflects the scope of the study. Based on this change, we also checked and updated the supporting references to ensure that they match the revised statement.

Revision: The sentence was revised to: “Soil quality and functioning are often positively related to soil organic carbon content (Giandon, 2015; Lal, 2004).” The corresponding references were updated accordingly.

Response: Thank you for this comment. We revised the introduction to clarify that we do not assume that biochar effects can be directly transferred to methane-derived solid carbon materials. We now explicitly state that SC_cat and SC_plas may differ from biochar in crystallinity, surface chemistry, porosity, and potential residual organics. We also clarified that biochar and graphite were used as reference amendments to test whether the novel solid carbon materials show similar or distinct effects.

Revision: The introduction was revised to clarify the rationale for comparing SC_cat and SC_plas with biochar and graphite and to state that material-specific, soil-dependent effects were expected.

“Building on the well-documented benefits of biochar, we do not assume that its effects can be directly transferred to solid carbon derived from methane pyrolysis because the materials differ in crystallinity, surface chemistry, porosity, and (for SC_plas) the potential presence of residual organics. We expected biochar to show stronger effects on water retention and heavy metal immobilization because of its higher specific surface area and porosity. In contrast, we expected SC_cat to be comparatively inert due to its lower specific surface area and more crystalline structure, resulting in smaller effects on soil hydraulic, biological, and chemical properties. For SC_plas, we expected stronger sorption-related effects than for SC_cat because of its higher specific surface area, but also potential negative effects on soil organisms and microbial processes due to residual organic compounds. We therefore hypothesize that the two solid carbon materials will exert distinct and potentially soil-dependent effects on (i) soil hydraulic properties (water retention and conductivity), (ii) soil biological and microbiological processes and (iii) the mobility and bioavailability of heavy metals. ”

Response: Thank you for this suggestion. We revised the hypothesis paragraph to state the expected direction of effects more clearly. We now explain that biochar was expected to show stronger effects on water retention and heavy metal immobilization, SC_cat was expected to behave more inertly, and SC_plas was expected to show stronger sorption-related effects than SC_cat but also possible negative effects on soil organisms and microbial processes.

Revision: The Introduction was revised to provide more directional hypotheses and clarify the expected differences among biochar, SC_cat, and SC_plas.

Line 88: “Fe@carbon catalyst,” Is this a typo or brand name?

Response: Regarding the name of the Fe@carbon catalyst, this is a common notation in materials science to indicate a certain structure. In this case, Fe@carbon means that it is a core-shell structure, with Fe in the center and carbon around it.

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.

Response: Thank you for the clarification. We have this data and have included it in the publication.

Revision in manuscript: This information has been added to Table 1, “Characteristics of the studied solid carbon materials.”

	Solid carbon from catalytic pyrolysis (SC_cat)	Solid carbon from plasma pyrolysis (SC_plas)	Biochar	Graphite
Starting material	methane	methane	Straw (Miscanthus x giganteus)	graphite
Structure	crystallized	crystallized	amorphous	crystallized
Specific surface area, (m² g^-1)	5.9	58.8	346.4	17.4
Mesoporosity	2< pore diameter <50 nm	n.d.	2< pore diameter <50 nm	2< pore diameter <50 nm
Particles size	< 200 μm	<200 µm	125 and 1000 µm	< 50 μm
Metallic iron	1.7 wt.%	n.d.	n.d.	n.d.
Phenanthrene, (mg kg⁻¹)	0.09 ± 0.02	2.5 ± 0.05	n.d.	0.1 ± 0.02
Pyrene, (mg kg⁻¹)	0.01 ± 0.002	1.7 ± 0.2	n.d.	0.02 ± 0.004
C_org(%)	98	100	77.44
N_tot(%)	n.d.	n.d.	0.59
C/N(%)	n.d.	n.d.	131

Line 222: “4.4% TOC” -> According Table 3 it is 5.0%, please check.

Response: Thank you for identifying this inconsistency. We checked the value and corrected the text to match the table.

Revision in manuscript: The TOC value was corrected to 5.0%.

Line 615: “, in contrast, had contrasting effects” Maybe change a “contrast” in a synonym or rewrite.

Response: Corrected. We rewrote the sentence to avoid the repetition of “contrast/contrasting” and to improve clarity.

Revision: The sentence was revised to: “SC_plas showed texture-dependent effects on plant-available water.”

Response: Thank you for this comment. We agree that the determination of C_org and N_tot needs to be described more clearly. We revised the Methods section to clarify that C_org and N_tot were determined separately from loss on ignition. We also revised the notation in the earthworm avoidance equation to avoid confusion with N_tot as total nitrogen.

Revision in manuscript: The Methods section was revised to clarify the determination of C_org and N_tot and to distinguish it from loss on ignition. The notation in the earthworm avoidance equation was revised to avoid ambiguity.

Table 3: Do you have data on the carbonate content? Would be good to add, if available.

Response: Thank you for this suggestion. Unfortunately, carbonate content was not available for all soils used in the study. To keep the soil-property table consistent across sites, we therefore did not add carbonate content. However, we clarified the available carbon measurements in the Methods section by specifying how total carbon, inorganic carbon, and organic carbon were determined where available.

Revision in manuscript: The Methods section was revised to clarify the determination of total carbon, inorganic carbon, and organic carbon where available. Carbonate content was not added to the soil-property table because comparable data were not available for all soils.

Citation: https://doi.org/10.5194/egusphere-2025-3866-AC1

CC1: 'Comment on egusphere-2025-3866', Line Broszka, 19 Dec 2025

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

AC3: 'Reply on CC1', Hermin Saki, 09 Jun 2026

Line Broszka

Saki and Kostiuk (2025) studied the effects of two novel (CO₂-emission free) solid carbon materials from methane pyrolysis, SC_plas (plasmalytic pyrolysis) and SC_cat (catalytic pyrolysis), 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; SC_plas 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. SC_cat 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.

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. Additionally, 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.

Response: We agree that the large number of figures can make it difficult to identify the key messages. To improve readability, we revised the Results text to more clearly guide the reader through the key findings and we revised the figure captions throughout to (i) explicitly define the error bars and (ii) standardize the interpretation of letter annotations across figures. We also re-checked post-hoc groupings and harmonized the letter notation so that treatments sharing at least one letter are not significantly different within a site.

Revision in manuscript: Figure captions and Results text revised; statistical lettering was checked and harmonized across figures.

Response: We appreciate this suggestion. We retained the water retention and hydraulic conductivity results because they directly address the first hypothesis (effects on soil physical functioning). However, we revised the corresponding text to more explicitly link these figures to the main conclusions and to highlight the effect directions and their magnitude, rather than repeating values.

Revision in manuscript: Revised Discussion text in Sections 4.1.

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.

Response: we agree that a large number of panels can make it difficult to identify key messages. Therefore, we removed graphs 5 and 7 and summarized the information in a table. As for graph 6, we would like to keep it, since the materials we studied have a statistically significant impact on the EOС.

Revision in manuscript: The information previously shown in Figures 5–7 has been summarized in two new tables (Table 5 and Table 6), which present microbial biomass, EOC, and nitrogen species across sampling times and treatments. This allows clearer comparison between treatments and time points. Table 5. Microbial biomass (C_mic) and extractable organic C (EOC) during 56 days of incubation after addition of 40 t ha^-1 solid amendment (mean and standard error, n = 4). Treatments were: C: control, SC_cat: solid carbon from catalytic pyrolysis, SC_plas: solid carbon plasma pyrolysis, BC: Biochar, and G: Graphite. Different lowercase letters represent a significant difference at p < 0.05 between treatments at a given sampling day (7, 14, 35, or 56).

	Treatments	Microbial biomass (µg g^-1 soil dw)	Extractable organic C (EOC)
days	days
7	14	35	56	7	14	35	56
Sandy soil	Control	165.8±45.4^a	217.9±20.6^ab	174.1±18.7^ab	199.4±13.9^a	79.3±22.3^a	61.5±5.2^a	55.9±1.3^ab	59.5±6.0^a
SC_cat	143.6±23.9^a	226.9±36.7^ab	213.9±35.6^ab	185.5±26.2^a	63.2±2.7^a	52.2±2.5^b	49.7±2.8^bc	57.0±5.2^a
SC_plas	150.0±43.4^a	235.2±15.2^a	192.9±26.7^a	201.8±26.9^a	26.2±1.7^b	19.4±4.0^c	21.2±4.1^d	25.3±7.7^b
Biochar	179.5±4.4^a	251.4±11.9^a	210.7±7.9^a	208.4±6.5^a	62.9±1.8^a	50.4±0.8^b	47.7±2.7^c	54.9±4.8^a
Graphite	103.3±15.4^a	171.5±8.3^b	147.3±12.8^b	169.3±11.2^a	74.2±17.3^a	56.4±1.5^ab	57.2±4.8^a	58.9±5.7^a
Silty loam soil	Control	277.5±19.1^a	359.3±27.9^a	292.8±40.0^a	324.3±44.9^a	39.5±2.5^ab	36.1±1.5^ab	38.9±0.8^a	46.3±4.6^a
SC_cat	268.3±31.5^a	312.9±33.5^a	252.3±55.1^a	292.6±30.5^a	31.6±2.9^c	30.8±1.5^b	32.4±2.7^a	32.1±1.1^b
SC_plas	247.3±10.3^a	333.6±20.4^a	259.3±24.5^a	309.9±9.5^a	15.8±3.7^d	14.3±6.1^c	18.3±9.9^b	17.6±3.6^c
Biochar	260.6±39.2^a	340.4±21.1^a	289.8±25.3^a	350.3±45.0^a	41.2±4.0^a	38.1±1.5^a	39.1±5.3^a	43.6±6.0^a
Graphite	222.2±55.2^a	310.6±56.8^a	244.3±13.5^a	285.8±53.3^a	33.7±1.4^bc	33.1±2.0^ab	36.0±4.8^a	33.2±0.6^b

Table 6. Concentrations of ammonium and nitrate ions during 56 days of incubation after addition of 40 t ha^-1 solid amendment (mean and standard error, n = 4). Treatments were: C: control, SC_cat: solid carbon from catalytic pyrolysis, SC_plas: solid carbon plasma pyrolysis, BC: Biochar, and G: Graphite. Different lowercase letters represent a significant difference at p < 0.05 between treatments at the four sampling days (7, 14, 35, 56).

	Treatments	Concentrations of ammonium (µg g^-1 soil dw)	Concentrations of nitrate (µg g^-1 soil dw)
days	days
7	14	35	56	7	14	35	56
Sandy soil	Control	0.6±0.98^c	0±0^b	0.8±0.7^b	7.5±6.6^ab	33.5±2.5^a	37.5±1.8^a	47.4±1.8^a	51.5±8.6^a
SC_cat	1.7±1.8^bc	2.9±3.9^ab	1.6±1.9^b	5.2±5.7^ab	31.2±3.3^ab	37.7±2.7^a	47.6±4.7^a	53.2±6.1^a
SC_plas	6.0±3.9^ab	8.6±4.5^a	12.7±6.5^a	15.3±4.0^a	24.7±1.6^c	27.7±0.8^b	31.5±1.9^c	32.2±0.2^b
Biochar	0±0^c	0±0^b	0±0^b	0.1±0.2^b	26.8±3.8^bc	30.0±2.4^b	36.7±2.0^bc	47.1±2.8^a
Graphite	6.5±0.7^a	8.6±2.2^a	15.3±3.8^a	12.5±6.6^a	27.8±1.1^abc	31.5±1.7^b	38.8±3.9^b	49.8±2.3^a
Silty loam soil	Control	3.7±6.9^a	0±0^a	0±0^b	0.9±1.1^a	17.3±1.6^b	19.5±0.6^b	24.8±1.4^b	32.7±0.9^b
SC_cat	0±0^a	0±0^a	0±0^b	0±0^a	15.5±0.6^b	16.8±0.5^c	23.4±0.4^bc	28.7±1.5^c
SC_plas	0±0^a	0±0^a	1.4±0.9^a	2.7±2.5^a	15.4±0.7^b	16.5±0.1^c	18.5±1.7^d	21.5±2.0^d
Biochar	8.0±9.0^a	0±0^a	0±0^b	0±0^a	16.1±0.6^b	15.7±0.3^c	21.2±1.2^c	25.4±1.9^c
Graphite	0.9±1.7^a	0±0^a	0±0^b	0.03±0.05^a	21.9±0.6^a	23.9±1.4^a	31.6±0.8^a	37.3±2.1^a

Across all figures, the letter annotation used to imply statistical differences is inconsistent. For example, in Figure 7C (nitrate ion concentration) SC_cat (‘ab’) is not significantly different (p. 14, l. 343) from the control (‘a’),

Response: thank you for clarifying. We have checked all statistical letter designations in all figures and would like to note that different lowercase letters indicate significant differences between treatment options within each plot/soil, and that treatment options sharing at least one letter do not differ significantly (post-hoc test, p < 0.05).

Revision in manuscript: we checked all statistical letter designations in all figures.

the same applies for Figure 11A (plant available heavy metals) where SC_plas (‘ab’) is not significantly different (p. 17, l. 399) from control (‘a’), whereas in Figure 10A (Cd concentrations) SC_cat ‘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.

Response: We agree and have made the statistical lettering consistent across figures. In all figure captions we now state explicitly that different lower-case letters indicate significant differences among treatments within each site/soil, and that treatments sharing at least one letter are not significantly different (post-hoc test, p < 0.05). This is now applied consistently, and the Results text refers to significant differences among treatments within each site.

Revision in manuscript: Updated captions for Figures 8 and 9; minor edits in Results text to align wording with the statistical output.

Overselling results and conclusions inconsistent with figures

An overarching issue in your manuscript is repeated claims that SC_plas “consistently” demonstrates “strong immobilization” (l. 28; 378; l. 397; l. 540; l. 618), however this is not reflected in figure 10 and 11. SC_plas is misrepresented as better and more effective on heavy metal immobilization than results show, thus overselling results. Figure 10 shows that SC_plas 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. SC_plas significantly increased Zn concentration at one site. Figure 11 showed no significance of SC_plas 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 SC_plas is consistent. I do agree that the effect is in some instances strong, however it seems to heavily depend on location and heavy metal.

Response: Thank you for pointing this out. We agree that our previous wording overstated the metal immobilisation effects of SC_plas. We re-checked Figures 8 and 9 and the associated statistical groupings and revised the manuscript to ensure that all statements are fully consistent with the results. Specifically, we removed the terms “consistently” and “strong” where these were not supported and now describe heavy-metal responses as metal- and site-specific. In the revised text, we report that SC_plas shows the clearest immobilisation effect for Cu (reduced concentrations in the soil-solution proxy at several sites and reduced plant-available Cu at specific sites), whereas Cd does not show consistent reductions across extraction pools. We also revised related sentences in the Results, Discussion, Conclusions, and Abstract accordingly.

Revision in manuscript: Revised Abstract; Results (Sections 3.2.1–3.2.2); Discussion (Sections 4.3); and Conclusions to remove unsupported claims of consistent/strong immobilisation and to align the interpretation with Figures 8–9.

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, SC_plas showed strong immobilization particularly for Cd and Cu, across several sites, outperforming the reference materials”. Figure 10 shows that (1) biochar performs better than SC_plas for Cd and Zn at two locations, (2) SC_plas performs better than biochar for Cu, (3) effects are quite similar for Ni (SC_plas 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 SC_plas, only for Cu this is true. Please rephrase this.

Response: We agree. We revised the abstract to remove the claim that SC_plas outperformed the reference materials. The revised wording emphasizes metal- and site-specific responses and states that SC_plas showed the clearest immobilisation for Cu, whereas Cd effects were not consistent across extraction pools.

Revision in manuscript: Abstract revised to remove “outperforming the reference materials” and to report metal-specific effects.

1, l. 30 (abstract). You highlight a promise for SC_plas 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 SC_plas in silty loam soil (Figure 9), whilst earthworms showed avoidance in sandy soils (Figure 8). Please add the ecotoxicological aspect to your promise for SC_plas to nuance it.

Response: Thank you for the clarification, we completely agree with you and have added the ecotoxicological aspect.

Revision in manuscript: The abstract has been revised to add an ecotoxicological aspect: “In addition, recorded ecotoxicological reactions - avoidance of sandy soil supplemented with SC_plas by earthworms and avoidance of treated loamy soil by springtails - require careful application.”

1, l. 31 (abstract). You claim that SC_cat has more negative ecotoxicological effects than positive ones depending on the soil. This is not in line with figures 8 and 9 showing that SC_cat 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 SC_cat is a safe soil amendment. Please adjust this incorrect claim.

Response: We agree that this statement was incorrect. Figures 6 and 7 show that SC_cat did not significantly influence avoidance behavior of earthworms or springtails in either soil, and our сonclusions describe SC_cat as a comparatively inert amendment. We therefore removed the claim that SC_cat has more negative ecotoxicological effects than positive ones and revised the abstract to state that SC_cat showed generally minor effects on the biological endpoints assessed, indicating comparatively limited ecotoxicological effects under our test conditions.

Revision in manuscript: Abstract revised to remove the unsupported negative ecotoxicology claim for SC_cat and to align with Figures 6–7 and the сonclusions. “In contrast, SC_cat was characterized by a generally neutral effect on soil biota and did not cause significant changes in the biological properties of the soil.“

16, l. 367. You claim that springtail behavior in silty loam soil was only different for SC_plas. 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.

Response: Thank you for clarifying. We stated that the behavior of springtails in silty loam soil differed only for SC_plas, meaning that in sandy soil they did not avoid soil with this additive, but in silty loam soil they began to avoid it. As for biochar and graphite, the behavior of springtails did not differ from the previous soil (avoiding both sandy soil and silty loam soil). However, for a better understanding of the text, we rephrased this sentence to make it clearer.

Revision in manuscript: we rephrased the sentence : “In the silty loam soil, unlike sandy soil, the behavior of F. candida was different only for SC_plas. Springtails showed avoidance of soils amended not only with biochar and graphite, but also with SC_plas, whereas no significant avoidance was observed for SC_cat (Fig. 6B).“

16, l. 378 (results):“(…) with SC_plas consistently demonstrating a strong immobilizing effect” (referring to figure 10). Remove ‘consistently’.

Response: We agree. The term “consistently” overstates the pattern shown in Fig. 8. We have removed “consistently” and revised the sentence to describe the response as metal- and site-specific, highlighting the effects that are supported by the figure and statistics.

Revision in manuscript: Results (Section 3.2.1; Fig. 8 text) revised to remove “consistently” and to reflect site-/metal-specific effects.

16, l. 383 (results): “SC_plas also reduced Ni concentrations (….), while SC_cat and other amendments had negligible effects (Fig. 10C).” Figure 10C shows that biochar had a significant effect at both Braunschweig locations as well as SC_plas. Please adjust, so that “other amendments” are not all said to have negligible effects.

Response: We agree. Figure 8C shows that Ni concentrations were reduced not only by SC_plas but also by biochar at the Braunschweig sites. We revised the text accordingly, so that “other amendments” are not described as negligible where significant effects are present.

Revision in manuscript: Results (Section 3.2.1; Ni paragraph, Fig. 8C) revised to state that SC_plas and biochar reduced Ni at the Braunschweig sites, whereas SC_cat and graphite did not differ significantly from the control.

17, l. 397 (results): “SC_plas consistently demonstrated a strong immobilizing effect” (referring to figure 11). Remove ‘consistently’.

Response: We agree. We removed “consistently” and revised the wording to reflect that effects in Fig. 9 are metal- and site-specific (e.g., Cu reductions at specific sites), rather than uniform across metals and locations.

Revision in manuscript: Results (Section 3.2.2; Fig. 9 text) revised to remove “consistently” and align the wording with the site-/metal-specific outcomes.

22, l. 540 (discussion): “SC_plas consistently demonstrated a strong immobilizing effect across sites, particularly for Cd”. Remove ‘consistently’.

Response: We agree. We removed “consistently” and revised the discussion to avoid overstatement. In addition, we corrected the text to reflect that the clearest immobilisation effect of SC_plas is for Cu (and solution-phase Ni at the Braunschweig sites), while Cd does not show consistent reductions across extraction pools.

Revision in manuscript: Discussion (Section 4.3 and related text) revised to remove “consistently” and to accurately reflect the metal- and site-specific patterns shown in Figs. 8–9.

P 24, l. 618 (conclusion): “Nevertheless, in soils contaminated with heavy metals, SC_plas consistently strongly immobilized heavy metals, particularly Cd, Cu and Ni.”Remove ‘consistently’.

Response: We agree. We removed “consistently” and revised the conclusion to avoid a broad claim of strong immobilisation across all metals. The revised conclusion now states that SC_plas shows the clearest immobilisation effect for Cu (and reduced solution-phase Ni in the Braunschweig soils), whereas Cd did not show consistent reductions across extraction pools.

Revision in manuscript: Conclusions revised to remove “consistently strongly immobilized” and to provide a metal-specific summary consistent with Figs. 8–9.

Missing discussion on soil-specific responses to heavy metal immobilization

Your results clearly show that the soil amendments SC_plas and SC_cat 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 having 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. SC_plas 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 SC_cat (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 SC_plas, 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 variability in your results cannot be explained with your current knowledge to acknowledge research gaps.

Response: Thank you for this important comment. We agree that the previous discussion did not sufficiently address the soil-specific responses in heavy metal immobilization and, in particular, did not adequately discuss cases where amendments increased extractable metal concentrations. We therefore merged the previous Sections 4.3 and 4.4 and substantially revised the heavy-metal discussion. The revised section now emphasizes that amendment effects were metal-, fraction-, and site-specific and discusses how soil properties such as pH, organic carbon content, texture, and initial metal concentration may have influenced amendment performance. We also added a paragraph comparing the stronger responses in the Braunschweig soils with the more limited responses in Gundelsheim and Neckarwestheim, and we now explicitly acknowledge that some site-specific responses cannot be fully explained with the available data because metal speciation, amendment-induced pH changes, dissolved organic matter composition, and competitive ion effects were not fully characterized.

Revision: The heavy-metal discussion was rewritten to include a more comprehensive interpretation of site-specific amendment responses, including both immobilization and possible mobilization effects, and to acknowledge remaining mechanistic uncertainties.

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 evidence. 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”.

Response: We thank the reviewer for this important point and agree that our study does not directly quantify long-term carbon storage (e.g., via changes in total organic carbon or long-term persistence), and that CO₂ fluxes alone are not sufficient to substantiate a “carbon storage” claim in the title. We therefore revised the title to avoid implying verified carbon sequestration and to better reflect the scope of the work, which focuses on soil hydraulic properties, soil biology/ecotoxicology, and heavy metal mobility/availability.

Revision in manuscript: We changed the title to remove the explicit “increase carbon storage” claim and to emphasize the assessed endpoints (hydraulics, soil biology/ecotoxicology, and heavy metal mobility/availability).

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 SC_cat, SC_plas 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.

Response: Thank you for this recommendation. We expanded the Materials section to better explain the differences among SC_cat, SC_plas, and biochar, including crystallinity, specific surface area, porosity, particle size, and surface chemistry. We also added text explaining how these properties may influence microbial activity and heavy metal sorption.

Revision: The Materials section was expanded to provide background on methane-derived solid carbon materials and their potential effects on microbial activity and heavy metal sorption.

“According to the characteristics of the studied materials presented in Table 1, biochar, which is characterized by an amorphous structure, high specific surface area, and developed porosity, as well as the presence of a significant number of functional groups (–COOH, –OH, –C=O), is a more biologically active matrix for microorganisms. The high specific surface area and porous structure provide more accessible sites for cell adhesion and colonization of microorganisms, as well as promote more efficient sorption of nutrients and metabolites. In addition, the larger particle size of biochar can promote the formation of stable biofilms, while solid carbons are characterized by finer dispersion, which, on the one hand, provides better contact with cells, but on the other hand, can cause physical stress to microorganisms.

The crystal structure of solid carbons indicates their relative chemical inertness. The specific surface area of these materials is an order of magnitude lower than that of biochar, which limits the number of potential sites for microorganism adhesion and reduces their sorption capacity for nutrients. The absence of mesopores in SC_plas indicates limited possibilities for the formation of microbial micro-niches, in particular with regard to the retention of water, substrates, and signalling molecules, as well as the absence of effective protection of microorganisms from physicochemical stress factors.

Thus, biochar is a more suitable material for long-term microbial colonization, while solid carbons are likely to be involved mainly in short-term interactions with microbial communities. At the same time, the presence of metallic iron in SC_plas may partially compensate for the low specific surface area of this material. Iron can serve as a cofactor for enzymes, catalyze redox reactions, and stimulate electron transport processes, which potentially distinguishes SC_plas from SC_cat and indicates the possibility of stimulating specific microbial processes. In contrast, SC_cat, given its combination of low specific surface area and lack of additional active components, is likely to exhibit the least biological 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.

Response: We thank the reviewer for this important comment. The experiment was not conducted in accordance with ISO 17512-1:2008, and we acknowledge that the duration of the test and the experimental design differ significantly from the avoidance test specified in the ISO standard. To avoid confusion, we have revised the manuscript to clearly describe the experimental procedure used without reference to ISO 17512-1:2008. The earthworm avoidance test used in this study was a modified experimental approach developed for the purposes of this work. Han et al. (2021) is now cited only as the source for the calculation of the avoidance coefficient and the 25% avoidance threshold used to interpret the results.

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.

Response: We thank the reviewer for this helpful clarification. We have carefully re-evaluated the applied methodology and agree that the experimental design corresponds to ISO 17512-2:2011 rather than OECD 232. The manuscript has therefore been corrected accordingly, and the methods section has been revised to accurately reflect the standardized avoidance test protocol used for Folsomia candida.

Revision in manuscript: The avoidance test with springtails has been revised to state compliance with ISO 17512-2:2011. “The avoidance test with springtails was carried out in accordance with ISO 17512-2:2011 in plastic boxes (10.3 × 8.5 × 4.1 cm (height × length × depth). Boxes were filled with 100 g of control soil in one half and 100 g of test soil (1.11 g of solid carbon + 100 g of dry soil) in the other half without a barrier in between. Five replicates were prepared for each treatment. At the beginning of the test, 2 mg of dry yeast was homogeneously added to the soil surface in each test box, and then 20 springtails were added. The boxes were closed with lids ensuring gas exchange and incubated at 20 ± 1 °C. Two days later, the animals in each half of the boxes were counted manually under a microscope. Avoidance was calculated using the formula:

(4)

where AR is the avoidance rate (in %), Nc is the number of F. candida in the control part of the soil, N_t is the number of F. candida in the tested part of the soil, and No is the total number of F. candida in the box (20 specimens). The results were considered statistically insignificant if the avoidance rate did not exceed 25% ( ISO 17512-2:2011, 2011).“

Unclarities and conflicting statement regarding PAHs

3, L. 96: You state that SC_plas is washed with dichloromethane to extract any residual PAHs, however later (p. 21, l. 501) you say that PAHs in SC_plas 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.

Response: We thank the reviewer for this important comment. We clarify that dichloromethane washing was applied to remove the majority of PAHs potentially formed during plasma-based methane conversion; however, it cannot be excluded that trace amounts of PAHs remained in the final SC_plas material. This residual presence may explain the discussion of possible toxic effects on microbial respiration.

Revision in manuscript: The text has been revised as follows: “The SC_plas was washed with dichloromethane to remove most of the PAHs that could have formed during plasma formation with methane, but traces remained.“

20, L. 481: Results for PAH levels in SC_cat 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.

Response: We thank the reviewer for pointing this out. We have added the PAH analytical method to the Materials and Methods section. Phenanthrene and pyrene in SC_cat were determined in triplicate by GC-MS/MS after accelerated solvent extraction (ASE) and expressed as mg kg⁻¹ dry material. Because these data characterize the carbon materials, the SC_cat PAH concentrations were added to Table 1, and the Discussion now refers explicitly to Table 1.

Revision in manuscript: PAH method added to Materials and Methods; SC_cat PAH data added to Table 1; Discussion revised to refer to Table 1.

21, L. 501: Results for PAH levels in SC_plas are mentioned. These were also not mentioned earlier. Please make sure this is corrected.

Response: We thank the reviewer for this comment. We have added the PAH analytical method to the Materials and Methods section. Phenanthrene and pyrene in SC_plas were determined in triplicate by GC-MS/MS after accelerated solvent extraction (ASE) and expressed as mg kg⁻¹ dry material. The SC_plas PAH concentrations were added to Table 1, and the Discussion now refers explicitly to Table 1. We also revised the wording to present residual PAHs only as a possible contributing factor to the biological responses.

Revision in manuscript: PAH method added to Materials and Methods; SC_plas PAH data added to Table 1; Discussion revised accordingly.

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)”.

Response: Corrected throughout the manuscript. Author names were moved outside parentheses where narrative citations were required, e.g., “Ingwersen (2001)”.

19, l. 435: “These findings align with (Bordoloi et al,. 2021).”

Response: Corrected. The sentence was revised to: “These findings align with Bordoloi et al. (2021).”

19, l. 436: (Bordoloi et al., 2021) investigated how (…)”.

Response: Corrected. The sentence was revised to: “Bordoloi et al. (2021) investigated how …”

20, l. 457: “The results are consistent with those of (Blanco-Canqui, 2017)”.

Response: Corrected. The sentence was revised to: “The results are consistent with those of Blanco-Canqui (2017).”

20, l. 471: “According to (Jing et al., 2022; Yao et al., 2014)”

Response: Corrected. The citation format and sentence structure were revised for clarity.

20, l. 484: “In the experiment of (Van Zwieten et al., 2010)”

Response: Corrected. The phrase was revised to: “In the study by Van Zwieten et al. (2010), …”

20, l. 515: “A study by (Wu et al, 2012) found…”

Response: Corrected. The citation was revised to: “A study by Wu et al. (2012) found …”

23, l. 566: “(…) previous research by (Edah et al., 2020) and (Villagra-Mendoza and Horn, 2018)”

Response: Corrected. The citation was revised to: “previous research by Edeh et al. (2020) and Villagra-Mendoza and Horn (2018).”

23, l. 576: “with (Edeh et al, 2020)”

Response: Corrected. The citation was revised to “consistent with Edeh et al. (2020).”

23, l. 590: ”(Wei et al, 2023) emphasized (…)”

Response: Corrected. We revised the sentence structure so that the citation now follows the correct format, with the author’s name outside the parentheses: “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.

Response: Thank you for pointing this out. We checked the cited references and corrected the attribution. Cao et al. (2008) was retained for EDTA extraction, while Meers et al. (2007) was cited for ammonium-nitrate extraction.

Revision: The citation for ammonium-nitrate extraction was changed from Cao et al. (2008) to Meers et al. (2007).

19, l. 421: “A plausible explanation for this pattern is that SC_plas 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 SC_plas. 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 SC_plas 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.

Response: Thank you for this comment. We agree that the previous wording could incorrectly imply that Ajayi and Horn (2016) and Hardie et al. (2014) investigated SC_plas. We revised the sentence to clarify that these studies examined biochar-amended soils. We also clarified that the “stone-like clusters” were our own visual observation during sample preparation, not a result reported in those studies. The proposed mechanism is now presented as a plausible but untested explanation.

Revision: The sentence was revised to distinguish our observation of SC_plas clustering from mechanisms reported for biochar-amended soils.

21, L. 521: “In the silty loam soil, the toxic effect of SC_plas 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.

Response: Thank you for this valuable comment. We agree that the original wording did not sufficiently account for the relative roles of soil organic matter (SOM) and clay minerals in PAH sorption, and that the citation used was not fully appropriate. We have therefore replaced Yu et al. (2024) with Zhao et al. (2022), which more appropriately describes PAH adsorption by clay minerals. Considering the relatively low organic carbon contents in both soils and the comparatively high clay content (22%) in the silty loam soil, we consider clay minerals to be the most likely important sorbent controlling PAH bioavailability under the conditions of this study. However, SOM may still contribute to sorption processes, although its role is considered secondary in this case. We have revised the manuscript accordingly to reflect this interpretation and to avoid overstatement regarding the relative importance of individual soil components.

Revision in manuscript: we have rephrased the text: “In the silty loam soil, the reduced toxicity of SCplas may be explained by decreased bioavailability of PAHs due to sorption processes, likely associated with clay minerals (22% clay content). Clay minerals are known to act as effective sorbents for polycyclic aromatic hydrocarbons (PAHs) in soils (Zhao et al., 2022). Soil organic matter (SOM) may also contribute to sorption, but due to the relatively low organic carbon content and the lack of mineralogical analysis, its role cannot be quantified in this study.“

22, L. 547: “Conversely, SC_cat 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 SC_cat. 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”.

Response: Thank you for pointing this out. We agree that the previous sentence was speculative and that the citation to Blanco-Canqui (2017) was not appropriate in this context. Since this part of the discussion was removed during revision, the problematic wording and citation are no longer included in the manuscript.

Revision: The sentence formerly at p. 22, l. 547 was deleted.

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)

Response: Thank you for your recommendation. We will use the information that PAHs may have a genotoxic effect on the DNA of earthworms for discussion.

Revision in manuscript: We used a reference to Han et al (2021) : “In addition, Han et al (2021) demonstrated the genotoxic effects of PAHs on the DNA of earthworms. (Han at al., 2021).“

Minor issues

1, l. 31: I suggest not only cautioning against hydrophobic effects in sandy soils, but also ecotoxicological effects.

Response: We agree. The abstract now mentions both hydrophobicity-related limitations and ecotoxicological responses.

Revision in manuscript: Abstract revised.

3, l. 72: Perhaps reference (TITAN Project) rather than inserting a link. The link should instead appear in the reference list.

Response: We agree. We revised the wording and moved the project link to the reference list or appropriate project citation format.

Revision in manuscript: Introduction/reference list revised.

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?

Response: Thank you for this comment. We agree that 40 t ha⁻¹ is relatively high compared with many practical biochar application rates, however, this is still within the range of biochar application rates in many studies (e.g. see the meta-analyses of Ye et al., 2020). We therefore added a clearer justification in the Methods. The rate was selected as a high-dose scenario to ensure detectable changes in soil hydraulic properties and contaminant mobility within the short- to medium-term laboratory experiments. We also clarified that this dose should be interpreted as a screening dose rather than as a general recommendation for field application.

Revision: The Methods section was revised to justify the 40 t ha⁻¹ application rate and to clarify its experimental purpose.

9, l. 247: I do not think it is appropriate to say “their” sandy soils when referencing Streck and Richer (1997).

Response: We agree and corrected the wording.

Revision in manuscript: Text revised.

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.

Response: We agree. Since porosity was calculated using the same assumed bulk density and particle density for all treatments, the repeated column was removed, and the common porosity value was stated in the table note.

Revision in manuscript: Table 5 revised.

10, l. 285: An ‘s’ appears at the end of the line

Response: Thank you. The typographical error was removed.

Revision in manuscript: Typographical correction made.

12, l. 316: You mention that SC_cat and SC_plas reduced CO₂ produced in silty loam soil. However, so did graphite and biochar. I think this should be included in the observation.

Response: we agree that graphite and biochar also reduced CO₂, so we included this in the observation.

Revision in manuscript: the text has been revised to include graphite and biochar, which also reduce CO₂ emissions: “In the silty loam soil, none of the amendments increased CO₂ production, but SC_plas and reference materials slightly reduced it (Fig. 4B).“

12, l. 317: You refer to Fig S1A and B, which do not exist in the paper. Do you mean Fig 4?

Response: Thank you for your clarification. No, we are not referring to Figure 4, but rather Figures S1A and B, which are included in the Supplement document.

14, Figures 7A/7B: Missing a legend, since the total figure (7A-D) is cut in half.

Response: Thank you for identifying this mismatch.

Revision in manuscript: The figure legend for Figure 7 has been updated.

15, l. 366: You have a space too much before “The avoidance”

Response: Thank you for identifying this mismatch.

Revision in manuscript: We removed the space before the word "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”.

Response: We agree. We revised the caption to define error bars and statistical letters, clarified that very small error bars may be hidden by symbols or bars, and removed redundant “C: control” wording.

Revision in manuscript: Figure 8 caption and legend revised.

17, l. 403: You state that for Ni, SC_plas, biochar and graphite reduced concentrations below detection limits at Neckarwestheim. This is inconsistent with figure 11C, which shows a detected level for biochar.

Response: We agree. We corrected the sentence so that it matches Figure 9C and does not state that biochar was below detection limit if a detected value is shown.

Revision in manuscript: Results/discussion text revised.

18, Figure 11: Refers to “C: control”, which is unnecessary, as the legend labels it “Control” and not “C”.

Response: We agree and removed the redundant label explanation.

Revision in manuscript: Figure 9 caption revised.

19, l. 440: Unnecessary to start the sentence by “Other than in the sandy soil, (…)”

Response: We agree and simplified the sentence.

Revision in manuscript: Text revised.

20, l. 490: Claiming that high crystallinity and low reaction surface of SC_cat 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 SC_Cat possibly ensure long-term stability.

Response: We agree that this statement is exaggerated, so we have removed it.

Revision in manuscript: Removal of the statement high crystallinity and low reaction surface of SC_cat likely ensure long-term stability as indicated by non-enhanced CO₂ production seems a bit of a stretch.

21, l. 491 and l. 493: Do not include the title of Table 1 in the bracket.

Response: Thank you for identifying this mismatch. We have removed the table title.

Revision in manuscript: Table title removed.

22 l. 542: Do not say “other studies” when you only reference one single study.

Response: We agree and changed the wording to singular.

Revision in manuscript: Text revised.

22 l. 562: Do not use “studies” in plural when you only refer to one single study.

Response: We agree and changed the wording to singular.

Revision in manuscript: Text revised.

23, l. 589: Once again you state that for Ni, SC_plas, biochar and graphite reduced concentrations below detection limits at Neckarwestheim. This is inconsistent with figure 11C, showing a detected level for biochar.

Response: We agree. We corrected the sentence to match Figure 9C.

Revision in manuscript: Discussion revised.

Citation: https://doi.org/10.5194/egusphere-2025-3866-AC3

RC2: 'Comment on egusphere-2025-3866', Anonymous Referee #2, 20 May 2026

General comments

With great interest I read the manuscript “Amending solid carbon from methane cracking to arable soils: A sustainable approach to increase carbon storage and heavy metal immobilization?” by Saki, Kostiuk et al. In light of climate change mitigation, there is a great need for both alternative energy sources and soil amendments that could sequester more carbon. The approach described in this manuscript could contribute to both, with solid carbon materials derived as a by-product from hydrogen production. The manuscript focuses on the use of this solid carbon as a soil amendment and how this would affect soil physical properties, ecotoxicology, and heavy metal availability.

While the research is undoubtedly interesting and, and the experiments generally seem sound, the manuscript itself could have been better written and appears sloppy / messy in some places. It lacks context for the audience, and for example information about the upscaling potential of this method. The discussion too closely follows the structure of the results without going into possible relevant mechanisms and the bigger picture. Methods are described that do not appear in the result section (EDTA extractions) or important information is lacking (e.g. which earthworm species), and basic changes in soil properties following amendments (e.g. pH) are not described. The number of figures is on the high side; perhaps some could be moved to an appendix. Recommendations for further research are not provided at all. I suggest a major revision of the manuscript to tackle these issues.

Detailed comments

Title: The title might not be too appealing to attract the right audience. What about something like: “The potential of a novel sustainable soil amendment for increasing carbon storage and heavy metal immobilization”?

L.20 (and later in methods): the application dose of 40 tons/ha is on the high side when compared with biochar, where often 10 or 20 tons/ha is used in research because in practice doses are even lower. In the methods, the choice for the dose needs to be better justified.

L. 30-32: from the above description of the results, I would conclude differently, e.g. because you write that SCcat had almost no effect on the studied biological properties.

L. 48-50: here I feel context is lacking. The idea to use these materials as a soil amendment must come from somewhere. What chemical/physical properties do they have that suggests they can be added to soils? Later on (L.53-54) you write that the materials are expected to behave as biochar, but why? And similarly, why is graphite also chosen as a reference material? (L. 78) In other words, the materials need to be introduced much better and the context better provided. The audience (the soil science community) needs to know what these novel materials are and how they compare to things they already know (such as biochar).

L. 51-52: does this statement refer to natural SOC or soil carbon in general? Not all C is equal.

L. 60-61: why does biochar reduce GHG emissions? Some more mechanistic explanations would be valuable.

L. 86-97: most readers will not know much about these methods. For example, which of the two is more common or scalable? Some context would be better.

L. 96: the mention of PAH is somewhat out of the blue here. Only later in the discussion is becomes clear why this is relevant and apparently common for such materials. I think the common possible risks of the solid materials should already be mentioned in the introduction.

L. 99: the pyrolysis T of the biochar is on the high side compared to the average pyrolysis T. How would that have influenced the properties of the biochar and your results?

Table 1: how were these characteristics determined? E.g. the iron content and SSA

Table 2: hoe were the Corg and Nt contents determined from the LOI described on L. 120-121?

L. 170: which endogeic species were introduced? That is key information to include.

L. 193: how was earthworm activity monitored?

L. 229: how was the Cd content determined? And why is only Cd shown and not the contents of the other metals from Figure 10?

L. 234-240: were the amendments mixed into the dried soil and then immediately extracted in the various extractions? How long was the equilibration phase with 0.0025 M CaCl2 and was this before or after addition of the amendments? This is unclear.

L. 397: I don’t agree that SCplas consistently demonstrated a strong immobilizing effect. For Cd, perhaps the element of most strong focus, this was not shown.

Discussion: I found the discussion in general too long and too strongly tied to the figures presented in the results. It would be better to discuss each material per category of investigation (soil physical properties, ecotoxicology, and heavy metal availability), rather than per characteristic within each of these categories. This would already shorten the discussion substantially because it avoids some repetitions (e.g. when discussing the separate heave metals). This means including more generic statements like those in L. 489-494. Then, a final section discussing the applicability in general would really strongly benefit the manuscript. This shortly summarizes the pros and cons of both novel materials relative to the references, talks about practical factors like upscaling, and makes recommendations for future research.

Section 4.2: this section lacks entirely a discussion on the possible effect of the native SOC (e.g. statement in L.462-463). If any effect (e.g. on soil respiration) is shown, it could come directly from the amendment, or indirectly because decomposition of the native SOC is primed, or the native SOC is stabilised via bonding with the amendment. Or both effects are prevalent but cancel each other out in the net effect. The minimum that would be required is discussing how the pH changed after amendment, as pH strongly links with microbial activity and could be a key driver for the observed changes in EOC, respiration and microbial biomass following amendment (and also for heavy metal availability). But the authors can also discuss more mechanistically how the materials could affect the native SOC. A nice starting point (for biochar) would be Han et al Geoderma 2020, see e.g. their Figure 5 (doi:10.1016/j.geoderma.2020.114184).

L. 479-482, L.501-503: the role and relevance of PAHs should already be introduced much earlier in the manuscript. I got confused that PAHs were detected even though SCplas was washed with DCM.

L. 523-524: earthworm biomass results are not presented in the manuscript, but are referred to here. Perhaps they can be added to an appendix if they are deemed key.

Section 4.3: I suggest to merge this section with 4.4, as it contains quite some duplication, especially when making the section more general and mechanistic. I also found this section quite weak content-wise. The section talks a lot about how enhanced CEC drives the observed patterns, but heavy metals mostly bind via ion-specific adsorption and many more mechanisms can play a role (see. e.g., Figure 2 and associated text in Joseph et al. GCB Bioenergy 2021, doi:10.1111/gcbb.12885). Also, the section talk a lot about functional group diversity without giving any details about this, which is not very compelling. If functional group diversity is mentioned, what is known about it from the reference materials can be discussed from the literature, and ideally it should be investigated for the novel materials. The same applies to the porosity for SCplas (porosity is not determined for SCplas in Table 1, but changes in it are given as an explanation in L.543). Finally, like with section 4.2, the section could be much more strongly linked to changes in pH induced by the amendments.

Minor and technical comments

L.15: why the quotation marks for “plasmalytic”?

L.18-19: the last part of the aim doesn’t read well, please rephrase

L.27: “had almost no effect” --> what does this mean? Did it or did it not have a statistically significant effect? Please rephrase.

L.39: “negative emissions technologies” --> the term Carbon Dioxide Removal (CDR) technologies is nowadays more common. Please rephrase.

L 61: “both soil nitrogen (N) dynamics”--> why both? I think this word can be removed.

L. 81-82: Information seems an odd word choice here, very non-specific. What about guidelines, or a framework?

L. 86: not relevant to mention the project name here.

L. 148: here 40 microcosms are mentioned, combining both soils, but later on in the methods (L.191) a total number per soil type is given. This is inconsistent.

L. 148: I assume the 115 g is only for the sandy soil? And for the silty loam it has to be 4*31.25 = 125 g?

L. 153-156: do I understand correctly from L.151-152 that the number of vials and thus the amount of soil declines over time in the soil respiration measurements? If yes, would be good to explicitly mention that

L. 216: what is the area associated with the 200 g Cd input per year?

L. 224: areas --> area

L. 283-284: this statement conflicts with L. 280-281.

Table 4: the footnote can be included in the table header for clarity

L. 293: “with values sharing the same letter not being significantly different..”

L. 293: “post-hoc Tukey test”.

L. 311-313: this is a weirdly phrased statement. Rather than using the word ‘pronounced’, it would make more sense to directly describe the positive / negative effect.

Figure 4 (and elsewhere): I find ‘test material’ an odd description. What about ‘solid amendment’ instead?

L. 347: SCcat mentioned twice

L. 356: endogenous --> endogeic

L. 368: I do not find ‘largely’ the right word here. The avoidance is only slightly higher than the statistically significant limit of 25%

L. 397: “also varied by metal and site” (because repetition from L. 377)

L. 471-474: the use of brackets makes this sentence confusing.

Section 4.3: no need to give the abbreviations of the metals again.

L. 625-627: this repeats L. 620-624 and could be removed.

Citation: https://doi.org/10.5194/egusphere-2025-3866-RC2

AC2: 'Reply on RC2', Hermin Saki, 09 Jun 2026

Response: We thank the reviewer for the careful evaluation and constructive assessment of the manuscript. We agree that the manuscript required substantial revision to improve clarity, context, and interpretation. We therefore revised the manuscript throughout to improve readability and consistency, expanded the Introduction to better introduce methane-derived solid carbon materials and their comparison with biochar and graphite, and clarified the rationale for testing these materials as soil amendments.

In the Discussion, we reduced repetition and moved away from a figure-by-figure interpretation. In particular, the previous Sections 4.3 and 4.4 were merged into one section discussing heavy metal concentrations in soil solution and operationally defined plant availability together. The revised section now provides a broader mechanistic interpretation, including ion-specific adsorption, surface complexation, precipitation, redox reactions, pH-mediated effects, and site-specific soil properties. We also added a new section on general applicability and future research needs, addressing the advantages and limitations of SC_cat and SC_plas relative to biochar and graphite, practical aspects such as application rate and field-scale use, and key knowledge gaps for future studies.

We further revised the Methods and Results to improve consistency, including clarification of extraction methods, amendment application rate, earthworm and springtail test details, and the presentation of figures and tables. Where appropriate, figures were reduced or moved into tables/supplementary material to improve readability. We also revised the conclusions to avoid overstatement and to emphasize that the effects of both novel solid carbon materials are site-, material-, and purpose-specific.

Revision: The Introduction, Methods, Discussion, figures/tables, and Conclusions were substantially revised to improve context, clarity, mechanistic interpretation, and practical applicability.

Response: Thank you for the suggestion. We revised the title to make it clearer and more appealing to the target audience while avoiding overstatement of carbon storage or heavy metal immobilization effects.

Revision: The title was revised to: “Assessing solid carbon from methane cracking as a soil amendment: effects on soil hydraulic properties, soil biology and heavy metal mobility.”

L.20 (and later in methods): the application dose of 40 tons/ha is on the high side when compared with biochar, where often 10 or 20 tons/ha is used in research because in practice doses are even lower. In the methods, the choice for the dose needs to be better justified.

Revision: The Methods section was revised to justify the 40 t ha⁻¹ application rate and to clarify its experimental purpose.

30-32: from the above description of the results, I would conclude differently, e.g. because you write that SC_cat had almost no effect on the studied biological properties.

Response: Thank you for pointing out this inconsistency. We agree that the previous final statement did not follow logically from the preceding results, particularly because SC_cat showed almost no effect on the biological endpoints. We revised the abstract to align the interpretation with the results. The revised text now states that SC_cat showed minor effects on the assessed endpoints and comparatively limited ecotoxicological effects under the tested conditions, but also limited benefits as a soil amendment. We also revised the interpretation of SC_plas to emphasize metal- and site-specific effects and to avoid overstatement.

Revision: The abstract was revised to provide a more balanced and internally consistent conclusion on SC_cat and SC_plas. “The solid carbon from catalytic pyrolysis (SC_cat) had no significant effect on the biological properties studied and showed no significant influence on plant-available water, although it moderately increased hydraulic conductivity in the sandy soil. ”

48-50: here I feel context is lacking. The idea to use these materials as a soil amendment must come from somewhere. What chemical/physical properties do they have that suggests they can be added to soils? Later on (L.53-54) you write that the materials are expected to behave as biochar, but why? And similarly, why is graphite also chosen as a reference material? (L. 78) In other words, the materials need to be introduced much better and the context better provided. The audience (the soil science community) needs to know what these novel materials are and how they compare to things they already know (such as biochar).

Response: Thank you for this comment. We agree that the introduction needed more context for the soil science audience. We revised the Introduction to better introduce methane-derived solid carbon materials and explain which material properties may make them relevant as soil amendments, including stability, particle size, surface area, porosity, hydrophobicity, and surface reactivity. We also clarified why biochar and graphite were selected as reference materials: biochar as a well-studied carbon-based soil amendment and graphite as a more crystalline and comparatively inert carbon reference.

Revision: The Introduction was expanded to explain how SC_cat and SC_plas compare conceptually with biochar and graphite and why their effects need to be tested rather than assumed.

51-52: does this statement refer to natural SOC or soil carbon in general? Not all C is equal.

Response: Corrected. We revised the sentence to clarify that the statement refers specifically to soil organic carbon, not to soil carbon in general. We also changed the wording from soil fertility to soil quality and functioning to better match the scope of the study.

Revision: The sentence now reads: “Soil quality and functioning are often positively related to soil organic carbon content (Giandon, 2015; Lal, 2004).”

60-61: why does biochar reduce GHG emissions? Some more mechanistic explanations would be valuable.

Response: Thank you for this suggestion. We added a mechanistic explanation of how biochar may reduce greenhouse gas emissions. The revised text now explains that biochar can contribute to carbon sequestration through the long-term stabilization of recalcitrant aromatic carbon and may reduce greenhouse gas emissions by influencing N₂O, CH₄, and CO₂ production through changes in nitrification, denitrification, soil redox conditions, labile carbon availability, and microbial respiration.

Revision: The Introduction was revised to include mechanistic explanations for the effects of biochar on greenhouse gas emissions.

86-97: most readers will not know much about these methods. For example, which of the two is more common or scalable? Some context would be better.

The production methods by plasma pyrolysis and catalytic pyrolysis using iron ore as catalyst are pursuing pilot plant demonstrations by the companies Monolith(USA) and Hazer (AUS), respectively (ref attached). While the two processes produce (decarbonized) H2 at low carbon intensity as major product, they also produce solid carbon as co-product in stoechiometric amount. The produced carbon of this study are made by plasma pyrolysis and catalytic pyrolysis using Microwave technologies. Plasma technology achieves higher temperature than catalytic pyrolysis. As a result the produced carbon has different crystaline and porous structures which may find different applications.

96: the mention of PAH is somewhat out of the blue here. Only later in the discussion is becomes clear why this is relevant and apparently common for such materials. I think the common possible risks of the solid materials should already be mentioned in the introduction.

Response: Thank you for this comment. We agree that the relevance of PAHs should be introduced earlier. We added a short explanation in the Introduction that PAHs can form during high-temperature carbonization processes and may condense on the surface of solid carbon materials. We also mention that similar PAH-related risks have been reported for improperly produced or insufficiently treated biochars, which explains why PAHs are relevant for assessing the safety of these materials as soil amendments.

Revision in manuscript: The Introduction was revised to briefly introduce PAHs as a potential risk associated with solid carbon materials and to explain why PAH residues are relevant for soil organisms and microbial processes.

L. 99: the pyrolysis T of the biochar is on the high side compared to the average pyrolysis T. How would that have influenced the properties of the biochar and your results?

Response: Thank you for this important comment. We selected a biochar produced at 850°C because our experimental materials obtained by catalytic pyrolysis and plasma processing of biogas consist predominantly of highly carbonized carbon structures. Therefore, a high-temperature biochar provided a more appropriate reference material than low-temperature biochars. We have added a discussion to the manuscript explaining that biochar produced at 850°C is characterized by a higher degree of carbonization and aromaticity, greater stability against microbial degradation, and a lower content of labile organic compounds and oxygen-containing functional groups. These properties make it more comparable to our carbon-rich test materials and more relevant for evaluating their potential for long-term carbon sequestration. However, the lower abundance of surface functional groups may also reduce some chemically mediated interactions compared with biochars produced at lower pyrolysis temperatures.

Revision: “This temperature was intentionally selected because the investigated carbon materials obtained from catalytic pyrolysis and plasma processing of biogas are composed predominantly of highly carbonized carbon structures.”

Table 1: how were these characteristics determined? E.g. the iron content and SSA

Structure : X-Ray diffraction

Specific surface area , mesoporosity : BET and BJH methods using N2 adsorption isotherm at 77K, respectively

Particle size : Scanning Electron Miscroscopy

Metallic Iron content : Thermal Gravimetry Analysis

Table 2: hoe were the Corg and Nt contents determined from the LOI described on L. 120-121?

Response: Thank you for pointing this out. We clarified that C_org and N_t were not determined from LOI. Total carbon and total nitrogen were measured by dry combustion, and C_org was calculated as the difference between total carbon and inorganic carbon.

Revision: The Methods section was revised to specify how C_org and N_t were determined.

170: which endogeic species were introduced? That is key information to include.

Response: We thank the reviewer for this important comment. We agree that the identity of the introduced endogeic species is key information and should be explicitly stated. Therefore, we have revised the manuscript to specify the earthworm species used in the experiment. For the sandy soil, the introduced endogeic species was Aporrectodea caliginosa, whereas for the silty loam soil, Aporrectodea icterica was used. These were the endgeic earthworms that occurred most frequently in the respective soil types. This information has been added to the Materials and Methods section.

Revision in manuscript: The following sentence has been added to the Materials and Methods section: “For the ecotoxicological study, we used juvenile endogeic earthworms (175-357 mg living biomass) that were collected from the sites where the soils were taken from (Aporrectodea caliginosa- the sandy soil, Aporrectodea icterica- the silty loam soil).”

193: how was earthworm activity monitored?

Response: We thank the reviewer for this comment. We have clarified the procedure used to monitor earthworm activity in the manuscript. Earthworm activity was assessed by daily monitoring of the earthworms' location within the microcosms. The position of each individual was marked on the outside surface of the microcosm using a marker, allowing us to track changes in its spatial distribution and activity over time.

Revision in manuscript: The following text has been added to the Materials and Methods section: “Earthworm activity and spatial distribution were monitored daily throughout the 28-day exposure period. The position of each individual (i.e., in the control or amended soil compartment) was recorded once per day. In addition, the external position of each earthworm within the transparent vessels was marked on the outer surface of the vessel to ensure consistent tracking of its location over time.”

229: how was the Cd content determined? And why is only Cd shown and not the contents of the other metals from Figure 10?

Response: Thank you for this comment. We clarified in the table footnote how Cd content was determined and why only Cd is shown in the site-characterization table. The Braunschweig soils originate from the raster cells described by Ingwersen (2001), where Cd contamination was the main basis for site characterization. Therefore, Cd was included in Table 3 as the primary contamination indicator used to describe and select the sites. Cu, Ni, and Zn were not included in Table 3 to keep the site-characterization table concise, because these metals were not used for site selection but were analysed as experimental response variables and are presented in the Results section.

Revision: A footnote was added to Table 3 to clarify the source and method of Cd determination and the reason for presenting Cd in the site-characterization table.

234-240: were the amendments mixed into the dried soil and then immediately extracted in the various extractions? How long was the equilibration phase with 0.0025 M CaCl2 and was this before or after addition of the amendments? This is unclear.

Response: Thank you for pointing out this lack of clarity. We revised the Methods section to clarify the sequence of amendment addition and extraction. The amendments were first mixed into the air-dried soils, and no separate pre-incubation was performed before extraction. The CaCl₂ solution was added after amendment addition, and the 24 h shaking period represented the equilibration/extraction phase.

Revision: The CaCl₂ extraction method was revised to clarify when the amendments were added and how long the CaCl₂ equilibration/extraction phase lasted.

397: I don’t agree that SC_plas consistently demonstrated a strong immobilizing effect. For Cd, perhaps the element of most strong focus, this was not shown.

Discussion: I found the discussion in general too long and too strongly tied to the figures presented in the results. It would be better to discuss each material per category of investigation (soil physical properties, ecotoxicology, and heavy metal availability), rather than per characteristic within each of these categories. This would already shorten the discussion substantially because it avoids some repetitions (e.g. when discussing the separate heave metals). This means including more generic statements like those in L. 489-494. Then, a final section discussing the applicability in general would really strongly benefit the manuscript. This shortly summarizes the pros and cons of both novel materials relative to the references, talks about practical factors like upscaling, and makes recommendations for future research.

Response: Thank you for this comment. We agree that the previous wording overstated the effect of SC_plas, especially because Cd immobilization was not consistently observed. We removed this statement and revised the text to state that SC_plas showed the clearest immobilization effect for Cu at selected sites, whereas Cd, Ni, and Zn responses were site-, metal-, and fraction-dependent. We also merged the previous Sections 4.3 and 4.4 to reduce repetition and to discuss heavy metal concentrations in soil solution and plant availability of heavy metal together in a more general and mechanistic way. In addition, we added a new section on general applicability and future research needs, summarizing the advantages and limitations of SC_cat and SC_plas relative to biochar and graphite, practical considerations such as application rate and field-scale use, and key knowledge gaps for future studies.

Revision: Sections 4.3 and 4.4 were merged and rewritten to avoid overstatement, reduce duplication, and strengthen the material- and site-specific interpretation. A new “General applicability and future research needs” section was added to synthesize the practical implications of the study.

Section 4.2: this section lacks entirely a discussion on the possible effect of the native SOC (e.g. statement in L.462-463). If any effect (e.g. on soil respiration) is shown, it could come directly from the amendment, or indirectly because decomposition of the native SOC is primed, or the native SOC is stabilised via bonding with the amendment. Or both effects are prevalent but cancel each other out in the net effect. The minimum that would be required is discussing how the pH changed after amendment, as pH strongly links with microbial activity and could be a key driver for the observed changes in EOC, respiration and microbial biomass following amendment (and also for heavy metal availability). But the authors can also discuss more mechanistically how the materials could affect the native SOC. A nice starting point (for biochar) would be Han et al Geoderma 2020, see e.g. their Figure 5 (doi:10.1016/j.geoderma.2020.114184).

Response: Importantly, in contrast to many biochar-based studies (e.g. Han et al., 2020, Geoderma, doi:10.1016/j.geoderma.2020.114184), the solid carbon materials made by methane pyrolysis used in our experiment did not induce measurable changes in soil pH. We therefore consider pH-driven shifts in microbial activity and nutrient availability to be unlikely drivers of the observed effects in our system.

479-482, L.501-503: the role and relevance of PAHs should already be introduced much earlier in the manuscript. I got confused that PAHs were detected even though SC_plas was washed with DCM.

Response: We agree that the role and relevance of PAHs should be introduced earlier in the manuscript to provide better context for the interpretation of the results. We also acknowledge that the detection of PAHs in SCplas may be unexpected given that the material was washed with DCM. To address this concern, we have included information on PAH content and the characteristics of the studied solid carbon materials in Table 1 (Characteristics of the studied solid carbon materials). This addition provides the necessary background information at an earlier stage of the manuscript and improves the clarity of the study design and results interpretation.

Revision in manuscript: PAH concentrations have been added to Table 1 (“Characteristics of the studied solid carbon materials”).

	Solid carbon from catalytic pyrolysis (SC_cat)	Solid carbon from plasma pyrolysis (SC_plas)	Biochar	Graphite
Starting material	methane	methane	Straw (Miscanthus x giganteus)	graphite
Structure	crystallized	crystallized	amorphous	crystallized
Specific surface area, (m² g^-1)	5.9	58.8	346.4	17.4
Mesoporosity	2< pore diameter <50 nm	n.d.	2< pore diameter <50 nm	2< pore diameter <50 nm
Particles size	< 200 μm	<200 µm	125 and 1000 µm	< 50 μm
Metallic iron	1.7 wt.%	n.d.	n.d.	n.d.
Phenanthrene, (mg kg⁻¹)	0.09 ± 0.02	2.5 ± 0.05	n.d.	0.1 ± 0.02
Pyrene, (mg kg⁻¹)	0.01 ± 0.002	1.7 ± 0.2	n.d.	0.02 ± 0.004
C_org(%)	98	100	77.44
N_t(%)	n.d.	n.d.	0.59
C/N(%)	n.d.	n.d.	131

523-524: earthworm biomass results are not presented in the manuscript, but are referred to here. Perhaps they can be added to an appendix if they are deemed key.

Response: Earthworm biomass was monitored during the experiment; however, these data were not included in the main text of the manuscript because they were not a primary endpoint of the study and were not directly related to the main research objectives. In addition, considering the overall length of the manuscript, we aimed to focus on the parameters most relevant to the study questions. However, to avoid confusion, we have revised the text accordingly and added the reference to the earthworm biomass results.

Revision in manuscript: A table presenting earthworm biomass data has been added to the Supplementary Information.

Section 4.3: I suggest to merge this section with 4.4, as it contains quite some duplication, especially when making the section more general and mechanistic. I also found this section quite weak content-wise. The section talks a lot about how enhanced CEC drives the observed patterns, but heavy metals mostly bind via ion-specific adsorption and many more mechanisms can play a role (see. e.g., Figure 2 and associated text in Joseph et al. GCB Bioenergy 2021, doi:10.1111/gcbb.12885). Also, the section talk a lot about functional group diversity without giving any details about this, which is not very compelling. If functional group diversity is mentioned, what is known about it from the reference materials can be discussed from the literature, and ideally it should be investigated for the novel materials. The same applies to the porosity for SC_plas (porosity is not determined for SC_plas in Table 1, but changes in it are given as an explanation in L.543). Finally, like with section 4.2, the section could be much more strongly linked to changes in pH induced by the amendments.

Response: Thank you for this detailed suggestion. We merged Sections 4.3 and 4.4 to reduce duplication and revised the section to provide a broader mechanistic interpretation of the heavy metal results. Following Joseph et al. (2021), we no longer explain the patterns mainly by CEC, but instead discuss multiple possible mechanisms, including ion-specific adsorption, surface complexation, ligand exchange, precipitation, redox reactions, pH-mediated changes in metal solubility, and interactions with mineral and organic soil constituents. We also removed unsupported statements about functional group diversity and porosity where these properties were not directly measured for SC_plas and SC_cat.

Revision: Sections 4.3 and 4.4 were merged and rewritten to emphasize metal-, fraction-, and site-specific responses and to present the proposed mechanisms as plausible explanations rather than confirmed processes.

Minor and technical comments

L.15: why the quotation marks for “plasmalytic”?

plasma-based approaches

Response: Corrected. We removed the quotation marks and replaced “plasmalytic” with “plasma-based” to avoid ambiguity and improve readability.

Revision: The sentence now reads: “Methane pyrolysis, including catalytic and plasma-based approaches, has attracted attention…”

L.18-19: the last part of the aim doesn’t read well, please rephrase

Response: Corrected. We rephrased the aim sentence to improve readability and clarify that the study assessed soil hydraulic properties, heavy metal mobility, and potential ecotoxicological effects on soil organisms.

Revision: The sentence was revised to: “This study investigated the potential of solid carbon materials from catalytic pyrolysis and plasmalysis, alongside biochar and graphite as reference materials, to modify soil hydraulic properties and heavy metal mobility, and to assess potential ecotoxicological effects on soil organisms.”

L.27: “had almost no effect” --> what does this mean? Did it or did it not have a statistically significant effect? Please rephrase.

Response: Corrected. We replaced the vague phrase “had almost no effect” with a statistically clearer statement. The revised text now specifies that SC_cat showed no significant effect on the biological endpoints assessed.

Revision: The abstract was revised to: “SC_catshowed no significant effect on the biological endpoints assessed and had no significant influence on plant-available water, although it moderately increased hydraulic conductivity in the sandy soil.”

L.39: “negative emissions technologies” --> the term Carbon Dioxide Removal (CDR) technologies is nowadays more common. Please rephrase.

Response: Corrected. We replaced “negative emissions technologies” with the more commonly used term “carbon dioxide removal (CDR) technologies.”

Revision: The sentence was revised to: “Carbon dioxide removal (CDR) technologies, such as bioenergy with carbon capture and storage (BECCS), have emerged as key strategies to achieve these targets.”

L 61: “both soil nitrogen (N) dynamics”--> why both? I think this word can be removed.

Response: Corrected. We removed “both” because it was unnecessary in this sentence.

Revision: The sentence was revised to “It can also influence soil nitrogen (N) dynamics.”

81-82: Information seems an odd word choice here, very non-specific. What about guidelines, or a framework?

Response: Corrected. We replaced the non-specific word “information” with a more precise formulation and revised the sentence to emphasize that the findings provide guidance/framework for soil-specific assessment.

Revision: The sentence was revised to: “Our findings provide a framework for evaluating whether, and under which soil-specific conditions, solid carbon from plasma and catalytic pyrolysis can be considered as an amendment to agricultural soils.”

86: not relevant to mention the project name here.

Response: Corrected. We removed the project name from the Methods section because it was not relevant to the synthesis description.

Revision: The project name (“TITAN project”) has been removed from the text.

148: here 40 microcosms are mentioned, combining both soils, but later on in the methods (L.191) a total number per soil type is given. This is inconsistent.

Response: We thank the reviewer for this comment. We would like to clarify that the numbers refer to two different experimental setups and therefore are not inconsistent. The statement on L.148 (40 microcosms) refers to the experiment used to assess soil respiration, microbial biomass, extractable organic carbon (EOC), and nitrogen mineralization across both soil types. In contrast, the description on L.191 refers to a separate earthworm experiment conducted in planar vessels, which included a total of 28 units.

148: I assume the 115 g is only for the sandy soil? And for the silty loam it has to be 4*31.25 = 125 g?

Response: We thank the reviewer for this careful observation. We agree that the originally reported value (115 g) was correct for the sandy soil only, but it did not accurately reflect the corresponding amount for the silty loam soil. As correctly pointed out by the reviewer, the appropriate value for the silty loam soil is 125 g (4 × 31.25 g). We have revised the manuscript accordingly to correct this inconsistency and to ensure clarity and accuracy of the reported soil amounts.

Revision in manuscript: The text has been revised as follows: “Each microcosm contained either 115 g of fresh sandy soil or 125 g of fresh silty loam soil, corresponding to 100 g of dry matter (DM) in each case.”

153-156: do I understand correctly from L.151-152 that the number of vials and thus the amount of soil declines over time in the soil respiration measurements? If yes, would be good to explicitly mention that

Response: The reviewer is correct that the number of vials decreased over time. However, the amount of soil used for each soil respiration measurement remained constant, since each measurement was performed using soil from a single vial. We have clarified this point in the Methods section to prevent potential confusion.

Revision in manuscript: The following clarification has been added to the Methods section: “Accordingly, the number of vials remaining per microcosm decreased over time. The amount of soil used for each measurement remained constant because each vial contained the same quantity of soil.”

216: what is the area associated with the 200 g Cd input per year?

Response: Thank you for pointing this out. The value refers to an area-based Cd load, not to the total Cd input for the whole irrigation area. We corrected the sentence to specify the unit as g ha⁻¹ yr⁻¹.

Revision: The sentence was revised to: “Initial heavy metal assessments in the early 1980s indicated Cd inputs of up to approximately 200 g ha⁻¹ yr⁻¹ in the wastewater irrigation area.”

224: areas --> area

Response: Checked. I have changed it accordingly.

283-284: this statement conflicts with L. 280-281.

Response: We revised the paragraph to remove the apparent conflict between the general statement about the water retention curves and the treatment-specific PAW results.

Revision: The Results section was revised to align the text with Fig. 2 and the statistical differences shown in Table 4. “The addition of SC_plas visibly altered the shape of the water retention curves in both the sandy (Fig. 2A) and silty loam soil (Fig. 2B), whereas the other amendments showed only minor deviations from the control curve. The effect of SC_plas was soil-specific. In the sandy soil, SC_plas reduced the volumetric water content between pF 0 and pF 3 and decreased the plant available water (PAW), whereas in the silty loam soil, SC_plas increased both quantities (Table 4). Similar to SC_plas, graphite significantly increased plant available water in the silty loam soil. In contrast, SC_cat and the biochar did not change the amount of plant available water compared to the control in the silty loam soil.”

Table 4: the footnote can be included in the table header for clarity

Response: Corrected. We moved the definitions of SC_cat and SC_plas into the Table 4 caption for clarity.

293: “with values sharing the same letter not being significantly different..”

Response: Corrected. We revised the wording to: “values sharing the same letter are not significantly different.”

293: “post-hoc Tukey test”.

Response: Corrected. We specified the statistical test as “ANOVA followed by Tukey’s post-hoc test.”

311-313: this is a weirdly phrased statement. Rather than using the word ‘pronounced’, it would make more sense to directly describe the positive / negative effect.

Response: We agree with the reviewer, so we have removed this sentence and now begin the paragraph directly with a description of the observed effects in each soil type.

Revision in manuscript: We have removed the first sentence from this paragraph.

Figure 4 (and elsewhere): I find ‘test material’ an odd description. What about ‘solid amendment’ instead?

Response: We thank the reviewer for this suggestion. We agree that the term “solid amendment” is more appropriate and clearer than “test material”. Therefore, we have revised Figure 4 and the corresponding text throughout the manuscript accordingly.

Revision in manuscript: The term “test material” has been replaced with “solid amendment” in all figure and table captions, including Figure 4.

347: SCcat mentioned twice

Response: We thank the reviewer for pointing this out. We confirm that SC_cat was inadvertently mentioned twice in this sentence. We have corrected this duplication in the revised manuscript.

Revision in manuscript: The sentence has been corrected as follows: “In silty loam soil, only the addition of graphite increased the content of nitrate significantly in comparison to the control, while the other materials (SC_cat, SC_plas, biochar) decreased nitrate contents starting from day 14 on (Table 6).”

356: endogenous --> endogeic

Response: We thank the reviewer for this comment. The term has been corrected to “endogeic” in the revised manuscript.

Revision in manuscript: The term has been corrected in the text as follows: “In terms of avoidance test with earthworms, all specimens survived the 28 days of incubation in control and amended soils. In the sandy soil, the avoidance test showed a significant avoidance of endogeic earthworms against soils amended with SC_plas and graphite, but not against soils amended with SC_cator biochar (Fig. 5A).”

368: I do not find ‘largely’ the right word here. The avoidance is only slightly higher than the statistically significant limit of 25%

Response: We thank the reviewer for this comment. We agree that the term “largely” was not appropriate in this context. Therefore, we have revised the sentence to more accurately reflect that the avoidance was only slightly above the statistically significant threshold of 25%.

Revision in manuscript: The sentence has been revised as follows: “Springtails showed avoidance of soils amended not only with biochar and graphite, but also with SCplas, whereas no significant avoidance was observed for SCcat (Fig. 6B).”

397: “also varied by metal and site” (because repetition from L. 377)

Response: Corrected. We rephrased the second sentence to avoid repeating “varied by metal and site.”

Revision: The sentence was revised to: “A similar site- and metal-dependent pattern was observed for plant-available heavy metal concentrations, as determined by ammonium nitrate extraction.” For L.377 the sentence was revised to: “The effects of soil amendments on heavy metal concentrations in 0.0025 M CaCl₂ solution were site- and metal-specific (Fig. 10).”

471-474: the use of brackets makes this sentence confusing.

Response: Thank you for your comment. We agree that the numerous parenthetical phrases made the sentence difficult to read. The sentence has been revised to improve clarity by incorporating the information previously provided in parentheses into the main text.

Revision: “According to Jing et al. (2022) and Yao et al. (2014), the presence of clay particles on pyrogenic carbon surfaces can increase adsorption capacity by approximately fivefold. This enhancement is mainly attributed to ion exchange associated with clay particles and electrostatic attraction associated with biochar. A decrease in nitrate ion concentration in the silty loam soil was observed after 14 days. Although the temporal pattern of the biochar effect on nitrate varied with soil texture, biochar generally decreased nitrate concentrations. This finding is consistent with previous studies showing that biochar can reduce inorganic nitrogen leaching through adsorption (Liu et al., 2018; Major et al., 2012; Sun et al., 2017; Yao et al., 2012).”

Section 4.3: no need to give the abbreviations of the metals again.

Response: Corrected. We removed repeated definitions of the metal abbreviations in Section 4.3 where these abbreviations had already been introduced earlier. The section now uses the metal names without redefining the abbreviations.

Revision: “For zinc (Zn)” and similar repeated abbreviation definitions were revised to “For zinc,” “For copper,” “For nickel,” and “For cadmium.”

625-627: this repeats L. 620-624 and could be removed.

Response: Corrected. We removed the repetitive final paragraph and revised the conclusion to avoid restating the same interpretation twice.

Revision: The conclusion was shortened and restructured to provide one concise final synthesis of the material-specific benefits and limitations of SC_cat and SC_plas.

Citation: https://doi.org/10.5194/egusphere-2025-3866-RC2

Citation: https://doi.org/10.5194/egusphere-2025-3866-AC2

Hermin Saki, Kateryna Kostiuk, Joachim Ingwersen, Galina Krauße, Valentin L’hospital, Nolven Guilhaume, Marilena Radoiu, David Farrusseng, Thilo Streck, Ellen Kandeler, and Sven Marhan

Supplement

https://doi.org/10.5194/egusphere-2025-3866-supplement

Hermin Saki, Kateryna Kostiuk, Joachim Ingwersen, Galina Krauße, Valentin L’hospital, Nolven Guilhaume, Marilena Radoiu, David Farrusseng, Thilo Streck, Ellen Kandeler, and Sven Marhan

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Short summary

Scientists tested two new solid carbon materials produced from hydrogen as soil improvers. These materials are produced by breaking down methane gas using different technologies. One material improved water retention and reduced heavy metals, but harmed soil organisms. The other offered fewer benefits and increased metal pollution. This research will help determine the conditions under which these carbon materials can safely be used to improve agricultural soils.


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