Agronomic Potential of Pyrochar and Hydrochar from Sewage Sludge: Effects of Carbonization Conditions

Comments 1: Abstract: Consider specifying the origin of the sewage sludge in the abstract (e.g., "municipal SS") as its composition can be highly variable.

Response 1: Thank you for the suggestion. We agree that specifying the origin of the sewage sludge improves clarity given its variability. Accordingly, we have revised the abstract to indicate that the sewage sludge used in this study is of municipal origin.

Comments 2: Introduction: It currently states the focus is on "agronomic quality," which is broad. Emphasizing the comparison of nutrient bioavailability as the central novel aspect would be more precise. Furthermore, the sentence "Hydrochar and pyrochar were produced under a range of operational conditions..." (Page 3) is somewhat methodological and might fit better in the Materials and Methods section.

Response 2: We have revised the introduction to clarify that the central novel aspect of this study is the comparison of nutrient bioavailability between hydrochar and pyrochar, rather than the broader concept of agronomic quality alone. We agree that the sentence describing the production of hydrochar and pyrochar under different operational conditions is methodological in nature. We have replaced the original text with the following:

“The novelty of this study lies in its integrated assessment of HTC and pyrolysis as val-orization routes for sewage sludge, with a focus on the agronomic quality of the result-ing hydrochar and pyrochar and a specific emphasis on comparing their nutrient bioa-vailability. This comparative perspective highlights how different carbonization path-ways influence the potential of the final solid product for sustainable agricultural use.”

Comments 3: Raw Material: You should introduce in detail，such as, the key initial characteristics of the dried SS

Response 3: Thank you for the suggestion. We have added text in the Raw Material section indicating that the key initial characteristics of the dried sewage sludge are provided in Table 1, where the detailed physicochemical properties are presented.

Comments 4: Section 2.2: The rationale for the different heating rates (3.5 K/min for pyrolysis vs. 1 K/min for HTC) should be briefly explained. Is this due to the inherent limitations of the different reactor systems?

Response 4: Thank you for the observation. We have added text in Section 2.2 explaining the rationale for the different heating rates used in the study. For both processes, the selected heating rate corresponds to the minimum rate required to ensure homogeneous heating conditions within each reactor system. The differences arise from the inherent operational characteristics and limitations of the respective reactors used for pyrolysis and HTC. The following text has been added at the end of Section 2.2.:

“The heating rates selected for pyrolysis (3.5 K/min) and HTC (1 K/min) correspond to the minimum rates required to ensure homogeneous and stable heating conditions within each reactor system, reflecting the inherent operational characteristics and limitations of the respective equipment.”

Comments 5: Section 2.5: This section must be expanded. It is crucial to state how the statistical results are reported. Currently, the absence of the detail statistical indicators in the results is a major omission

Response 5: We thank the reviewer for this valuable comment. Following your suggestion, we have improved the statistical description in the Materials and Methods section. The revised text now provides a clearer explanation of the tests used to assess normality (Shapiro–Wilk), homogeneity of variances (Levene), and the criteria applied in the ANOVA, ensuring greater transparency and methodological detail. We have replaced the original text with the following:

“Statistical analyses were performed to verify that the data met the assumptions required for parametric testing. Normality of the residuals was assessed using the Shapiro–Wilk test, which evaluates deviations from a normal distribution; no violation of this assumption was detected (p > 0.001). Homogeneity of variances across groups was examined with the Levene test, which tests the equality of group variances, and the results confirmed that this assumption was also satisfied.

Differences among treatments were evaluated using an analysis of variance (ANOVA). The F-statistic was calculated as the ratio between the mean square of the factor (MS) and the mean square of the residuals (MS), and evaluated at a significance level of α = 0.05. Significant differences were detected among treatments (p < 0.05).”

Error bars and significance letters have also been included in the figure. We have also incorporated the statistical interpretation into the Results and Discussion section.

Comments 6: Section 3.4: For sulfur, the explanation could be deepened. Why does available S decrease in hydrochars but increase in pyrochars with temperature? The hypothesis about the formation of metal sulfides in hydrochars is interesting but speculative. Are there analytical techniques that could confirm the speciation of S (e.g., X-ray Absorption Near Edge Structure spectroscopy)?

Response 6: We thank the reviewer for this insightful suggestion and for pointing out the relevance of techniques such as XANES for confirming sulfur speciation. Unfortunately, all original samples associated with this study were fully consumed during the analyses already reported, and no remaining material is available to perform additional spectroscopic characterization. Therefore, it is not possible for us to obtain experimental speciation data (e.g., XANES, XPS) for inclusion in the revised version of the manuscript.

Given this analytical limitation, it is also important to note that from an agronomic perspective, the most relevant parameter is the concentration of plant-available sulfate, as sulfate is the predominant form in which sulfur is taken up by plants; therefore, our analysis focuses on this fraction rather than on total sulfur speciation. We now clarify this point in the manuscript and acknowledge that future work using dedicated spectroscopic techniques will be necessary to confirm the specific S transformations proposed here.

Although we recognize that direct confirmation of sulfur speciation is not possible without additional spectroscopic analyses, the hypothesis regarding the formation of metal sulfides in hydrochars is supported by previous studies showing that hydrothermal conditions promote the stabilization of sulfur into reduced, mineral-bound forms such as metal sulfides.

However, we have revised the text to improve clarity and ensure that this explanation is more easily understood.

Comments 7: References: Some references are too outdated (e.g.,Rulkens,2008; Gómez,2008; IGAC,2006; Channiwala,2002; ). The references are relevant but could be updated to include more recent studies (10.1016/j.cscm.2025.e05047; 10.3390/buildings15142471), and ensure all citations in the text match the reference list.

Response 7: Thank you for the suggestion. The references Gómez (2008) and IGAC (2006) have been retained because they remain fundamental methodological sources that are still widely applied. Additionally, the reference 10.3390/buildings15142471 has been incorporated as literature supporting. We have also ensured that all in-text citations are consistent with the reference list

4. Response to Comments on the Quality of English Language

Point 1: The English could be improved to more clearly express the research.

Response 1: A full language revision has been performed to ensure consistency, proper grammar, and improved readability across the entire text.

Comments 1: Inconsistency in units for N-NO₃⁻ (mg/L vs mg·kg⁻¹) In the methodology (section 2.4), nitrate concentrations in the filtrate were determined chromatographically with LOD/LOQ in mg/L

Response 1: Thank you for pointing this out. However, in the research results (section 3.4, description of Fig. 8), the values for N-NO₃⁻ are presented as mg·kg⁻¹ of material. The described conversion from solution to sample dry mass (extraction factor, volume/weight, moisture content) is also missing. It is proposed to add a conversion equation (with the sample mass, extract volume, and moisture content provided) or to report in mg/L of solution.

We thank the reviewer for pointing out the inconsistency in the units used for N–NO₃⁻. We acknowledge that the LOD and LOQ values were mistakenly reported in mg·L⁻¹, whereas the correct units are mg·kg⁻¹. This was a typographical error, and it has now been corrected in the revised manuscript.

In addition, we have updated the methodology to ensure full consistency in units throughout the text. We have also added a detailed explanation of the conversion from solution concentration (mg·L⁻¹) to dry-mass basis (mg·kg⁻¹), including extraction volume, sample dry mass, moisture correction, and blank subtraction. The following paragraph and the corresponding conversion equation have been incorporated into Section 2.4 to address this point.

Quantification was performed by external calibration using certified nitrate standard solution traceable to SRM from NIST NaNO₃ in H₂O 1000 mg·L⁻¹, NO₃⁻ Certipur®. The calibration range was 0.02–10 mg·L⁻¹, with excellent linearity (R² = 0.9993). Method repeatability, expressed as the relative standard deviation (RSD) of six replicate injections of a 5 mg·L⁻¹ standard, was 3.84%, which is consistent with commonly accepted performance criteria (<5%) for suppressed ion chromat graphy methods (ISO 10304).

The unit conversion from mg·L⁻¹measured in the extract to mg·kg⁻¹ dry sample was performed according to Equation (1), including blank subtraction:

                                                       (1)

Where:

C_IC (mg N- NO₃^- kg⁻¹) is the concentration obtained from ion chromatography,

C_blank (mg NO₃^- L⁻¹) is the nitrate concentration measured in the reagent blank,

V _extract (L)is the extraction solution volume,

DF is the dilution factor,

m_dry (kg) is the dry-weight equivalent of the sample,

14 (mg) is the atomic weight of Nitrogen,

and 62 (mg) is the molecular weight of the nitrate ion (NO₃⁻)

The evaluation of measurement uncertainty was carried out in accordance with the recommendations of the Guide to the Expression of Uncertainty in Measurement (GUM). The combined standard uncertainty Ucwas obtained by identifying and quantifying all relevant sources of uncertainty and propagating them using the law of propagation of uncertainty as described in the GUM. The expanded uncertainty U was then calculated as: U=K. Uc, where a coverage factor K=2 was applied, corresponding to an approximate confidence level of 95%.

Comments 2: Equation (2) for the "distribution of metal elements" The equation describes the element's participation in the product "relative to the initial SS mass," but the notation in the denominator includes the dry mass of solid product y, which suggests an accidental insertion of the yield symbol "y" into the denominator. It is recommended to clearly define whether the numerator represents the mass of the element in char, and the denominator the initial dry mass of SS, or to express it as a percentage in char × (y/100) as a metric normalized to SS. Add units to the formula.

Response 2: Thank you for the suggestion. The correction has been made, and the equation is now expressed as a percentage in the char x (y/100) as a metric normalized to the initial SS. The notation has been clarified accordingly, and units have been added to the formula.

Comments 3: Statistical justification for n=3 (paragraph "Statistical analysis") The Shapiro-Wilk test and one-way ANOVA, along with Tukey's test with three replicates per group, can be supplemented with additional statistical analyzes. It was not stated whether the homogeneity of variance was checked (e.g., Levene's test). The effect size and confidence intervals are missing from the graphs. It is proposed to add a test for homogeneity of variance (Levene/Brown–Forsythe), report η²/ω² for ANOVA, and add error bars and letter designations for significantly different groups to the graphs (Figs. 2–3, 8). Alternatively, non-parametric alternatives (Kruskal–Wallis with Dunn) can be considered in the absence of normality.

Response 3: We thank the reviewer for this valuable comment. Following your suggestion, we have improved the statistical description in the Materials and Methods section. The revised text now provides a clearer explanation of the tests used to assess normality (Shapiro–Wilk), homogeneity of variances (Levene), and the criteria applied in the ANOVA, ensuring greater transparency and methodological detail

We have replaced the original text with the following:

“Statistical analyses were performed to verify that the data met the assumptions required for parametric testing. Normality of the residuals was assessed using the Shapiro–Wilk test, which evaluates deviations from a normal distribution; no violation of this assumption was detected (P > 0.001). Homogeneity of variances across groups was examined with the Levene test, which tests the equality of group variances, and the results confirmed that this assumption was also satisfied.”

Differences among treatments were evaluated using an analysis of variance (ANOVA). The F-statistic was calculated as the ratio between the mean square of the factor (MS) and the mean square of the residuals (MS), and evaluated at a significance level of α = 0.05. Significant differences were detected among treatments (P < 0.05).

Error bars and significance letters have also been included in the figure. We have also incorporated the statistical interpretation into the Results and Discussion section

Comments 4: Interpretation of heavy metal mobility based primarily on total content. Claims of "lower pollutant content" and potential metal mobility in hydrochar are speculative without leaching tests and speciation (e.g., TCLP, BCR). It is recommended to supplement the discussion of the results with leaching tests (e.g., using water, acid, brine) or to state that this is a hypothesis requiring further research. You can move overly risky statements to "limitations."

Response 4: We thank the reviewer for this important observation. We agree that interpretations regarding heavy metal mobility based solely on total metal content are limited, and that statements about reduced pollutant content or potential metal mobility in hydrochars cannot be confirmed without dedicated leaching or speciation analyses (e.g., TCLP, BCR). Since leaching tests were not performed in this study and no material remains available to conduct them.

Comments 5: Lack of control over the influence of the "residence time" factor on agronomic traits In section 3.4, it was stated that "the time effect was not investigated" because the differences were insignificant in the proximal analyzes, which is an assumption. The availability of N, P, and S can respond independently of proximal time. It is worth either showing the full data (time effect) for N-NH₄⁺/P/S and pH, or clearly limiting the generalizations to temperature

Response 5: We appreciate the reviewer’s insightful comment. As clarified in Section 3.4, the analysis in this study was intentionally centered on temperature, since preliminary evaluations indicated that temperature exerted a substantially stronger influence on the transformation processes than residence time. However, we acknowledge the reviewer’s point that residence time may also affect agronomic traits such as N–NH₄⁺, P and S availability, as well as pH, independently of its limited impact on proximal analyses. In response, the manuscript has been revised to explicitly recognize this potential influence and to clearly limit the interpretation of our findings to temperature-driven effects. This clarification has been incorporated into Section 3.4.

“Bioavailable nutrient content and pH of hydrochar and pyrochar obtained at different temperatures were determined. The effect of residence time was not studied, as no notable differences were observed in the proximate and ultimate analyses of the chars (Table 1). Nevertheless, we acknowledge that residence time may still influence agronomic traits such as N–NH₄⁺, P, S availability, and pH. For this reason, the discus-sion is focused on temperature, which showed a substantially stronger effect on mate-rial transformation, while recognizing that residence time could exert a secondary in-fluence that remains beyond the scope of the present analysis.”

Comments 6: pH Consequences for "Fit" to Soil Types

The statement "hydrochar for alkaline soils, pyrochar for acidic soils" seems too simplistic without CEC results, ash alkalinity, soil buffering capacity, and dosage. It is worth changing the theses by suggesting that "selection depends on soil pH/CEC and dosage – pot/field trials required.

Response 6: We thank the reviewer for this valuable suggestion, and we have revised the manuscript accordingly to reflect a more nuanced and evidence-based recommendation. In response to this comment, we have added the following sentence to the manuscript to provide a more accurate and balanced recommendation.

“However, the choice between hydrochar and pyrochar should be guided by soil pH, cation-exchange capacity, and application rate, and must ultimately be validated through pot- or field-scale trials.”) Methodological remarks and clarity of description”

Comments 7: Methodological remarks and clarity of description

·       Process parameters: for pyrolysis, a heating rate of 3.5 K·min⁻¹ and manual vacuum control were specified. It's worth adding the temperature profile (time to T_f), measurement uncertainty T/p.

·       HTC: "autogenic pressure monitored over time" is provided, but the pressure range and its uncertainty are missing. It would be good to include the T-p-t curve in the supplementary materials.

·       BET: only SBET values were recorded, while pore size distribution (BJH) and hysteresis are missing. The statement "no clear trend" is not supported by a statistical test.

·       FTIR: the description assigns bands to minerals and organics, but the spectra are not normalized (e.g., relative to the quartz band), and the spectra of SS and chars are not shown on a common intensity scale (which makes them harder to compare).

Response 7:

·       Process parameters: for pyrolysis, a heating rate of 3.5 K·min⁻¹ and manual vacuum control were specified. It's worth adding the temperature profile (time to T_f), measurement uncertainty T/p.

The requested information regarding the temperature profile has been added to the Supplementary Material for clarity and completeness. Uncertainty is associated with the measurement instruments themselves (e.g., thermocouples, pressure gauges), rather than with the pyrolysis or hydrothermal process. For this reason, no additional uncertainty values are reported beyond the specifications already provided by the equipment manufacturers.

·       HTC: "autogenic pressure monitored over time" is provided, but the pressure range and its uncertainty are missing. It would be good to include the T-p-t curve in the supplementary materials.

The pressure range during the HTC process, along with the corresponding temperature–pressure–time (T–p–t) curve, has been added to the Supplementary Material.

·       BET: only SBET values were recorded, while pore size distribution (BJH) and hysteresis are missing. The statement "no clear trend" is not supported by a statistical test.

Thank you for this comment. In the revised manuscript, we have included an explanatory paragraph with the corresponding supporting data to address this point. Specifically, both the BJH pore surface area and the analysis of the hysteresis loop have now been incorporated. Regarding the comment on the statement “no clear trend,” this phrase has been removed, as the trends are now explicitly described and supported by the pore structure analysis and isotherm interpretation. The text included:

“As shown in Table 1, the specific surface area of all chars produced in this study ranges between 12,7 and 96,5 m²·g⁻¹ when determined by the BET method and between 11,74 and 79,17 m²·g⁻¹ when determined by the BJH method. These relatively low values indicate an incipient development of porosity. This observation is corroborated by the nitrogen sorption isotherms at 77 K presented in Figure S4.1 of the Supplementary Ma-terial (Section S4). Low values of the amount adsorbed (i.e., pore volume) at low relative pressures, together with the slope observed at relative pressures up to approximately 0,9 and the presence of a hysteresis loop, indicate the predominance of mesoporosity. For both processes, hydrothermal carbonization and pyrolysis, similar trends are observed, as shown in Figure S4.2 of the Supplementary Material. An increase in specific surface area and mesopore volume with increasing final processing temperature is observed (Fig. S4.1 (a) and (c) and Fig. S4.2 (a) and (c)), whereas a decrease in these parameters occurs with increasing residence time (Fig. S4.1 (b) and (d) and Fig. S4.2 (b) and (d)). These effects are more pronounced in the case of hydrothermal carbonization. The re-duction in porosity with increasing residence time may be associated with structural transformations of the chars that occur without significant variations in yield or chemical composition. Additionally, Figure S4.1 (c) shows a decrease in specific surface area and pore volume at 550 °C compared with the values obtained at 430 °C, which may be attributed to loss of porosity due to volatilization of the pyrolyzed material at higher temperatures. Moreover, the SBET values obtained in this study are consistent with those reported in the literature for pyrochars and hydrochars produced from SS [52, 53, 54].”

·       FTIR: the description assigns bands to minerals and organics, but the spectra are not normalized (e.g., relative to the quartz band), and the spectra of SS and chars are not shown on a common intensity scale (which makes them harder to compare).

We thank the reviewer for this valuable comment. In response, we have added a new figure in which all FTIR spectra (SS and chars) are presented on a common intensity scale and normalized relative to the quartz band, as suggested. This modification improves the comparability of the spectra and clarifies the assignment of mineral and organic functional groups.

Comments 8: Data presentation, figures, tables

·       Table 1: the header contains typographical artifacts ("LHVd", "SBET,d A /m²·g⁻¹ d"), and the columns mix values; in row P520-20, "0,05 ± 0.09" appears (mixed decimal separator and dot). It is recommended to standardize the separator, use consistent units (SI), move "d" (dry basis) to the table caption, and separate rows and columns (e.g., with lines) to improve the table's readability.

·       Figs. 2-3: add error bars.

Response 8:

·       Table 1: the header contains typographical artifacts ("LHVd", "SBET,d A /m²·g⁻¹ d"), and the columns mix values; in row P520-20, "0,05 ± 0.09" appears (mixed decimal separator and dot). It is recommended to standardize the separator, use consistent units (SI), move "d" (dry basis) to the table caption, and separate rows and columns (e.g., with lines) to improve the table's readability.

Thank you for the detailed recommendations. All suggested revisions have been implemented. Table 1 has been reformatted according to the journal’s style guidelines: typographical artifacts have been removed, decimal separators and units have been standardized (using SI units), the “d” designation for dry basis has been moved to the table caption

·       Figs. 2-3: add error bars.

Error bars have been added to Figures 2 and 3 as requested.

Comments 9: Language and typographical errors, and suggestions for corrections Below are examples for improvement:

·       „ISO ISO 18123:2023” → „ISO 18123:2023”

·       „ThermoFischer Flash 2000” → „Thermo Fisher Flash 2000”

·       „hydrocars pH were measured …” → „hydrochars pH were measured …” 3

·       "mobile phase... 1.2 mL·min⁻¹ and 20 µL..." → remove "f"

·       "FRX" in the caption of Fig. 5 → "XRF"

·       "sewage sludge" in Conclusions → "sewage sludge"

·       "Effect of operating conditions..." in the literature → "Effect of operating conditions..."

·       „Pirólisisde Biomasa” (item 30) → „Pirólisis de Biomasa”

·       "Global Nest Journal, ... 24th" (item 35) → add the full year (e.g., (2024)

·       Consistency of the software name: "SigmaPlot 12.0", not "Sigmaplot"

·       Unit notation: m²·g⁻¹, r·min⁻¹ (no period after "min"), "K·min⁻¹" → better "K·min⁻¹" or "°C·min⁻¹" depending on the intention. Unify everyplace. Libardo

·       There are missing commas in many places and minor grammatical errors ("the hydrochars exhibit..."). I suggest a language edit.

Response 9: Thank you for the comprehensive list of language and typographical corrections. All the suggested changes have been implemented throughout the manuscript. Additionally, a full language revision has been performed to ensure consistency, proper grammar, and improved readability across the entire text.

Comments 10: Suggestions for expanding and supplementing the work

·       Environmental tests: leachability can be added (e.g., TCLP/BCR) and a simple phytotoxicity test (germination) to support the claims of agronomic safety.

·       Phosphorus balance: in addition to available and total P, it is worth providing speciation (PO₄³⁻/P₂O₇⁴⁻/organic P) in a few representative samples, which will increase the value of the conclusions.

·       Energy: since LHVd is reported, it would be worthwhile to also show HHV (with the Channiwala–Parikh correlation) or justify why LHV is relevant here from an agronomic perspective.

·       Measurement uncertainty: a table with uncertainty values for the determinations (N-NH₄⁺, P, S, pH) can be added in the supplement

·       Title: you can leave it as is or consider clarifying "...effects of carbonization temperature and residence time.”

·       Section 2.4: you can add the unit conversion scheme and IC parameters (column, eluent, calibration range, R², repeatability)

·       Conclusions: generalizations regarding soil suitability can be softened and "requires pot trials" can be added.

Response 10:

·       Environmental tests: leachability can be added (e.g., TCLP/BCR) and a simple phytotoxicity test (germination) to support the claims of agronomic safety.

We appreciate this valuable suggestion; however, because no material remains available for additional analyses, we are unable to conduct leachability or phytotoxicity tests at this stage. We have therefore revised the manuscript to acknowledge this limitation and to state that future studies should include TCLP/BCR leaching assays and germination tests to more robustly assess agronomic safety.

·       Phosphorus balance: in addition to available and total P, it is worth providing speciation (PO₄³⁻/P₂O₇⁴⁻/organic P) in a few representative samples, which will increase the value of the conclusions.

We are grateful for the reviewer’s suggestion; however, no material remains available for additional analyses, and therefore phosphorus speciation (e.g., PO₄³⁻, P₂O₇⁴⁻, organic P) cannot be performed at this stage. From an agronomic perspective, we emphasize that the most relevant fraction is orthophosphate (PO₄³⁻), as it represents the predominant form in which plants absorb phosphorus (Amadou et al., 2022; Mimura et al., 2024). Accordingly, our discussion focuses on this available P fraction. We have clarified this point in the revised manuscript and acknowledge that future studies including detailed P speciation would strengthen the conclusions.

Mimura, T., Reid, R. Phosphate environment and phosphate uptake studies: past and future. J Plant Res 137, 307–314 (2024). https://doi.org/10.1007/s10265-024-01520-9

Issifou, A., Faucon, M.-P., & Houben, D. (2022). Role of soil minerals on organic phosphorus availability and phosphorus uptake by plants. Geoderma, 428, 116125. https://doi.org/10.1016/j.geoderma.2022.116125

In response to this comment, we have added the following sentence:

“Although the speciation of P and N in hydrochars and pyrochars may be valuable for future studies, in the present work we only evaluated the bioavailable forms of S and P (specifically sulfates and phosphates), with the aim of determining their plant-available fractions”

·       Energy: since LHVd is reported, it would be worthwhile to also show HHV (with the Channiwala–Parikh correlation) or justify why LHV is relevant here from an agronomic perspective.

The LHV has been removed from the manuscript because energy-related parameters (LHV or HHV) are not relevant to the agronomic perspective of this study. Since the objective is to evaluate the soil-related and nutrient characteristics of the pyrochars and hydrochars rather than their energetic potential, including HHV (via the Channiwala–Parikh correlation) or discussing LHV does not contribute to the agronomic interpretation. For clarity and focus, all energy-based metrics have therefore been excluded from the revised version.

·       Measurement uncertainty: a table with uncertainty values for the determinations (N-NH₄⁺, P, S, pH) can be added in the supplement.

Thank you for your valuable comment. We agree that providing the measurement uncertainty values improves the transparency and robustness of the analytical results. Following your suggestion, we have added a supplementary table that includes the uncertainty estimates for all determinations (N–NH₄⁺, P, S, and pH). The table reports the combined standard uncertainty u_c, the expanded uncertainty U(k=2), and the corresponding confidence level (~95%), calculated according to the GUM (JCGM 100:2008). In response to this comment, we have added the following sentence:

“The evaluation of measurement uncertainty was carried out in accordance with the recommendations of the Guide to the Expression of Uncertainty in Measurement (GUM). The combined standard uncertainty u_cwas obtained by identifying and quantifying all relevant sources of uncertainty and propagating them using the law of propagation of uncertainty as described in the GUM. The expanded uncertainty U was then calculated as: U=K. Uc, where a coverage factor K=2 was applied, corresponding to an approximate confidence level of 95%.”

JCGM 104:2009. Evaluation of Measurement Data — An Introduction to the “Guide to the Expression of Uncertainty in Measurement” and Related Documents.

·       Title: you can leave it as is or consider clarifying "...effects of carbonization temperature and residence time."

Thank you for the suggestion. We have adopted the proposed clarification and updated the title to include “effects of carbonization temperature and residence time.”

·       Section 2.4: you can add the unit conversion scheme and IC parameters (column, eluent, calibration range, R², repeatability)

Thank you for the suggestion. The corresponding information has been added to Section 2.4, including the unit conversion scheme and the IC parameters.

Quantification was performed by external calibration using certified nitrate standard solution traceable to SRM from NIST NaNO₃ in H₂O 1000 mg·L⁻¹, NO₃⁻ Certipur®. The calibration range was 0.02–10 mg·L⁻¹, with excellent linearity (R² = 0.9993). Method repeatability, expressed as the relative standard deviation (RSD) of six replicate injections of a 5 mg·L⁻¹ standard, was 3.84%, which is consistent with commonly accepted performance criteria (<5%) for suppressed ion chromat graphy methods (ISO 10304).

The unit conversion from mg·L⁻¹measured in the extract to mg·kg⁻¹ dry sample was performed according to Equation (1), including blank subtraction:

                                                        (1)

Where:

C_IC (mg N- NO₃^- kg⁻¹) is the concentration obtained from ion chromatography,

C_blank (mg NO₃^- L⁻¹) is the nitrate concentration measured in the reagent blank,

V _extract (L)is the extraction solution volume,

DF is the dilution factor,

m_dry (kg) is the dry-weight equivalent of the sample,

14 (mg) is the atomic weight of Nitrogen,

and 62 (mg) is the molecular weight of the nitrate ion (NO₃⁻)

·       Conclusions: generalizations regarding soil suitability can be softened and "requires pot trials" can be added.

In response to this comment, we have added the following sentence in conclusions:

“It is recommended that future research include greenhouse and field trials to validate the agronomic performance of these materials under real soil conditions.”

4. Response to Comments on the Quality of English Language

Point 1: There are missing commas in many places and minor grammatical errors ("the hydrochars exhibit..."). I suggest a language edit.

Response 1: A full language revision has been performed to ensure consistency, proper grammar, and improved readability across the entire text.